RDF Primer
RDF Primer
W3C Recommendation 10 February 2004
New Version
Available: "RDF 1.1 Primer"
(Document Status Update, 25 February 2014)
The RDF Working Group has produced
a W3C Recommendation for a new version of RDF which adds
features to this 2004 version, while remaining compatible.
Please see
"RDF 1.1
Primer"
for a new version of this document, and the
"What's New in RDF 1.1"
document for the differences between this
version of RDF and RDF 1.1.
This version:
Latest version:
Previous version:
Editors:
Frank Manola,
fmanola@acm.org
Eric Miller, W3C,
em@w3.org
Series Editor:
Brian McBride, Hewlett-Packard Laboratories,
bwm@hplb.hpl.hp.com
Please refer to the
errata
for this document, which may include some normative corrections.
See also
translations
W3C
MIT
ERCIM
Keio
),
liability
trademark
document
use
and
software
licensing
rules apply.
Abstract
The Resource Description Framework (RDF) is a language for
representing information about resources in the World Wide Web.
This Primer is designed to provide the reader with the basic
knowledge required to effectively use RDF. It introduces the basic
concepts of RDF and describes its XML syntax. It describes how to
define RDF vocabularies using the RDF Vocabulary Description
Language, and gives an overview of some deployed RDF applications.
It also describes the content and purpose of other RDF
specification documents.
Status of this Document
This document has been reviewed by W3C Members and other interested
parties, and it has been endorsed by the Director as a
W3C
Recommendation
. W3C's role in making the Recommendation is to
draw attention to the specification and to promote its widespread
deployment. This enhances the functionality and interoperability of
the Web.
This is one document in a
set
of six
Primer
Concepts
Syntax
Semantics
Vocabulary
and
Test
Cases
) intended to jointly replace the original Resource
Description Framework specifications,
RDF Model and Syntax (1999
Recommendation)
and
RDF Schema
(2000 Candidate Recommendation)
. It has been developed by the
RDF Core Working Group
as part of the
W3C Semantic Web
Activity
Activity
Statement
Group
Charter
) for publication on 10 February 2004.
Changes to this document since the
Proposed Recommendation Working Draft
are detailed in
the
change log
The public is invited to send comments to
www-rdf-comments@w3.org
archive
and to participate in general discussion of related technology on
www-rdf-interest@w3.org
archive
).
A list of
implementations
is available.
The W3C maintains a list of
any patent disclosures related to this work
This section describes the status of this document at the time of its
publication. Other documents may supersede this document. A list of current W3C
publications and the latest revision of this technical report can be found in
the
W3C technical reports index
at
Table of Contents
1.
Introduction
2.
Making Statements About
Resources
2.1
Basic Concepts
2.2
The
RDF Model
2.3
Structured Property Values and Blank
Nodes
2.4
Typed Literals
2.5
Concepts Summary
3.
An XML Syntax for RDF:
RDF/XML
3.1
Basic Principles
3.2
Abbreviating and Organizing RDF
URIrefs
3.3
RDF/XML Summary
4.
Other RDF
Capabilities
4.1
RDF
Containers
4.2
RDF
Collections
4.3
RDF
Reification
4.4
More
on Structured Values: rdf:value
4.5
XML
Literals
5.
Defining RDF Vocabularies: RDF
Schema
5.1
Describing Classes
5.2
Describing Properties
5.3
Interpreting RDF Schema
Declarations
5.4
Other Schema Information
5.5
Richer Schema Languages
6.
Some RDF Applications: RDF
in the Field
6.1
Dublin Core Metadata Initiative
6.2
PRISM
6.3
XPackage
6.4
RSS 1.0:
RDF Site Summary
6.5
CIM/XML
6.6
Gene
Ontology Consortium
6.7
Describing Device Capabilities and User
Preferences
7.
Other Parts of the RDF
Specification
7.1
RDF
Semantics
7.2
Test
Cases
8.
References
8.1
Normative References
8.2
Informational References
9.
Acknowledgments
Appendices
A.
More on
Uniform Resource Identifiers (URIs)
B.
More on the Extensible Markup
Language (XML)
C.
Changes
1.
Introduction
The Resource Description Framework (RDF) is a language for
representing information about resources in the World Wide Web.
It is particularly intended for representing metadata about Web
resources, such as the title, author, and modification date of a
Web page, copyright and licensing information about a Web
document, or the availability schedule for some shared resource.
However, by generalizing the concept of a "Web resource", RDF can
also be used to represent information about things that can be
identified
on the Web, even when they cannot be directly
retrieved
on the Web. Examples include information about
items available from on-line shopping facilities (e.g.,
information about specifications, prices, and availability), or
the description of a Web user's preferences for information
delivery.
RDF is intended for situations in which this information
needs to be processed by applications, rather than being
only displayed to people.
RDF provides a common framework for expressing this
information so it can be exchanged between applications without
loss of meaning. Since it is a common framework, application
designers can leverage the availability of common RDF parsers and
processing tools. The ability to exchange information between
different applications means that the information may be made
available to applications other than those for which it was
originally created.
RDF is based on the idea of identifying things using Web
identifiers (called
Uniform Resource Identifiers
or
URIs
), and describing resources in terms of simple
properties and property values. This enables RDF to represent
simple statements about resources as a
graph
of nodes
and arcs representing the resources, and their properties and
values. To make this discussion somewhat more concrete as soon as
possible, the group of statements "there is
a Person
identified by
whose name is
Eric Miller, whose email address is em@w3.org, and whose title is
Dr." could be represented as the RDF graph in
Figure 1
Figure 1: An RDF Graph
Describing Eric Miller
Figure 1
illustrates that RDF uses URIs
to identify:
individuals, e.g., Eric Miller, identified by
kinds of things, e.g., Person, identified by
properties of those things, e.g., mailbox, identified by
values of those properties, e.g.
mailto:em@w3.org
as the value of the mailbox property (RDF also uses character
strings such as "Eric Miller", and values from other datatypes
such as integers and dates, as the values of properties)
RDF also provides an XML-based syntax (called
RDF/XML
) for
recording and exchanging these graphs.
Example 1
is a small chunk of RDF in RDF/XML
corresponding to the graph in
Figure
Example 1: RDF/XML
Describing Eric Miller
Note that this RDF/XML also contains URIs, as well as
properties like
mailbox
and
fullName
(in an
abbreviated form), and their respective values
em@w3.org
, and
Eric Miller
Like HTML, this RDF/XML is machine processable and, using
URIs, can link pieces of information across the Web. However,
unlike conventional hypertext, RDF URIs can refer to any
identifiable thing, including things that may not be directly
retrievable on the Web (such as the person Eric Miller). The
result is that in addition to describing such things as Web
pages, RDF can also describe cars, businesses, people, news
events, etc. In addition, RDF properties themselves have URIs, to
precisely identify the relationships that exist between
the linked items.
The following documents contribute to the specification of
RDF:
RDF Concepts
and Abstract Syntax
[RDF-CONCEPTS]
RDF/XML
Syntax Specification
[RDF-SYNTAX]
RDF Vocabulary
Description Language 1.0: RDF Schema
[RDF-VOCABULARY]
RDF Semantics
[RDF-SEMANTICS]
RDF Test
Cases
[RDF-TESTS]
RDF Primer
(this document)
This Primer is intended to provide an introduction to RDF and
describe some existing RDF applications, to help information
system designers and application developers understand the
features of RDF and how to use them. In particular, the Primer is
intended to answer such questions as:
What does RDF look like?
What information can RDF represent?
How is RDF information created, accessed, and
processed?
How can existing information be combined with RDF?
The Primer is a
non-normative
document, which means
that it does not provide a definitive specification of RDF. The
examples and other explanatory material in the Primer are
provided to help readers understand RDF, but they may not always
provide definitive or fully-complete answers. In such cases, the
relevant normative parts of the RDF specification should be consulted.
To help in doing this, the Primer describes the roles these other
documents play in the complete specification of RDF, and provides
links pointing to the relevant parts of the normative specifications,
at appropriate places in the discussion.
It should also be noted that these RDF documents update and clarify
previously-published RDF specifications
, the
Resource
Description Framework (RDF) Model and Syntax Specification
[RDF-MS]
and the
Resource Description Framework (RDF) Schema Specification 1.0
[RDF-S]
As a result, there have been some changes in terminology, syntax, and concepts.
This Primer reflects the newer set of RDF specifications given in the
bulleted list of RDF documents cited above. Hence, readers
familiar with the older specifications, and with earlier tutorial and
introductory articles based on them, should be aware that there may be
differences between the current specifications and those previous documents.
The
RDF Issue
Tracking
document
[RDFISSUE]
can be consulted for a list of issues
raised concerning the previous RDF specifications, and their resolution in
the current specifications.
2. Making Statements
About Resources
RDF is intended to provide a simple way to make statements
about Web resources, e.g., Web pages. This section
describes the basic ideas behind the way RDF provides these
capabilities (the normative specification describing these
concepts is
RDF
Concepts and Abstract Syntax
[RDF-CONCEPTS]
).
2.1 Basic
Concepts
Imagine trying to state that someone named
John Smith created a particular Web page. A straightforward way
to state this in a natural language such as English would be in the form of a simple
statement such as:
has a
creator
whose value is
John Smith
Parts of this statement are emphasized to illustrate that,
in order to describe the properties of something, there need to be ways
to name, or identify, a number of things:
the thing the statement describes
(the Web page, in this case)
a specific property (creator,
in this case) of the thing the statement describes
the thing the statement says is
the value of this property (who the creator is), for the
thing the statement describes
In this statement, the Web page's URL (Uniform
Resource Locator) is used to identify it. In addition, the
word "creator" is used to identify the property,
and the two words "John Smith" to identify the thing (a person)
that is the value of this property.
Other properties of this Web page could be described by writing
additional English statements of the same general form, using
the URL to identify the page, and words (or other expressions)
to identify the properties and their values. For example, the
date the page was created, and the language in
which the page is written, could be described using the additional
statements:
has a
creation-date
whose value is
August 16,
1999
has a
language
whose value is
RDF is based on the idea that the things being described
have
properties
which have values, and that resources can be described by
making statements, similar to those above, that specify those
properties and values. RDF uses a particular terminology for
talking about the various parts of statements. Specifically,
the part that identifies the thing the statement is about (the
Web page in this example) is called the
subject
The part that identifies the property or characteristic of the
subject that the statement specifies (creator, creation-date,
or language in these examples) is called the
predicate
and the part that identifies the value of that property is
called the
object
So, taking the English statement
has a
creator
whose value is
John Smith
the RDF terms for the various parts of the statement
are:
the
subject
is the URL
the
predicate
is the word "creator"
the
object
is the phrase "John Smith"
However, while English is good for communicating between
(English-speaking) humans, RDF is about making
machine-processable
statements. To make these kinds of
statements suitable for processing by machines, two
things are needed:
a system of machine-processable identifiers
for identifying a subject, predicate, or object in a statement
without any possibility of confusion with a similar-looking
identifier that might be used by someone else on the
Web.
a machine-processable language for representing these
statements and exchanging them between machines.
Fortunately, the existing Web architecture provides both
these necessary facilities.
As illustrated earlier, the Web already provides one form of
identifier, the
Uniform Resource Locator
(URL).
A URL was used in the original example to identify the Web page
that John Smith created. A URL is a character string that
identifies a Web resource by representing its primary access
mechanism (essentially, its network "location"). However, it
is also important to be able to record information about many
things that, unlike Web pages, do not have network locations or
URLs.
The Web provides a more general form of identifier for these
purposes, called the
Uniform Resource
Identifier
(URI). URLs are a particular kind of URI. All
URIs share the property that different persons or organizations
can independently create them, and use them to identify things.
However, URIs are not limited to identifying things that have
network locations, or use other computer access mechanisms. In
fact, a URI can be created to refer to anything that needs
to be referred to in a statement, including
network-accessible things, such as an electronic
document, an image, a service (e.g., "today's weather report
for Los Angeles"), or a group of other resources.
things that are not network-accessible, such as human
beings, corporations, and bound books in a library.
abstract concepts that do not physically exist, such as the
concept of a "creator".
Because of this generality, RDF uses URIs as the basis of
its mechanism for identifying the subjects, predicates, and
objects in statements. To be more precise, RDF uses
URI
references
[URIS]
. A URI reference
(or
URIref
) is a URI, together with an optional
fragment identifier
at the end. For example, the URI
reference
consists of the URI
and (separated by the "#" character) the fragment identifier
Section2
RDF URIrefs can contain Unicode
[UNICODE]
characters (see
[RDF-CONCEPTS]
), allowing many languages to be reflected in URIrefs.
RDF defines a
resource
as anything
that is identifiable by a URI reference, so using URIrefs
allows RDF to describe practically anything, and to state
relationships between such things as well. URIrefs and fragment
identifiers are discussed further in
Appendix A
, and in
[RDF-CONCEPTS]
To represent RDF statements in a machine-processable way,
RDF uses the
Extensible
Markup Language
[XML]
. XML was
designed to allow anyone to design their own document format
and then write a document in that format. RDF defines a
specific XML markup language, referred to as
RDF/XML
for use in representing RDF information, and for exchanging it
between machines. An example of RDF/XML was given in
Section 1
. That example (
Example 1
) used tags such as
and
to delimit the text
content
Eric Miller
and
Dr.
, respectively.
Such tags allow programs written with an understanding of what
the tags mean to properly interpret that content.
Both XML content and (with certain exceptions) tags can contain Unicode
[UNICODE]
characters, allowing information from many languages to be directly represented.
Appendix B
provides further background on
XML in general. The specific RDF/XML syntax used for RDF is
described in more detail in
Section 3
and is normatively defined in
[RDF-SYNTAX]
2.2 The RDF Model
Section 2.1
has introduced
RDF's basic statement concepts, the idea of using
URI references to identify the things referred to
in RDF statements, and RDF/XML as a machine-processable way to
represent RDF statements. With that background, this section
describes how RDF uses URIs to make statements about resources. The
introduction said that RDF was based on the idea of
expressing simple statements about resources, where each
statement consists of a subject, a predicate, and an object.
In RDF, the English statement:
has a
creator
whose value is
John Smith
could be represented by an RDF statement having:
a subject
a predicate
and an object
Note how URIrefs are used to identify not only the
subject of the original statement, but also the predicate and
object, instead of using the words "creator" and "John Smith",
respectively (some of the effects of using URIrefs in this
way will be discussed later in this section).
RDF models statements as nodes and arcs in a graph. RDF's
graph
model
is defined in
[RDF-CONCEPTS]
. In this notation,
a statement is represented by:
a node for the subject
a node for the object
an arc for the predicate,
directed from the subject node to the object node.
So the RDF statement above would be represented by the graph
shown in
Figure 2
Figure 2: A Simple RDF
Statement
Groups of statements are represented by corresponding groups
of nodes and arcs. So, to reflect the additional English statements
has a
creation-date
whose value is
August 16,
1999
has a
language
whose value is
in the RDF graph, the graph shown in
Figure 3
could be used
(using suitable URIrefs to name the properties
"creation-date" and "language"):
Figure 3: Several Statements
About the Same Resource
Figure 3
illustrates that objects
in RDF statements may be either URIrefs, or constant values (called
literals
represented by character strings, in order to represent certain
kinds of property values.
(In the case of the predicate
the literal is an international standard two-letter code for English.)
Literals may not be used as subjects or
predicates in RDF statements. In drawing RDF
graphs, nodes that are URIrefs are shown as ellipses,
while nodes that are literals are shown as boxes.
(The simple character string
literals used in these examples are called
plain
literals
, to distinguish them from the
typed
literals
to be introduced in
Section 2.4
. The various kinds of
literals that can be used in RDF statements are defined in
[RDF-CONCEPTS]
Both plain and typed literals can contain Unicode
[UNICODE]
characters, allowing information from many languages to be directly represented.)
Sometimes it is not convenient to draw graphs when
discussing them, so an alternative way of writing down the
statements, called
triples
is also used. In the triples notation, each statement in the
graph is written as a simple triple of subject, predicate, and
object, in that order. For example, the three statements shown in
Figure 3
would be written in the triples notation as:
Each triple corresponds to a single arc in the graph,
complete with the arc's beginning and ending nodes (the subject
and object of the statement). Unlike the drawn graph (but like
the original statements), the triples notation requires that a
node be separately identified for each statement it appears in.
So, for example,
appears three times (once in each triple) in the triples
representation of the graph, but only once in the drawn graph.
However, the triples represent exactly the same information as
the drawn graph, and this is a key point: what is fundamental
to RDF is the
graph model
of the statements. The
notation used to represent or depict the graph is
secondary.
The full triples notation requires that URI references be
written out completely, in angle brackets, which, as the
example above illustrates, can result in very long lines on a page. For
convenience, the Primer uses a shorthand way of writing triples
(the same shorthand is also used in other RDF specifications).
This shorthand substitutes an XML
qualified name
(or
QName
) without angle brackets as an abbreviation
for a full URI reference (QNames are discussed further in
Appendix B
).
A QName contains a
prefix
that has
been assigned to a namespace URI, followed by a colon, and then
local name
. The full URIref is formed
from the QName by appending the local name to the
namespace URI assigned to the prefix. So, for example, if the
QName prefix
foo
is assigned to the namespace URI
, then the QName
foo:bar
is shorthand for the URIref
Primer examples will also use several "well-known" QName
prefixes (without explicitly specifying them
each time), defined as follows:
prefix
rdf:
, namespace URI:
prefix
rdfs:
, namespace URI:
prefix
dc:
, namespace URI:
prefix
owl:
, namespace URI:
prefix
ex:
, namespace URI:
(or
prefix
xsd:
, namespace URI:
Obvious variations on the "example" prefix
ex:
will also be used as needed in the examples, for instance,
prefix
exterms:
, namespace URI:
(for terms used by an
example organization),
prefix
exstaff:
, namespace URI:
(for the example
organization's staff identifiers),
prefix
ex2:
, namespace URI:
(for a second example
organization), and so on.
Using this new shorthand, the previous set of
triples can be written as:
ex:index.html dc:creator exstaff:85740 .
ex:index.html exterms:creation-date "August 16, 1999" .
ex:index.html dc:language "en" .
Since
RDF uses URIrefs instead of words to name things in statements, RDF refers to a set of URIrefs (particularly a set intended for a specific purpose) as a
vocabulary
. Often, the URIrefs in such vocabularies are organized so that they can be represented as a set of QNames using a common prefix. That is, a common namespace URIref will be chosen for all terms in a vocabulary, typically a URIref under the control of whoever is defining the vocabulary. URIrefs that are contained in the vocabulary are formed by appending individual local names to the end of the common URIref. This forms a set of URIrefs with a common prefix. For instance, as illustrated by the previous examples, an organization such as example.org might define a vocabulary consisting of URIrefs starting with the prefix
for terms it uses in its business, such as "creation-date" or "product", and another vocabulary of URIrefs starting with
to identify its employees. RDF uses this same approach to define its own vocabulary of terms with special meanings in RDF. The URIrefs in this RDF vocabulary all begin with
, conventionally associated with the QName prefix
rdf:
The RDF Vocabulary Description Language (described in
Section 5
) defines an additional set of terms having URIrefs that begin with
, conventionally associated with the QName prefix
rdfs:
. (Where a specific QName prefix is commonly used in connection with a given set of terms in this way, the QName prefix itself is sometimes used as the name of the vocabulary. For example, someone might refer to "the
rdfs:
vocabulary".)
Using common URI prefixes provides a convenient way to organize the URIrefs for a related set of terms. However, this is just a convention. The RDF model only recognizes full URIrefs; it does not "look
inside" URIrefs or use any knowledge about their structure. In particular, RDF does not assume there is any relationship between URIrefs just because they have a common leading prefix (see
Appendix A
for further discussion). Moreover, there is nothing that says that URIrefs with different leading prefixes cannot be considered part of the same vocabulary. A particular organization, process, tool, etc. can define a vocabulary that is significant for it, using URIrefs from any number of other vocabularies as part of its vocabulary.
In addition, sometimes an organization will use a vocabulary's namespace URIref as the URL of a Web resource that provides further information about that vocabulary. For example, as noted earlier, the QName prefix
dc:
will be used in Primer examples, associated with the namespace URIref
. In fact, this refers to the Dublin Core vocabulary described in
Section 6.1
. Accessing this namespace URIref in a Web browser will retrieve additional information about the Dublin Core vocabulary (specifically, an RDF schema). However, this is also just a convention. RDF does not assume that a namespace URI identifies a retrievable Web resource (see
Appendix B
for further discussion).
In the rest of the Primer, the term
vocabulary
will be used when referring to a set of URIrefs defined for some specific purpose, such as the set of URIrefs defined by RDF for its own use, or the set of URIrefs defined by example.org to identify its employees. The term
namespace
will be used only when referring specifically to the syntactic concept of an XML namespace (or in describing the URI assigned to a prefix in a QName).
URIrefs from different vocabularies can be freely mixed in RDF graphs. For example, the graph in
Figure 3
uses URIrefs from the
exterms:
exstaff:
, and
dc:
vocabularies. Also, RDF imposes no restrictions on
how many statements
using a given URIref as predicate can appear in a graph to describe the same resource. For example, if the resource
ex:index.html
had been created by the cooperative efforts of several staff members in addition to John Smith, example.org might have written the statements:
ex:index.html dc:creator exstaff:85740 .
ex:index.html dc:creator exstaff:27354 .
ex:index.html dc:creator exstaff:00816 .
These examples of RDF statements begin to
illustrate some of the advantages of using URIrefs as RDF's
basic way of identifying things. For instance, in the first
statement, instead of
identifying the creator of the Web page by
the character string "John Smith", he has been assigned a URIref,
in this case (using a URIref based on his employee number)
. An advantage of
using a URIref in this case is that the identification
of the statement's subject can be more precise.
That is, the creator of the page is not the
character string "John Smith", or any one of the thousands of
people named John Smith, but the particular John Smith
associated with that URIref (whoever created the URIref defines
the association). Moreover, since there is a URIref to refer to
John Smith, he is a full-fledged resource, and
additional information can be recorded about him, simply by adding
additional RDF statements with John's URIref as the subject.
For example,
Figure 4
shows some additional
statements giving John's name and age.
Figure 4: More Information
About John Smith
These examples also illustrate that RDF uses URIrefs as
predicates
in RDF statements. That is, rather than
using character strings (or words) such as "creator" or "name"
to identify properties, RDF uses URIrefs. Using URIrefs to
identify properties is important for a number of reasons.
First, it distinguishes the properties one person may use from
different properties someone else may use that would otherwise be
identified by the same character string. For instance, in the
example in
Figure
, example.org uses "name" to mean someone's full name
written out as a character string literal (e.g., "John Smith"),
but someone else may intend "name" to mean something different
(e.g., the name of a variable in a piece of program text). A
program encountering "name" as a property identifier on the Web
(or merging data from multiple sources) would not necessarily be
able to distinguish these uses. However, if example.org writes
for its "name"
property, and the other person writes
for hers, it is clear that there are distinct
properties involved (even if a program cannot automatically
determine the distinct meanings). Also, using URIrefs to identify properties
enables the properties to be treated as resources themselves.
Since properties are resources, additional
information can be recorded about them (e.g., the English description of what
example.org means by "name"), simply by adding additional RDF
statements with the property's URIref as the subject.
Using URIrefs as subjects, predicates, and objects in RDF
statements supports the development and use of shared
vocabularies on the Web, since people can discover and begin using
vocabularies already used by others to describe things, reflecting
a shared understanding of those concepts. For example, in
the triple
ex:index.html dc:creator exstaff:85740 .
the predicate
dc:creator
, when fully expanded as a
URIref, is an unambiguous reference to the "creator" attribute
in the Dublin Core metadata attribute set (discussed further in
Section 6.1
), a widely-used set of
attributes (properties) for describing information of all
kinds. The writer of this triple is effectively saying that the
relationship between the Web page (identified by
) and the creator of
the page (a distinct person, identified by
) is exactly the
concept identified by
Another
person familiar with the Dublin Core vocabulary,
or who finds out what
dc:creator
means (say by looking up its definition on the Web)
will know what is meant by this relationship.
In addition, based on this understanding, people can
write programs to behave in accordance with
that meaning when processing triples containing the predicate
dc:creator
Of course,
this depends on increasing the general use of URIrefs to refer to things instead of using literals; e.g., using URIrefs like
exstaff:85740
and
dc:creator
instead of character string literals like
John Smith
and
creator
Even then, RDF's use of URIrefs does not solve all identification
problems because, for example, people can still use different
URIrefs to refer to the same thing.
For this reason, it is a good idea to try to use terms from existing vocabularies (such as the
Dublin Core) where possible, rather than making up new terms that might overlap with those of
some other vocabulary. Appropriate vocabularies for use in specific application areas are
being developed all the time, as illustrated by the applications described in
Section 6
However, even when synonyms are created, the fact that
these different URIrefs are used in the commonly-accessible
"Web space" provides the opportunity both to identify
equivalences among these different references, and to migrate
toward the use of common references.
In addition, it is important to distinguish between any meaning that
RDF itself
associates with terms (such as
dc:creator
in the previous example) used in RDF statements and additional,
externally-defined
meaning that people (or programs written by those people) might associate with those terms.
As a language, RDF directly defines only the graph syntax of subject, predicate, and object triples, certain meanings associated with URIrefs in the
rdf:
vocabulary, and certain other concepts to be described later. These things are normatively defined in
[RDF-CONCEPTS]
and
[RDF-SEMANTICS]
. However, RDF does not define the meanings of terms from other vocabularies, such as
dc:creator
, that might be used in RDF statements. Specific vocabularies will be created, with specific meanings assigned to the URIrefs defined in them, externally to RDF. RDF statements using URIrefs from these vocabularies may convey the specific meanings associated with those terms to people familiar with these vocabularies, or to RDF applications written to process these vocabularies, without conveying any of these meanings to an arbitrary RDF application
not
specifically written to process these vocabularies.
For example, people can associate meaning with
a triple such as
ex:index.html dc:creator exstaff:85740 .
based on the meaning they associate with the appearance of the
word "creator" as part of the URIref
dc:creator
, or based on
their understanding of the specific definition of
dc:creator
in the Dublin Core vocabulary.
However, as far as an arbitrary RDF application is concerned the triple might as
well be something like
fy:joefy.iunm ed:dsfbups fytubgg:85740 .
as far as any built-in meaning is concerned. Similarly, any
natural language text describing the meaning of
dc:creator
that might be found on the Web provides no
additional meaning that an arbitrary RDF application can directly use.
Of course, URIrefs from a particular vocabulary can be used in RDF statements even though a given application may not be able to associate any special meanings with them.
For example,
generic RDF software would recognize that
the above expression is an RDF statement, that
ed:dsfbups
is the
predicate, and so on. It will simply not associate with the triple any special meaning that the vocabulary developer might have associated with a URIref like
ed:dsfbups
. Moreover, based on their understanding of a given vocabulary, people can write RDF applications to behave in accordance with the special meanings assigned to URIrefs from that vocabulary, even though that meaning will not be accessible to RDF applications not written in that way.
The result of all this is that RDF provides a way to make
statements that applications can more easily process. An
application cannot actually "understand" such statements, as noted
already,
any more than a database system "understands" terms like "employee" or "salary" in processing a query like
SELECT NAME FROM EMPLOYEE WHERE SALARY > 35000
However, if an application is appropriately written,
it can deal with RDF statements in a way that makes it seem
like it does understand them,
just as a database system and its applications can do useful work in processing employee and payroll information without understanding "employee" and "payroll".
For example, a user could search the Web for all
book reviews and create an average rating for each book. Then,
the user could put that information back on the Web. Another
Web site could take that list of book rating averages and
create a "Top Ten Highest Rated Books" page. Here, the
availability and use of a shared vocabulary about ratings, and
a shared group of URIrefs identifying the books they apply to,
allows individuals to build a mutually-understood and
increasingly-powerful (as additional contributions are made)
"information base" about books on the Web. The same principle
applies to the vast amounts of information that people create
about thousands of subjects every day on the Web.
RDF statements are similar to a number of other formats for
recording information, such as:
entries in a simple record or catalog listing describing
the resource in a data processing system.
rows in a simple relational database.
simple assertions in formal logic
and information in these formats can be treated as RDF
statements, allowing RDF to be used to integrate data from many
sources.
2.3 Structured Property Values
and Blank Nodes
Things would be very simple if the only types of information
to be recorded about things were obviously in the form of the
simple RDF statements illustrated so far. However, most
real-world data involves structures that are more complicated
than that, at least on the surface. For instance, in the
original example, the date the Web page was created is recorded
as a single
exterms:creation-date
property, with a
plain literal as its value. However, suppose
the value of the
exterms:creation-date
property
needed to record
the month, day, and year as separate pieces of information? Or,
in the case of John Smith's personal information, suppose
John's address was being described. The whole address could be
written out as a plain literal, as in the triple
exstaff:85740 exterms:address "1501 Grant Avenue, Bedford, Massachusetts 01730" .
However, suppose John's address needed to be recorded as a
structure
consisting of separate street, city, state,
and postal code values? How would this be done in RDF?
Structured information like this is represented in RDF by
considering the aggregate thing to be described (like
John Smith's address) as a resource, and then making statements
about that new resource. So, in the RDF graph, in order to
break up John Smith's address into its component parts,
a new node is created to represent the concept of John Smith's
address, with a new URIref to identify it,
say
(abbreviated as
exaddressid:85740
).
RDF statements (additional arcs and nodes) can then be
written with that
node as the subject, to represent the additional information,
producing the graph shown in
Figure
Figure 5: Breaking Up John's
Address
or the triples:
exstaff:85740 exterms:address exaddressid:85740 .
exaddressid:85740 exterms:street "1501 Grant Avenue" .
exaddressid:85740 exterms:city "Bedford" .
exaddressid:85740 exterms:state "Massachusetts" .
exaddressid:85740 exterms:postalCode "01730" .
This way of representing structured information in RDF can
involve generating numerous "intermediate" URIrefs
such as
exaddressid:85740
to represent aggregate concepts such as
John's address. Such concepts may never need to be referred to
directly from outside a particular graph, and hence may not
require "universal" identifiers. In addition, in the
drawing
of the graph representing the group of
statements shown in
Figure 5
the URIref assigned to identify "John Smith's
address" is not really needed, since the graph could just as easily
have been drawn as in
Figure 6
Figure 6: Using a Blank
Node
Figure 6
, which is a perfectly
good RDF graph, uses a node without a URIref to stand for
the concept of "John Smith's address". This
blank
node
serves its purpose in the drawing without needing a
URIref, since the node itself provides the necessary
connectivity between the various other parts of the graph.
(Blank nodes were called
anonymous resources
in
[RDF-MS]
.) However, some
form of explicit identifier for that node is needed in order to
represent this graph as triples. To see this, trying to
write the triples corresponding to what is shown in
Figure 6
would produce something like:
exstaff:85740 exterms:address ??? .
??? exterms:street "1501 Grant Avenue" .
??? exterms:city "Bedford" .
??? exterms:state "Massachusetts" .
??? exterms:postalCode "01730" .
where ??? stands for something that indicates the presence
of the blank node. Since a complex graph might contain more
than one blank node, there also needs to be a way to differentiate
between these different blank nodes in a triples representation
of the graph. As a result, triples use
blank
node identifiers
, having the form
_:name
, to
indicate the presence of blank nodes. For instance,
in this example a blank node identifier
_:johnaddress
might be used to refer to the blank node, in which
case the resulting triples might be:
exstaff:85740 exterms:address _:johnaddress .
_:johnaddress exterms:street "1501 Grant Avenue" .
_:johnaddress exterms:city "Bedford" .
_:johnaddress exterms:state "Massachusetts" .
_:johnaddress exterms:postalCode "01730" .
In a triples representation of a graph, each distinct blank
node in the graph is given a different blank node identifier.
Unlike URIrefs and literals, blank node identifiers are not
considered to be actual parts of the RDF graph (this can be
seen by looking at the drawn graph in
Figure
and noting that the blank node has no blank node
identifier). Blank node identifiers are just a way of
representing the blank nodes in a graph (and distinguishing one
blank node from another) when the graph is written in triple
form. Blank node identifiers also have significance only within
the triples representing a
single
graph (two different
graphs with the same number of blank nodes might independently
use the same blank node identifiers to distinguish them, and it
would be incorrect to assume that blank nodes from different
graphs having the same blank node identifiers are the same). If
it is expected that a node in a graph will need to be
referenced from outside the graph, a URIref should be assigned
to identify it. Finally, because blank node identifiers represent
(blank)
nodes
, rather than arcs, in the triple form of
an RDF graph, blank node identifiers may only appear as subjects or
objects in triples; blank node identifiers may not be used as
predicates in triples.
The beginning of this section noted that aggregate structures,
like John Smith's address, can be represented by
considering the aggregate thing to be described as a separate
resource, and then making statements about that new resource.
This example illustrates an important aspect of RDF: RDF
directly represents only
binary
relationships, e.g.
the relationship between John Smith and the literal
representing his address. Representing the
relationship between John and the group of separate
components
of this address involves dealing with an
n-ary
(n-way) relationship (in this case, n=5) between
John and the street, city, state, and postal code components. In order
to represent such structures directly in RDF (e.g., considering
the address as a group of street, city, state, and postal code
components), this n-way relationship must be broken up
into a group of separate binary relationships. Blank nodes provide
one way to do this. For each n-ary relationship,
one of the participants is chosen as the subject of the
relationship (John in this case), and a blank node is created to
represent the rest of the relationship (John's address in this
case). The remaining participants in the
relationship (such as the city in this example) are then
represented as separate
properties of the new resource represented by the blank
node.
Blank nodes also provide a way to more accurately make
statements about resources that may not have URIs, but that are
described in terms of relationships with other resources that
do
have URIs. For example, when making statements
about a person, say Jane Smith, it may seem natural to use a
URI based on that person's email address as her URI, e.g.,
mailto:jane@example.org
. However, this approach can
cause problems. For example, it may be necessary to record information
both about
Jane's mailbox
(e.g., the server it is on) as well as
about
Jane herself
(e.g., her current physical address), and using a
URIref for Jane based on her email address makes it difficult
to know whether it is Jane or her mailbox that is being described.
The same problem exists when a company's Web page URL, say
, is used as the URI of the
company itself. Once again, it may be necessary to record information
about the Web page itself (e.g., who created it and when) as well as
about the company, and using
as an identifier for both makes it difficult to know which
of these is the actual subject.
The fundamental problem is that using Jane's
mailbox
as a stand-in for
Jane
is not really
accurate: Jane and her mailbox are not the same thing, and
hence they should be identified differently. When Jane herself
does not have a URI, a blank node provides a more accurate way
of modeling this situation. Jane can be represented by a blank
node, and that blank node used as the subject of a statement
with
exterms:mailbox
as the property
and the URIref
mailto:jane@example.org
as
its value. The blank node could also be described with an
rdf:type
property having a value of
exterms:Person
(types are discussed in more detail
in the following sections), an
exterms:name
property
having a value of
"Jane Smith"
, and any other
descriptive information that might be useful, as shown in
the following triples:
_:jane exterms:mailbox
_:jane rdf:type exterms:Person .
_:jane exterms:name "Jane Smith" .
_:jane exterms:empID "23748" .
_:jane exterms:age "26" .
(Note that
mailto:jane@example.org
is written within angle brackets in the first triple. This is because
mailto:jane@example.org
is a full URIref in the
mailto
URI scheme, rather than a QName abbreviation,
and full URIrefs must be enclosed in angle brackets in the triples
notation.)
This says, accurately, that "there is a resource of type
exterms:Person
, whose electronic mailbox is identified
by
mailto:jane@example.org
, whose name is
Jane
Smith
, etc." That is, the blank node can be read as "there
is a resource". Statements with that blank node as subject then
provide information about the characteristics of that
resource.
In practice, using blank nodes instead of URIrefs in these
cases does not change the way this kind of
information is handled very much. For example, if it is known
that an email address uniquely identifies someone at
example.org (particularly if the address is unlikely to be
reused), that fact can still be used to associate information
about that person from multiple sources, even though the email
address is not the person's URI. In this case, if some
RDF is found on the Web that describes a book, and
gives the author's contact information as
mailto:jane@example.org
, it might be reasonable,
combining this new information with the previous set of
triples, to conclude
that the author's name is Jane Smith. The point is that saying
something like "the author of the book is
mailto:jane@example.org
" is typically a shorthand for
"the author of the book is someone whose mailbox is
mailto:jane@example.org
". Using a blank node to
represent this "someone" is just a more accurate way to
represent the real world situation. (Incidentally, some
RDF-based schema languages allow specifying that certain
properties are
unique identifiers
of the resources they
describe. This is discussed further in
Section 5.5
.)
Using blank nodes in this way can also help avoid the use of
literals in what might be inappropriate situations. For example,
in describing Jane's book, lacking a URIref to identify the author,
the publisher might have written (using the
publisher's own
ex2terms:
vocabulary):
ex2terms:book78354 rdf:type ex2terms:Book .
ex2terms:book78354 ex2terms:author "Jane Smith" .
However, the author of the book is not really the character
string "Jane Smith", but a person whose
name
is
Jane Smith. The same information might be more accurately
given by the publisher using a blank node, as:
ex2terms:book78354 rdf:type ex2terms:Book .
ex2terms:book78354 ex2terms:author _:author78354 .
_:author78354 rdf:type ex2terms:Person .
_:author78354 ex2terms:name "Jane Smith" .
This essentially says "resource
ex2terms:book78354
is of type
ex2terms:Book
, and its author is a resource of type
ex2terms:Person
, whose name is
Jane
Smith
." Of course, in this particular case the publisher might instead
have assigned its own URIrefs to its authors instead of using blank nodes to
identify them, in order to encourage external
references to its authors.
Finally, the example above giving Jane's age as 26 illustrates the fact that sometimes the value of a property may appear to be simple, but actually may be more complex. In this case, Jane's age is actually 26
years
, but the units information (years) is not explicitly given. Such information is often omitted in contexts where it can be safely assumed that anyone accessing the property value will understand the units being used. However, in the wider context of the Web, it is generally
not
safe to make this assumption. For example, a U.S. site might give a weight value in pounds, but someone accessing that data from outside the U.S. might assume that weights are given in kilograms. In general, careful consideration should be given to explicitly representing units and similar information. This issue is discussed further in
Section 4.4
, which describes an RDF feature for representing such information as structured values, as well as some other techniques for representing such information.
2.4 Typed
Literals
The last section described how to handle situations
in which property values represented by plain
literals had to be broken up into structured values to
represent the individual parts of those literals. Using
this approach, instead of, say, recording the date a Web page
was created as a single
exterms:creation-date
property, with a single plain literal as its value,
the value would be represented as a structure consisting of the month,
day, and year as separate pieces of information, using separate
plain literals to represent the corresponding values.
However, so
far, all constant values that serve as objects in RDF statements
have been represented by these plain
(untyped) literals, even when the intent is probably for the value
of the property to be a number (e.g., the value of a
year
or
age
property) or some other kind of
more specialized value.
For example,
Figure 4
illustrated an RDF graph recording information about John
Smith. That graph recorded the value of John Smith's
exterms:age
property as the plain literal "27", as
shown in
Figure 7
Figure 7: Representing John
Smith's Age
In this case, the hypothetical organization example.org
probably intends for "27" to be interpreted as a number, rather
than as the string consisting of the character "2" followed by
the character "7"
(since the literal represents the value of an "age"
property). However, there is no information in Figure 7's graph
that explicitly indicates that "27" should be interpreted as
a number. Similarly, example.org also
probably intends for "27" to be interpreted as a
decimal
number, i.e., the value
twenty seven
, rather than, say,
as an
octal
number, i.e., the value
twenty three
However, once again there is no information in Figure 7's graph that
explicitly indicates this. Specific applications might be written
with the understanding that they should
interpret values of the
exterms:age
property as decimal
numbers, but this would mean that proper interpretation of this
RDF would depend on information not explicitly provided
in the RDF graph, and hence on information that would not necessarily
be available to other applications that might need to interpret this RDF.
The common practice in
programming languages or database systems is to provide this
additional information about how to interpret a literal
by associating a
datatype
with the
literal, in this case, a datatype like
decimal
or
integer
. An application that understands the datatype
then knows, for example, whether the literal "10" is intended
to represent the number
ten
, the number
two
or the string consisting of the character "1" followed by the
character "0", depending on whether the specified datatype is
integer
binary
, or
string
(More specialized datatypes could also be used to include the units information mentioned
at the end of
Section 2.3
, e.g., a datatype
integerYears
, although the Primer will not elaborate on this idea.)
In RDF,
typed
literals
are used to provide this kind of information.
An RDF typed literal is formed by pairing a string
with a URIref that identifies a particular datatype. This
results in a single literal node in the RDF graph with the pair as the literal.
The value represented by the typed literal is the value that
the specified datatype associates with the specified string.
For example, using a typed literal, John Smith's age could be described as
being the integer number
27
using the triple:
or, using the QName simplification for writing long
URIs:
exstaff:85740 exterms:age "27"^^xsd:integer .
or as shown in
Figure 8
Figure 8: A Typed Literal for
John Smith's Age
Similarly, in the graph shown in
Figure
describing information about a Web page, the
value of the page's
exterms:creation-date
property
was written as
the plain literal "August 16, 1999". However, using a typed
literal, the creation date of the Web page could be explicitly described as
being the date
August 16, 1999
, using the triple:
ex:index.html exterms:creation-date "1999-08-16"^^xsd:date .
or as shown in
Figure 9
Figure 9: A Typed Literal for
a Web Page's Creation Date
Unlike typical programming languages and database systems,
RDF has no built-in set of datatypes of its own, such as
datatypes for integers, reals, strings, or dates.
Instead,
RDF typed literals simply provide a way to explicitly
indicate, for a given literal, what datatype should be used to
interpret it. The datatypes used in typed literals are defined
externally to RDF, and identified by their
datatype
URIs
(There is one exception: RDF defines a built-in datatype with the
URIref
rdf:XMLLiteral
to represent XML content as a literal
value. This datatype is defined in
[RDF-CONCEPTS]
, and its use is
described in
Section 4.5
.)
For instance, the examples in
Figure 8
and
Figure 9
use the datatypes
integer
and
date
from the XML Schema datatypes defined in
XML Schema Part 2:
Datatypes
[XML-SCHEMA2]
An advantage of this approach is that it
gives RDF the flexibility to directly represent information
coming from different sources without the need to perform type
conversions between these sources and a native set of RDF
datatypes. (Type conversions would still be required when
moving information between systems having different sets of datatypes,
but RDF would impose no extra conversions into and out
of a native set of RDF datatypes.)
RDF datatype concepts are based on
a conceptual framework
from XML Schema datatypes
[XML-SCHEMA2]
, as described
in
RDF
Concepts and Abstract Syntax
[RDF-CONCEPTS]
This conceptual framework defines a datatype as consisting of:
A set of values, called the
value space
, that
literals of the datatype are intended
to represent. For example, for the XML Schema datatype
xsd:date
, this set of values is a set of dates.
A set of character strings, called the
lexical space
, that
the datatype uses to represent its values. This set determines
which character strings can legally be used to represent
literals of this datatype. For example, the datatype
xsd:date
defines
1999-08-16
as being a legal
way to write a literal of this type
(as opposed, say, to
August 16, 1999
).
As defined in
[RDF-CONCEPTS]
, the lexical space of a datatype is a set of Unicode
[UNICODE]
strings, allowing information from many languages to be directly represented.
lexical-to-value mapping
from the lexical
space to the value space. This determines the value that a
given character string from the lexical space represents
for this particular datatype. For example,
the lexical-to-value mapping for datatype
xsd:date
determines that, for this datatype, the string
1999-08-16
represents the date
August 16, 1999
. The lexical-to-value
mapping is a factor because the same character string may represent
different values for different datatypes.
Not all datatypes are suitable for use in RDF. For a datatype
to be suitable for use in RDF, it
must conform to the conceptual framework just described. This basically means
that, given a character string, the datatype must
unambiguously define
whether or not the string is in its lexical space, and
what value in its value space the string represents.
For example, the basic XML Schema datatypes
such as
xsd:string
xsd:boolean
xsd:date
etc. are suitable
for use in RDF. However, some of the built-in XML Schema datatypes
are not suitable for use in RDF. For example,
xsd:duration
does
not have a well-defined value space, and
xsd:QName
requires an
enclosing XML document context. Lists of the XML Schema datatypes
that are currently considered suitable and unsuitable for use in
RDF are given in
[RDF-SEMANTICS]
Since the value that a given typed literal denotes is defined
by the typed literal's datatype, and, with the exception of
rdf:XMLLiteral
, RDF does not define any datatypes,
the actual interpretation of a
typed literal appearing in an RDF graph (e.g., determining
the value it denotes) must be performed by software
that is written to correctly process not only RDF, but the
typed literal's datatype as well. Effectively, this software must
be written to process an extended language that includes not
only RDF, but also the datatype, as part of its built-in
vocabulary.
This raises the issue of which datatypes will be generally available in
RDF software.
Generally, the XML Schema datatypes that are listed as suitable
for use in RDF in
[RDF-SEMANTICS]
have a "first among equals" status in RDF.
As noted already, the examples in
Figure 8
and
Figure 9
used some of these XML Schema datatypes, and the Primer will be
using these datatypes in
most of its other examples of typed literals as well (for one thing, XML Schema
datatypes already have assigned URIrefs that can be used to refer to them, specified
in
[XML-SCHEMA2]
). These XML Schema datatypes are
treated no differently than any other datatype, but they are
expected to be the most widely used, and therefore the most
likely to be interoperable among different software. As a
result, it is expected that much RDF software will also be
written to process these datatypes. However, RDF software
could be written to process other sets of datatypes as
well, assuming they were determined to be suitable for use
with RDF, as described already.
In general, RDF software may be called on to process RDF
data that contains references to datatypes that the software
has not been written to
process, in which case there are some things the software
will not be able to do.
For one thing, with the exception of
rdf:XMLLiteral
RDF itself does not define the URIrefs that identify datatypes.
As a result, RDF software, unless it has been written to recognize specific
URIrefs, will not be able to determine whether or not
a URIref written in a typed literal actually identifies a datatype.
Moreover, even when a URIref does identify a datatype, RDF
itself does not define the validity of pairing that datatype
with a particular literal.
This validity can only be determined
by software written to correctly process that particular datatype.
For example, the typed literal in the triple:
exstaff:85740 exterms:age "pumpkin"^^xsd:integer .
or the graph shown in
Figure 10
Figure 10: An Invalid Typed
Literal for John Smith's Age
is valid RDF, but obviously an error as far as the
xsd:integer
datatype is concerned, since
"pumpkin"
is
not defined as being in the lexical space of
xsd:integer
RDF software not written to
process the
xsd:integer
datatype would not be able
to recognize
this error.
However, proper use of RDF typed literals provides more information about
the intended interpretation of literal values, and hence makes
RDF statements a better means of information exchange among applications.
2.5 Concepts
Summary
Taken as a whole, RDF is basically simple: nodes-and-arcs diagrams
interpreted as statements about things identified by URIrefs.
This section has presented an introduction to these concepts.
As noted earlier, the normative (i.e., definitive) RDF
specification describing these concepts is
RDF Concepts and
Abstract Syntax
[RDF-CONCEPTS]
, which should be
consulted for further information. The formal semantics (meaning)
of these concepts is defined in the (normative)
RDF Semantics
[RDF-SEMANTICS]
document.
However, in addition to the basic techniques for
describing things using RDF statements discussed
so far, it should be clear that people or organizations also need a way
to describe the
vocabularies
(terms) they intend to use in those
statements, specifically, vocabularies for:
describing types of things (like
exterms:Person
describing properties (like
exterms:age
and
exterms:creation-date
), and
describing the types of things that can serve as the
subjects or objects of statements involving those properties
(such as specifying that the value of an
exterms:age
property should always be an
xsd:integer
).
The basis for describing such vocabularies in RDF is the
RDF Vocabulary
Description Language 1.0: RDF Schema
[RDF-VOCABULARY]
, which will be
described in
Section 5
Additional background on the basic ideas underlying RDF, and
its role in providing a general language for describing Web
information, can be found in
[WEBDATA]
. RDF draws upon ideas from
knowledge representation, artificial intelligence, and data
management, including Conceptual Graphs, logic-based knowledge
representation, frames, and relational databases. Some possible
sources of background information on these subjects include
[SOWA]
[CG]
[KIF]
[HAYES]
[LUGER]
, and
[GRAY]
3. An XML Syntax for RDF:
RDF/XML
As described in Section 2, RDF's conceptual model is a
graph. RDF provides an XML syntax for writing down and exchanging
RDF graphs, called
RDF/XML
. Unlike triples, which are
intended as a shorthand notation, RDF/XML is the normative syntax
for writing RDF. RDF/XML is defined in the
RDF/XML Syntax
Specification
[RDF-SYNTAX]
This section describes this RDF/XML syntax.
3.1 Basic
Principles
The basic ideas behind the RDF/XML syntax can be illustrated
using some of the examples presented already. Take as an example
the English statement:
has a
creation-date
whose value is
August 16,
1999
The RDF graph for this single statement, after assigning a
URIref to the
creation-date
property, is shown in
Figure 11
Figure 11: Describing a Web
Page's Creation Date
with a triple representation of:
ex:index.html exterms:creation-date "August 16, 1999" .
(Note that a typed literal is not used for the date value in
this example. Representing typed literals in RDF/XML will be
described later in this section.)
Example 2
shows the RDF/XML syntax
corresponding to the graph in
Figure
11
Example 2: RDF/XML for the
Web Page's Creation Date
1.
2.
4.
5.
6.
7.
(Line numbers are added to help in explaining the
example.)
This seems like a lot of overhead. It is easier to understand
what is going on by considering each part of this XML in turn
(a brief introduction to XML is provided in
Appendix B
).
Line 1,
, is the
XML
declaration
, which indicates that the following content is XML,
and what version of XML it is.
Line 2 begins an
rdf:RDF
element. This indicates
that the following XML content (starting here and ending with
the
in line 7) is intended to
represent RDF. Following the
rdf:RDF
on this same line
is an XML namespace declaration, represented as an
xmlns
attribute of the
rdf:RDF
start-tag.
This declaration
specifies that all tags in this content
prefixed with
rdf:
are part of the namespace
identified by the URIref
URIrefs beginning with the string
are used for terms from the RDF vocabulary.
Line 3 specifies another XML namespace declaration, this
time for the prefix
exterms:
. This is expressed as
another
xmlns
attribute of the
rdf:RDF
element, and specifies that the namespace URIref
is to be associated with
the
exterms:
prefix.
URIrefs beginning with the string
are used for terms from the vocabulary defined by the example organization,
example.org.
The ">" at the end of line 3 indicates the end
of the
rdf:RDF
start-tag. Lines 1-3 are general
"housekeeping" necessary to indicate that this is
RDF/XML content, and to identify the namespaces being used
within the RDF/XML content.
Lines 4-6 provide the RDF/XML for the specific statement
shown in
Figure
11
. An obvious way to talk about any RDF
statement is to say it is a
description
, and that it is
about
the subject of the statement (in this case,
about http://www.example.org/index.html), and this is the way
RDF/XML represents the statement. The
rdf:Description
start-tag in line 4 indicates the start of a
description
of a resource, and goes on to identify the
resource the statement is
about
(the subject of the
statement) using the
rdf:about
attribute to specify
the URIref of the subject resource.
Line 5 provides a
property element
, with the QName
exterms:creation-date
as its tag, to
represent the predicate and object of the statement.
The QName
exterms:creation-date
is chosen
so that appending
the local name
creation-date
to the URIref of the
exterms:
prefix (
gives the statement's predicate URIref
The content of this property element is the object of the
statement, the plain literal
August 19, 1999
(the value of the creation-date property of the subject resource).
The property element is nested within the containing
rdf:Description
element, indicating that this property
applies to the resource specified in the
rdf:about
attribute of the
rdf:Description
element. Line 6
indicates the end of this particular
rdf:Description
element.
Finally, Line 7 indicates the end of the
rdf:RDF
element started on line 2. Using an
rdf:RDF
element
to enclose RDF/XML content is optional in situations where
the XML can be identified as RDF/XML by context. This is discussed
further in
[RDF-SYNTAX]
. However, it
does not hurt to provide the
rdf:RDF
element in any case,
and Primer examples will generally (but not always) provide one.
Example 2
illustrates the basic
ideas used by RDF/XML to encode an RDF graph as XML elements,
attributes, element content, and attribute values. The URIrefs
of predicates (as well as some nodes) are written as XML
QNames
, consisting of a short
prefix
denoting
a namespace URI, together with a
local name
denoting a
namespace-qualified element or attribute, as described in
Appendix B
. The (namespace URIref, local
name) pair is chosen so that concatenating them forms the
URIref of the original node or predicate. The URIrefs of subject nodes are
written as XML attribute values (URIrefs of object nodes may sometimes be
written as attribute values as well). Literal nodes
(which are always object nodes) become element text content or
attribute values. (Many of these options are described later in
the Primer; all of these options are described in
[RDF-SYNTAX]
.)
An RDF graph consisting of multiple statements can be represented
in RDF/XML by using RDF/XML similar to Lines 4-6 in
Example 2
to separately represent each
statement. For example, to write the following two
statements:
ex:index.html exterms:creation-date "August 16, 1999" .
ex:index.html dc:language "en" .
the RDF/XML in
Example
could be used:
Example 3: RDF/XML for Two
Statements
1.
2.
4. xmlns:exterms="http://www.example.org/terms/">
5.
6.
7.
8.
9.
10.
11.
Example 3
is the same as
Example 2
, with the addition of
a second
rdf:Description
element (in lines 8-10)
to represent the second statement. (An additional namespace declaration
is also given in line 3
to identify the additional namespace used in this statement.)
An arbitrary number of
additional statements could be written in the same way, using a separate
rdf:Description
element for each additional statement.
As
Example 3
illustrates, once the
overhead of writing the XML and namespace declarations is dealt
with, writing each additional RDF statement in RDF/XML is both
straightforward and not too complicated.
The RDF/XML syntax provides a number of abbreviations to
make common uses easier to write. For example, it is typical
for the same resource to be described with several properties
and values at the same time, as in
Example
, where the resource
ex:index.html
is the subject
of several statements. To handle such cases, RDF/XML allows
multiple property elements representing those properties to be
nested within the
rdf:Description
element that
identifies the subject resource. For example, to
represent the following group of statements about
ex:index.html dc:creator exstaff:85740 .
ex:index.html exterms:creation-date "August 16, 1999" .
ex:index.html dc:language "en" .
whose graph (the same as
Figure 3
) is
shown in
Figure 12
Figure 12: Several
Statements About the Same Resource
the RDF/XML shown in
Example 4
could be written:
Example 4: Abbreviating
Multiple Properties
1.
2.
4. xmlns:exterms="http://www.example.org/terms/">
5.
6.
7.
8.
9.
10.
Compared with the previous two examples,
Example 4
adds an additional
dc:creator
property element (in line 8). In addition, the
property elements for the three properties whose subject is
are nested within a single
rdf:Description
element identifying that subject,
rather than writing a separate
rdf:Description
element
for each statement.
Line 8 also introduces a new form of property element. The
dc:language
element in line 7 is similar to the
exterms:creation-date
element used in
Example 2
. Both these elements represent
properties with plain literals as property values, and such
elements are written by enclosing the literal within start-
and end-tags corresponding to the property name. However, the
dc:creator
element on line 8 represents a property
whose value is
another resource
, rather than a
literal. If the URIref of this resource were written as a
plain literal within start- and end-tags in the same way as
the literal values of the other elements, this would
say that the value of the
dc:creator
element was
the
character string
, rather than the
resource identified by that literal interpreted as a URIref. In
order to indicate the difference, the
dc:creator
element is written using what XML calls an
empty-element tag
(it has no separate end-tag), and
the property value is written using an
rdf:resource
attribute within that empty element. The
rdf:resource
attribute indicates that the property element's value is
another resource, identified by its URIref. Because the URIref
is being used as an attribute
value
, RDF/XML requires
the URIref to be written out (as an absolute or relative URIref),
rather than abbreviating it as a
QName as was done in writing element and attribute
names
(absolute and relative URIrefs are discussed in
Appendix A
).
It is important to understand that the RDF/XML in
Example 4
is an
abbreviation
. The
RDF/XML in
Example 5
, in which each
statement is written separately, describes exactly the same RDF
graph (the graph of
Figure 12
):
Example 5: Writing Example
4 as Separate Statements
xmlns:exterms="http://www.example.org/terms/">
The following sections will describe a few additional
RDF/XML abbreviations.
[RDF-SYNTAX]
provides a more thorough
description of the abbreviations that are available.
RDF/XML can also represent graphs that include
nodes that have no URIrefs, i.e., the
blank nodes
described in
Section 2.3
. For
example,
Figure 13
(taken from
[RDF-SYNTAX]
) shows a graph saying
"the document 'http://www.w3.org/TR/rdf-syntax-grammar' has a
title 'RDF/XML Syntax Specification (Revised)' and has an
editor, the editor has a name 'Dave Beckett' and a home page
'http://purl.org/net/dajobe/' ".
Figure 13: A Graph
Containing a Blank Node
This illustrates an idea discussed in
Section 2.3
: the use of a
blank node to represent something that does not have a URIref,
but can be described in terms of other information. In this
case, the blank node represents a person, the editor of the
document, and the person is described by his name and home
page.
RDF/XML provides several ways to represent graphs
containing blank nodes. These are all described in
[RDF-SYNTAX]
. The approach
illustrated here, which is the most direct approach, is to
assign a
blank node identifier
to each blank node. A
blank node identifier serves to identify a blank node within a
particular RDF/XML document but, unlike a URIref, is unknown
outside the document in which it is assigned. A blank node is
referred to in RDF/XML using an
rdf:nodeID
attribute,
with a blank node identifier as its value, in places where the
URIref of a resource would otherwise appear. Specifically,
a statement with a blank node as its
subject
can be written in
RDF/XML using an
rdf:Description
element with
an
rdf:nodeID
attribute instead of an
rdf:about
attribute. Similarly, a statement with a
blank node as its
object
can be written using a property
element with an
rdf:nodeID
attribute instead of an
rdf:resource
attribute. Using
rdf:nodeID
Example 6
shows the RDF/XML corresponding
to
Figure 13
Example 6: RDF/XML
Describing a Blank Node
1.
2.
4. xmlns:exterms="http://example.org/stuff/1.0/">
5.
6.
7.
8.
9.
10.
11.
12.
13.
In
Example 6
, the blank node
identifier
abc
is used in line 9 to identify the blank
node as the subject of several statements, and is used in line
7 to indicate that the blank node is the value of a resource's
exterms:editor
property. The advantage of using a
blank node identifier over some of the other approaches
described in
[RDF-SYNTAX]
is that
using a blank node identifier allows the same blank node to be
referred to in more than one place in the same RDF/XML
document.
Finally, the
typed literals
described in
Section 2.4
may be used as property
values instead of the plain literals used in the
examples so far. A typed literal is represented in RDF/XML by
adding an
rdf:datatype
attribute specifying a datatype
URIref to the property element containing the literal.
For example, to change the statement in
Example 2
to use a typed literal instead
of a plain literal for the
exterms:creation-date
property, the
triple representation would be:
ex:index.html exterms:creation-date "1999-08-16"^^xsd:date .
with corresponding RDF/XML syntax shown in
Example 7
Example 7: RDF/XML Using a
Typed Literal
1.
2.
4.
5.
6.
7.
In line 5 of
Example 7
, a typed
literal is given as the value of the
exterms:creation-date
property element by adding an
rdf:datatype
attribute
to the element's start-tag to specify the datatype. The value
of this attribute is the URIref of the datatype, in this case,
the URIref of the XML Schema
date
datatype. Since this
is an attribute value, the URIref must be written out, rather
than using the QName abbreviation
xsd:date
used in the triple. A literal appropriate to this datatype is
then written as the element content, in this case, the literal
1999-08-16
, which is the literal representation for
August 16, 1999 in the XML Schema
date
datatype.
In the
rest of the Primer,
the examples will
use typed literals from appropriate datatypes rather than
plain (untyped) literals, in order to emphasize the value of
typed literals in conveying more information about the intended
interpretation of literal values. (The exceptions will be that plain
literals will continue to be used in
examples taken from actual applications that do not currently use
typed literals, in order to accurately reflect the usage in those
applications.)
In RDF/XML, both plain and typed literals (and, with certain exceptions, tags) can contain Unicode
[UNICODE]
characters, allowing information from many languages to be directly represented.
Example 7
illustrates that using typed literals requires writing an
rdf:datatype
attribute with
a URIref identifying the datatype for each element whose value is a typed literal. As noted earlier, RDF/XML requires that URIrefs used as attribute values
must be written out, rather than abbreviated as a QName.
XML
entities
can be used in RDF/XML to improve readability
in such cases, by providing an additional abbreviation
facility for URIrefs. Essentially, an XML entity declaration
associates a name with a string of characters. When the entity
name is referenced elsewhere within an XML document, XML processors replace
the reference with the corresponding string. For example, the
ENTITY
declaration (specified as part of a
DOCTYPE
declaration at the beginning of the RDF/XML document):
]>
defines the entity
xsd
to be the string representing the
namespace URIref for XML Schema datatypes. This declaration allows
the full namespace URIref to be abbreviated elsewhere in the XML
document by the
entity reference
&xsd;
Using this abbreviation,
Example 7
could also be written as shown in
Example 8
Example 8: RDF/XML Using a
Typed Literal and an XML Entity
1.
2. ]>
3.
5.
6.
7.
8.
The
DOCTYPE
declaration in line 2 defines the entity
xsd
, which is used in line 6.
The use of XML entities as an abbreviation mechanism is optional
in RDF/XML, and hence the use of an XML
DOCTYPE
declaration
is also optional
in RDF/XML. (For readers familiar with XML, RDF/XML is only required
to be "well-formed" XML. RDF/XML is not designed to be
validated against a DTD by a validating XML processor. This is
discussed more fully in
Appendix B
, which
provides additional information about XML.)
For readability purposes, examples in the rest of the
Primer will use the XML entity
xsd
as just described.
XML entities are discussed
further in
Appendix B
As illustrated in
Appendix B
other URIrefs (and, more generally, other strings)
can also be abbreviated using XML entities.
However, the URIrefs for XML Schema datatypes are the only ones that will be
abbreviated in this way in Primer examples.
Although additional abbreviated forms for writing RDF/XML
are available, the facilities illustrated so far
provide a simple but general way to express graphs in
RDF/XML. Using these facilities, an RDF graph is written in
RDF/XML as follows:
All blank nodes are assigned blank node identifiers.
Each node is listed in turn as the subject of an
un-nested
rdf:Description
element, using an
rdf:about
attribute if the node has a URIref, or
an
rdf:nodeID
attribute if the node is
blank.
For each triple with this node as subject, an appropriate
property element is created, with either literal content
(possibly empty), an
rdf:resource
attribute
specifying the object of the triple (if the object node has a
URIref), or an
rdf:nodeID
attribute specifying
the object of the triple (if the object node is blank).
Compared to some of the more abbreviated
approaches described in
[RDF-SYNTAX]
, this simple
approach provides the most direct representation
of the actual graph structure, and is particularly recommended
for applications in which the output RDF/XML is to be used in
further RDF processing.
3.2 Abbreviating
and Organizing RDF URIrefs
So far, the examples have assumed that the resources
being described have been given URIrefs already. For instance, the initial
examples provided descriptive information about
example.org's Web page, whose URIref was
. This resource was identified
in RDF/XML
using an
rdf:about
attribute citing its full URIref.
Although RDF does not specify or control how URIrefs are
assigned to resources, sometimes it is desirable to achieve the
effect of assigning URIrefs to resources that are part
of an organized group of resources. For example, suppose a
sporting goods company, example.com, wanted to provide an
RDF-based catalog of its products, such as tents, hiking boots,
and so on, as an RDF/XML document, identified by (and located
at)
. In that
resource, each product might be given a separate RDF
description. This catalog, along with one of these
descriptions, the catalog entry for a model of tent called the
"Overnighter", might be written in RDF/XML as shown in
Example 9
Example 9: RDF/XML for
example.com's Catalog
1.
2. ]>
3.
5.
6.
7.
8.
9.
10.
...other product descriptions...
11.
Example 9
is similar to previous
examples in the way it represents the properties (model,
sleeping capacity, weight) of the resource (the tent) being
described.
(The surrounding xml, DOCTYPE, RDF, and namespace
information is included in lines 1 through 4, and line 11, but this
information would only need to be provided once for the whole
catalog, not repeated for each entry in the catalog.
Note also that although the
datatypes
associated with the various property values
are given explicitly, the
units
associated with some of these property values are not, even
though this information should be available to properly interpret the values. Representing units and similar information that may be associated with property values is discussed in
Section 4.4
. In this example, the value of
exterms:sleeps
is the number of persons the tent can sleep, the value of
exterms:weight
is given in kilograms, and the value of
exterms:packedSize
is given in square centimeters, the area the tent occupies on a backpack.)
An important
difference
from previous examples
is that, in line 5, the
rdf:Description
element has an
rdf:ID
attribute instead of an
rdf:about
attribute. Using
rdf:ID
specifies
fragment identifier
, given by the
value of the
rdf:ID
attribute (
item10245
in
this case, which might be the catalog number assigned by
example.com), as an abbreviation of the complete URIref of the
resource being described. The fragment identifier
item10245
will be interpreted relative to a
base
URI
, in this case, the URI of the containing catalog
document. The full URIref for the tent is formed by taking the
base URI (of the catalog), and appending the character
" (to
indicate that what follows is a fragment identifier) and then
item10245
to it, giving the absolute URIref
The
rdf:ID
attribute is somewhat similar to the
ID
attribute in XML and HTML, in that it defines a name which must
be unique relative to the current base URI (in this example, that of the catalog).
In this case, the
rdf:ID
attribute appears to be assigning a name (
item10245
to this particular kind of tent. Any other RDF/XML within this
catalog could refer to the tent by using either the absolute URIref
or the
relative URIref
#item10245
. The
relative URIref would be understood as being a URIref defined relative to the
base URIref of the catalog. Using a similar abbreviation,
the URIref of the tent could also be given by specifying
rdf:about="#item10245"
in the catalog entry (i.e., by
specifying the relative URIref directly) instead of
rdf:ID="item10245"
. As an abbreviation mechanism,
the two forms are essentially
synonyms: the full URIref formed by RDF/XML is the same in
either case:
However, using
rdf:ID
provides an additional check
when assigning a set of distinct names, since a given value of the
rdf:ID
attribute can only appear once relative to the
same base URI (the catalog document, in this example). Using
either form, example.com would be giving the URIref for the
tent in a two-stage process, first assigning the URIref for the
whole catalog, and then using a relative URIref in the
description of the tent in the catalog to indicate the URIref
that has been assigned to this particular kind of tent.
Moreover, this use of a relative URIref can be thought of either
as being an abbreviation for a full URIref that has been
assigned to the tent independently of the RDF, or as being the
assignment of the URIref to the tent within the catalog.
RDF located
outside
the catalog could refer to this
tent by using the full URIref, i.e., by concatenating the
relative URIref
#item10245
of the tent to the base URI
of the catalog, forming the absolute URIref
. For
example, an outdoor sports Web site exampleRatings.com might
use RDF to provide ratings of various tents. The (5-star)
rating given to the tent described in
Example 9
might then be represented on
exampleRatings.com's Web site as shown in
Example 10
Example 10:
exampleRatings.com's Rating of the Tent
1.
2. ]>
3.
5.
6.
7.
8.
9.
In
Example 10
, line 5 uses an
rdf:Description
element with an
rdf:about
attribute whose value is the full URIref of the tent. The use
of this URIref allows the tent being referred to in the rating
to be precisely identified.
These examples illustrate several points. First, even though
RDF does not specify or control how URIrefs are assigned to
resources (in this case, the various tents and other items in
the catalog), the
effect
of assigning URIrefs to
resources in RDF can be achieved by combining a process
(external to RDF) that identifies a single document (the
catalog in this case) as the source for descriptions of those
resources, with the use of relative URIrefs in descriptions of
those resources within that document. For instance, example.com
could use this catalog as the central source where its products
are described, with the understanding that if a product's item
number is not in an entry in this catalog, it is not a product
known to example.com. (Note that RDF does not assume any
particular relationship exists between two resources just
because their URIrefs have the same base, or are otherwise
similar. This relationship may be known to example.com, but it
is not directly defined by RDF.)
These examples also illustrate one of the basic
architectural principles of the Web,
which is that
anyone
should be able to freely add information
about an existing resource, using any vocabulary they
please
[BERNERS-LEE98]
. The examples
further illustrate that the RDF describing a particular
resource does not need to be located all in one place; instead,
it may be distributed throughout the Web. This is true not only
for situations like this one, in which one organization is
rating or commenting on a resource defined by another, but also
for situations in which the original definer of a resource (or
anyone else) wishes to amplify the description of that resource
by providing additional information about it. This may be done
by modifying the RDF document in which the resource was
originally described, to add the properties and values needed
to describe the additional information. Or, as this example
illustrates, a separate document could be created, providing the
additional properties and values in
rdf:Description
elements that refer to the original resource via its URIref
using
rdf:about
The discussion above indicated that relative URIrefs
such as
#item10245
will be interpreted relative to a
base URI
. By default, this base URI would be the URI
of the resource in which the relative URIref is used.
However, in some cases it is desirable to be able to explicitly
specify this base URI. For instance, suppose that in addition
to the catalog located at
, example.org
wanted to provide a duplicate catalog on a mirror site, say at
. This could
create a problem, since if the catalog was accessed from the
mirror site, the URIref for the example tent would be generated
from the URI of the containing document, forming
rather than
, and
hence would apparently refer to a different resource than the
one intended. Alternatively, example.org might want to assign a
base URIref for its set of product URIrefs
without
publishing a
single source document whose location defines the base.
To deal with such cases, RDF/XML supports
XML
Base
[XML-BASE]
, which allows
an XML document to specify a base URI other than the URI of the
document itself.
Example 11
shows how
the catalog would be described using XML Base:
Example 11: Using XML
Base in example.com's Catalog
1.
2. ]>
3.
5. xml:base="http://www.example.com/2002/04/products">
6.
7.
8.
9.
10.
11.
...other product descriptions...
12.
In
Example 11
, the
xml:base
declaration in line 5 specifies that the base
URI for the content within the
rdf:RDF
element (until
another
xml:base
attribute is specified) is
, and all
relative URIrefs cited within that content will be interpreted
relative to that base, no matter what the URI of the containing
document is. As a result, the relative URIref of the tent,
#item10245
, will be interpreted as the same absolute
URIref,
, no
matter what the actual URI of the catalog document is, or
whether the base URIref actually identifies a particular
document at all.
So far, the examples have used a single product
description, a particular model of tent, from example.com's
catalog. However, example.com will probably offer several
different models of tents, as well as multiple instances of
other categories of products, such as backpacks, hiking boots,
and so on. This idea of things being classified into different
kinds
or
categories
is similar to the
programming language concept of objects having different
types
or
classes
. RDF supports this concept
by providing a predefined property,
rdf:type
. When an
RDF resource is described with an
rdf:type
property,
the value of that property is considered to be a resource that
represents a category or
class
of things, and the
subject of that property is considered to be an
instance
of that category or class. Using
rdf:type
Example 12
shows
how example.com might indicate that the product description is
that of a tent:
Example 12: Describing a
Tent with
rdf:type
1.
2. ]>
3.
5. xml:base="http://www.example.com/2002/04/products">
6.
7.
8.
9.
10.
11.
12.
...other product descriptions...
13.
In
Example 12
, the
rdf:type
property in line 7 indicates that the
resource being described is an instance of the class identified by the URIref
. This assumes
that example.com has described its classes as part of
the same vocabulary that it uses to describe its other terms
(such as the property
exterms:weight
), so the
absolute URIref of the class is used to refer to it. If example.com had
described these classes as part of the product catalog itself,
the relative URIref
#Tent
could have been used to refer
to it.
RDF itself does not provide facilities for defining
application-specific classes of things, such as
Tent
in this example, or their properties, such as
exterms:weight
Instead, such classes would be described in an
RDF schema
using the
RDF Schema
language
discussed in
Section 5
. Other such
facilities for describing classes can also be defined, such as
the
DAML+OIL
and
OWL
languages described in
Section 5.5
It is fairly common in RDF for resources to have
rdf:type
properties that describe the resources as instances of specific types
or classes. Such resources are called
typed nodes
in the
graph, or
typed node elements
in the RDF/XML.
RDF/XML provides a special abbreviation for describing
these typed nodes. In this abbreviation, the
rdf:type
property and its value are removed, and the
rdf:Description
element for the node is replaced by an element
whose name is the QName corresponding to the value of the
removed
rdf:type
property (a URIref that names a class).
Using this abbreviation, example.com's tent from
Example 12
could also be described as
shown in
Example 13
Example 13: Abbreviating
the Tent's Type
1.
2. ]>
3.
5. xml:base="http://www.example.com/2002/04/products">
6.
7.
8.
9.
10.
11.
...other product descriptions...
12.
Since a resource may be described as an instance of more
than one class, a resource may have more than one
rdf:type
property. However, only one of these
rdf:type
properties can be abbreviated in this way.
The others must be written out using
rdf:type
properties,
in the manner illustrated by the
rdf:type
property in
Example 12
In addition to its use in describing instances of user-defined
classes such as
exterms:Tent
, the typed node abbreviation
is also commonly used in RDF/XML when describing instances of
the built-in RDF classes (such as
rdf:Bag
) to be
described in
Section 4
, and
the built-in RDF Schema classes (such as
rdfs:Class
) to be described in
Section 5
Both
Example 12
and
Example 13
illustrate that RDF statements
can be written in RDF/XML in a way that closely resembles
descriptions that might have been written directly in (non-RDF) XML. This
is an important consideration, given the increasing use of XML
in all kinds of applications, since it suggests that RDF could
be used in these applications without requiring major changes
in the way their information is structured.
3.3 RDF/XML
Summary
The examples above have illustrated some of the basic ideas
behind the RDF/XML syntax. These examples provide enough
information to begin writing useful RDF/XML.
A more thorough discussion of the principles behind the
modeling of RDF statements in XML (known as
striping
),
together with a presentation of the other RDF/XML abbreviations
available, and other details and examples about writing RDF in
XML, is given in the (normative)
RDF/XML Syntax
Specification
[RDF-SYNTAX]
4.
Other RDF Capabilities
RDF provides a number of additional capabilities,
such as
built-in types and properties for representing groups of
resources and RDF statements, and capabilities for representing
XML fragments as property values.
These additional capabilities
are described in the following sections.
4.1 RDF
Containers
There is often a need to describe
groups
of things:
for example, to say that a book was created by
several authors, or to list the students in a course, or the
software modules in a package. RDF provides several predefined
(built-in) types and properties that can be used to describe such
groups.
First, RDF provides a
container vocabulary
consisting of three predefined types (together with some
associated predefined properties). A
container
is a
resource that contains things. The contained things are called
members
. The members of a container may be resources
(including blank nodes) or literals. RDF defines three types of containers:
rdf:Bag
rdf:Seq
rdf:Alt
Bag
(a resource having type
rdf:Bag
represents
a group of resources or literals, possibly including duplicate
members, where there is no significance in the order of the
members. For example, a Bag might be used to describe a group
of part numbers in which the order of entry or processing of
the part numbers does not matter.
Sequence
or
Seq
(a resource having type
rdf:Seq
represents
a group of resources or literals, possibly
including duplicate members, where the order of the members is
significant. For example, a Sequence might be used to describe
a group that must be maintained in alphabetical order.
An
Alternative
or
Alt
(a resource having
type
rdf:Alt
represents
a group of resources or literals that
are
alternatives
(typically for a single value of a
property). For example, an Alt might be used to describe
alternative language translations for the title of a book, or
to describe a list of alternative Internet sites at which a
resource might be found. An application using a property whose
value is an Alt container should be aware that it can choose
any one of the members of the group as appropriate.
To describe a resource as being one of these types of
containers, the resource is given an
rdf:type
property
whose value is one of the predefined resources
rdf:Bag
rdf:Seq
, or
rdf:Alt
(whichever is appropriate). The container resource (which may
either be a blank node or a resource with a URIref) denotes the
group as a whole. The
members
of the container can be
described by defining a
container membership property
for each member with the container resource as its subject and
the member as its object. These container membership properties
have names of the form
rdf:_
, where
is a decimal integer greater than zero, with no
leading zeros, e.g.,
rdf:_1
rdf:_2
rdf:_3
, and so on, and are used specifically for
describing the members of containers. Container resources may
also have other properties that describe the container, in
addition to the container membership properties and the
rdf:type
property.
It is important to understand that while these types of
containers are described using predefined RDF types and
properties, any special meanings associated with these
containers, e.g., that the members of an Alt container are
alternative values, are only
intended
meanings. These
specific container types, and their definitions, are provided
with the aim of establishing a shared convention among those
who need to describe groups of things. All RDF does is provide
the types and properties that can be used to construct the RDF
graphs to describe each type of container. RDF has no more
built-in understanding of what a resource of type
rdf:Bag
is than it has of what a resource of type
ex:Tent
(discussed in
Section 3.2
) is. In each case,
applications must be written to behave according to the
particular meaning involved for each type. This point will be
expanded on in the following examples.
A typical use of a container is to indicate that the value
of a property is a group of things. For example, to represent
the sentence "Course 6.001 has the students Amy, Mohamed, Johann,
Maria, and Phuong", the course could be described by giving it a
s:students
property (from an appropriate vocabulary)
whose value is a container of type
rdf:Bag
representing
the group of students). Then, using the
container membership properties, individual
students could be identified as being members of that group,
as in the RDF
graph shown in
Figure 14
Figure 14: A Simple Bag
Container Description
Since the value of the
s:students
property in this
example is described as a Bag, there is no intended
significance in the order given for the URIrefs of the
students, even though the membership properties in the graph have integers
in their names. It is up to applications creating and
processing graphs that include
rdf:Bag
containers to
ignore any (apparent) order in the names of the membership
properties.
RDF/XML provides some special syntax and abbreviations to
make it simpler to describe such containers. For example,
Example 14
describes the graph shown in
Figure 14
Example 14: RDF/XML for
a Bag of Students
Example 14
shows that RDF/XML
provides
rdf:li
as a convenience element to avoid having
to explicitly number each membership property. The numbered
properties
rdf:_1
rdf:_2
, and so on are
generated from the
rdf:li
elements in forming the
corresponding graph. The element name
rdf:li
was chosen to
be mnemonic with the term "list item" from HTML. Note also the
use of a
element nested within the
property element. The
element is another example of the
abbreviation used in
Example 13
that replaces both an
rdf:Description
element
and an
rdf:type
element with a single element
when describing an instance of a type (an instance of
rdf:Bag
in this case). Since
no URIref is specified, the Bag is a blank node. Its nesting
within the
property element is an
abbreviated way of indicating that the blank node is the value
of this property. These abbreviations are described further in
[RDF-SYNTAX]
The graph structure for an
rdf:Seq
container, and
the corresponding RDF/XML, are similar to those for an
rdf:Bag
(the only difference is in the type,
rdf:Seq
). Once again, although an
rdf:Seq
container is intended to describe a sequence, it is up to
applications creating and processing the graph to appropriately
interpret the sequence of integer-valued property names.
To illustrate an Alt container, the sentence "The
source code for X11 may be found at ftp.example.org,
ftp1.example.org, or ftp2.example.org" could be expressed in
the RDF graph shown in
Figure 15
Figure 15: A Simple Alt
Container Description
Example 15
shows how the graph in
Figure 15
could be written in
RDF/XML:
Example 15: RDF/XML for
an Alt Container
An Alt container is intended to have at least one member,
identified by the property
rdf:_1
. This member is
intended to be considered as the default or preferred value.
Other than the member identified as
rdf:_1
, the order
of the remaining elements is not significant.
The RDF in
Figure 15
as
written
states simply that the value of the
s:DistributionSite
site property is the Alt container
resource itself. Any additional meaning that is to be read into
this graph, e.g., that one of the
members
of the Alt
container is to be considered as the value of the
s:DistributionSite
site property, or that
ftp://ftp.example.org
is the default or preferred
value, must be built into an application's understanding of
the intended meaning of
an Alt container, and/or into the meaning defined
for the particular property (
s:DistributionSite
in
this case), which also must be understood by the
application.
Alt containers are frequently used in conjunction with
language tagging. (RDF/XML permits the use of the
xml:lang
attribute defined in
[XML]
to indicate
that the element content is in a specified language. The use
of
xml:lang
is described in
[RDF-SYNTAX]
, and illustrated later in
Section 6.2
.) For example, a work whose title has been
translated into several languages might have its
title
property pointing to an Alt container holding literals representing
the titles expressed in each of the language variants.
The distinction between the intended meanings of a Bag and
an Alt can be further illustrated by considering the authorship
of the book "Huckleberry Finn". The book has exactly one
author, but the author has two names (Mark Twain and Samuel
Clemens). Either name is sufficient to specify the author. Thus
using an Alt container for the author's names more accurately
represents the relationship than using a Bag (which might
suggest there are two
different
authors).
Users are free to choose their own ways to describe groups of
resources, rather than using the RDF container vocabulary. These RDF
containers are merely provided as common definitions that, if
generally used, could help make data involving groups of
resources more interoperable.
Sometimes there are clear alternatives to using these RDF
container types. For example, a relationship between a
particular resource and a group of other resources could be
indicated by making the first resource the subject of multiple
statements using the same property. This is structurally different
from the resource being the subject of a single
statement whose object is a container containing multiple
members. In some cases, these two structures may have
equivalent meaning, but in other cases they may not. The choice
of which to use in a given situation should be made with this
in mind.
Consider as an example the relationship between a writer and
her publications, as in the sentence:
Sue has written "Anthology of Time", "Zoological
Reasoning", and "Gravitational Reflections".
In this case, there are three resources each of which was
written independently by the same writer. This could be
expressed using repeated properties as:
exstaff:Sue exterms:publication ex:AnthologyOfTime .
exstaff:Sue exterms:publication ex:ZoologicalReasoning .
exstaff:Sue exterms:publication ex:GravitationalReflections .
In this example there is no stated relationship between the
publications other than that they were written by the same
person. Each of the statements is an independent fact, and so
using repeated properties would be a reasonable choice.
However, this could just as reasonably be represented as a
statement about the group of resources written by Sue:
exstaff:Sue exterms:publication _:z .
_:z rdf:type rdf:Bag .
_:z rdf:_1 ex:AnthologyOfTime .
_:z rdf:_2 ex:ZoologicalReasoning .
_:z rdf:_3 ex:GravitationalReflections .
On the other hand, the sentence:
The resolution was approved by the Rules Committee, having
members Fred, Wilma, and Dino.
says that the committee
as a whole
approved the resolution;
it does not necessarily state that each committee member
individually
voted in favor of the resolution. In this case, it
would be potentially misleading to model this sentence as three
separate
exterms:approvedBy
statements, one for each
committee member, as shown below:
ex:resolution exterms:approvedBy ex:Fred .
ex:resolution exterms:approvedBy ex:Wilma .
ex:resolution exterms:approvedBy ex:Dino .
since these statements say that each member individually
approved the resolution.
In this case, it would be better to model the sentence as a
single
exterms:approvedBy
statement whose subject is
the resolution and whose object is the committee itself. The
committee resource could then be described as a Bag whose
members are the members of the committee, as in the following
triples:
ex:resolution exterms:approvedBy ex:rulesCommittee .
ex:rulesCommittee rdf:type rdf:Bag .
ex:rulesCommittee rdf:_1 ex:Fred .
ex:rulesCommittee rdf:_2 ex:Wilma .
ex:rulesCommittee rdf:_3 ex:Dino .
When using RDF containers, it is important to
understand that the statements are not
constructing
containers,
as in a programming language data structure. Instead,
the statements are
describing
containers (groups of things) that
presumably exist. For instance, in the Rules Committee example
just given, the Rules Committee is an unordered group of
people, whether it is described in RDF that way or not.
Saying that the resource
ex:rulesCommittee
has type
rdf:Bag
is not saying that the Rules Committee is a data
structure, or constructing a particular data structure
to hold the members of the group (the
Rules Committee could be described as a Bag without
describing any members at
all). Instead, it is describing the Rules Committee as having
characteristics corresponding to those associated with a Bag
container, namely that it has members, and their order of description
is not significant.
Similarly, using the container membership
properties simply describes a container resource as
having certain things as members. This does not necessarily
say that the things described as members are the
only
members that exist. For example, the triples
given above to describe the Rules Committee say only that Fred,
Wilma, and Dino are members of the committee, not that they are the
only
members of the committee.
Also,
Example 14
and
Example 15
illustrated a common "pattern"
in describing containers, regardless of the type of container
involved (e.g., use of a blank node with
an appropriate
rdf:type
property to
represent the container itself, and use of
rdf:li
to
generate sequentially-numbered container membership properties).
However, it is important to understand that RDF does not
enforce
this particular way of using the RDF
container vocabulary, and so it is possible to use this
vocabulary in other ways. For example, in some cases
it might be appropriate to
use a container resource having a URIref rather
than using a blank node.
Moreover,
it is possible
to use the container vocabulary in ways that may not
describe graphs with the "well-formed" structures
shown in the previous examples.
For example,
Example 16
shows
the RDF/XML for a graph similar to the Alt container shown in
Figure 15
, but which writes the container
membership properties explicitly, rather than using
rdf:li
to generate them:
Example 16: RDF/XML for
an "Ill-Formed" Alt Container
As noted in
[RDF-SEMANTICS]
RDF imposes no "well-formedness" conditions on the use of
the container vocabulary, so
Example 16
is perfectly legal, even though the container
is described as
both
a Bag and an Alt, it is
described as having
two distinct values of the
rdf:_2
property,
and it does not have
rdf:_1
rdf:_3
or
rdf:_4
properties.
As a result, RDF applications that require containers
to be "well-formed" should be written to check that the
container vocabulary is being used appropriately, in order to
be fully robust.
4.2 RDF
Collections
A limitation of the containers described in
Section 4.1
is that there is no way to
close
them, i.e., to say "these are all the members of
the container". As noted in
Section 4.1
, a container
only says that certain identified
resources are members; it does not say that other members do
not exist. Also, while one graph may describe
some of the members, there is no way to exclude the possibility
that there is another graph somewhere that describes additional
members. RDF provides support for describing groups containing
only the specified members, in the form of RDF
collections
. An RDF collection is a group of things
represented as a list structure in the RDF graph. This list
structure is constructed using a predefined
collection
vocabulary
consisting of the predefined type
rdf:List
, the predefined properties
rdf:first
and
rdf:rest
, and the predefined resource
rdf:nil
To illustrate this, the sentence "The
students in course 6.001 are Amy, Mohamed, and Johann" could be represented
using the
graph shown in
Figure 16
Figure 16: An RDF
Collection (list structure)
In this graph, each member of the collection, such as
s:Amy
, is the object of an
rdf:first
property whose
subject is a resource (a blank node in this example) that
represents a list.
This list resource is linked to the rest of the list by an
rdf:rest
property. The end of the list is indicated by
the
rdf:rest
property having as its object the resource
rdf:nil
(the resource
rdf:nil
represents the empty
list, and is defined as being of type
rdf:List
).
This structure will be familiar to those who
know the Lisp programming language. As in Lisp, the
rdf:first
and
rdf:rest
properties allow
applications to traverse the structure.
Each of the blank nodes forming this list structure is
implicitly of type
rdf:List
(that is, each of these nodes implicitly has an
rdf:type
property whose
value is the predefined type
rdf:List
),
although this is not explicitly shown in the graph.
The RDF Schema language
[RDF-VOCABULARY]
defines the properties
rdf:first
and
rdf:rest
as having subjects of type
rdf:List
, so the information about
these nodes being lists can generally be inferred, rather than the corresponding
rdf:type
triples being written out all the time.
RDF/XML provides a special notation to make it easy to
describe collections using graphs of this form.
In RDF/XML, a collection can be described by
a property element that has the attribute
rdf:parseType="Collection"
, and that contains a group
of nested elements representing the members of the collection.
RDF/XML provides the
rdf:parseType
attribute to
indicate that the contents of an element are to be interpreted
in a special way. In this case, the
rdf:parseType="Collection"
attribute indicates that the enclosed elements are to be used to
create the corresponding list structure in the RDF graph
(other values of the
rdf:parseType
attribute will
be described in later sections of the Primer).
To illustrate how
rdf:parseType="Collection"
works,
the RDF/XML from
Example 17
would result in the RDF graph
shown in
Figure 16
Example 17: RDF/XML for
a Collection of Students
The use of
rdf:parseType="Collection"
in RDF/XML always
defines a list structure like the one
shown in
Figure 16
, i.e., a
fixed finite list of items with a given length and terminated
by
rdf:nil
, and which uses "new" blank nodes that are
unique to the list structure itself. However, RDF does not
enforce
this particular way of using the RDF
collection vocabulary, and so it is possible to use this
vocabulary in other ways, some of which may not describe lists
or closed collections.
To see why, note that the graph shown in
Figure 16
could also be written in RDF/XML by writing out the same triples "in longhand"
(without using
rdf:parseType="Collection"
) using the
collection vocabulary, as in
Example 18
Example 18: RDF/XML for
a Collection of Students in "Longhand"
As noted in
[RDF-SEMANTICS]
(and as was the case for the container vocabulary described
in
Section 4.1
),
RDF imposes
no "well-formedness"
conditions on the use of the collection vocabulary so, when
writing triples in longhand, it is possible to define RDF
graphs with structures other than the well-structured graphs
that would be automatically
generated by using
rdf:parseType="Collection"
For example, it is not illegal to assert that a given node has
two distinct values of the
rdf:first
property, to
create structures that have forked or non-list tails, or to
simply omit part of the description of a collection.
Also, graphs defined by using the collection vocabulary
in longhand could use URIrefs to identify the components of the list
instead of blank nodes unique to the list structure. In this case,
it would be possible to create triples in other graphs that
effectively added elements to the collection, making it non-closed.
As a result, RDF applications that require collections
to be well-formed should be written to check that the
collection vocabulary is being used appropriately, in order to
be fully robust. In addition, languages such as
OWL
[OWL]
which can define additional constraints on
the structure of RDF graphs, can rule out some of these cases.
4.3 RDF
Reification
RDF applications sometimes need to describe other RDF statements
using RDF, for instance, to record information about when
statements were made, who made them, or other similar
information (this is sometimes referred to as "provenance"
information). For example,
Example 9
in
Section 3.2
described a particular tent with URIref
exproducts:item10245
offered for sale by example.com.
One of the triples from that description, describing the weight
of the tent, was:
exproducts:item10245 exterms:weight "2.4"^^xsd:decimal .
and it might be useful for example.com to record who provided that particular
piece of information.
RDF provides a built-in vocabulary intended for describing RDF statements.
A description of a statement using this vocabulary is called a
reification
of the statement.
The RDF reification vocabulary consists of the type
rdf:Statement
, and the properties
rdf:subject
rdf:predicate
, and
rdf:object
. However, while RDF provides this
reification vocabulary, care is needed in using it, because it
is easy to imagine that the vocabulary defines some things that are
not actually defined. This point will be discussed further later in
this section.
Using the reification vocabulary, a
reification
of the statement
about the tent's weight would be given by assigning the statement
a URIref such as
exproducts:triple12345
(so statements can be written describing it), and then
describing the statement using the statements:
exproducts:triple12345 rdf:type rdf:Statement .
exproducts:triple12345 rdf:subject exproducts:item10245 .
exproducts:triple12345 rdf:predicate exterms:weight .
exproducts:triple12345 rdf:object "2.4"^^xsd:decimal .
These statements say that the resource identified by the URIref
exproducts:triple12345
is an RDF statement, that the subject
of the statement refers to the resource identified by
exproducts:item10245
the predicate of the statement refers to the resource identified by
exterms:weight
and the object of the statement refers to the decimal value identified by the typed literal
"2.4"^^xsd:decimal
Assuming that the original statement is actually identified by
exproducts:triple12345
, it should be clear by comparing the original
statement with the reification that the reification
actually does describe it. The conventional use of the RDF reification
vocabulary always involves describing a statement using four statements
in this pattern; the four statements are sometimes referred to as a
"reification quad" for this reason.
Using reification according to this convention, example.com could
record the
fact that John Smith made the original statement about the tent's weight
by first assigning the original statement a URIref (such as
exproducts:triple12345
as before), describing that statement
using the reification just described, and then adding an additional
statement that
exproducts:triple12345
was
written by John Smith (using a URIref to identify which John
Smith is being referred to). The resulting statements would be:
exproducts:triple12345 rdf:type rdf:Statement .
exproducts:triple12345 rdf:subject exproducts:item10245 .
exproducts:triple12345 rdf:predicate exterms:weight .
exproducts:triple12345 rdf:object "2.4"^^xsd:decimal .
exproducts:triple12345 dc:creator exstaff:85740 .
The original statement, together with the reification
and the attribution of the statement to John Smith,
forms the graph shown in
Figure 17
Figure 17: A Statement, Its
Reification, and Its Attribution
This graph
could be written in RDF/XML as shown in
Example 19
Example 19: RDF/XML for
the Reification Example
]>
xmlns:exterms="http://www.example.com/terms/"
xml:base="http://www.example.com/2002/04/products">
Section 3.2
introduced the use of the
rdf:ID
attribute in RDF/XML in an
rdf:Description
element to abbreviate the URIref of the subject of a statement.
rdf:ID
can also be used in a
property element to automatically produce a reification of the triple that
the property element generates.
Example 20
shows how this could be used to produce the same graph
as
Example 19
Example 20: Generating Reifications
using
rdf:ID
]>
xmlns:exterms="http://www.example.com/terms/"
xml:base="http://www.example.com/2002/04/products">
In this case, specifying the attribute
rdf:ID="triple12345"
in the
exterms:weight
element results in
the original triple describing the tent's weight:
exproducts:item10245 exterms:weight "2.4"^^xsd:decimal .
plus the reification triples:
exproducts:triple12345 rdf:type rdf:Statement .
exproducts:triple12345 rdf:subject exproducts:item10245 .
exproducts:triple12345 rdf:predicate exterms:weight .
exproducts:triple12345 rdf:object "2.4"^^xsd:decimal .
The subject of these reification triples
is a URIref formed by concatenating the base URI of the document
(given in the
xml:base
declaration), the character
" (to indicate that what follows is a fragment
identifier), and the value of the
rdf:ID
attribute;
that is, the triples have the same subject
exproducts:triple12345
as in the previous examples.
Note that asserting the reification is not
the same as asserting the original statement, and
neither implies the other. That is, when someone says that
John said something about the weight of a tent, they are
not making a statement about the weight of a tent themselves,
they are making a statement about something John said.
Conversely, when someone describes
the weight of a tent, they are
not also making a statement about a statement they made (since they
may have no intention of talking about things called "statements").
The text above deliberately referred in a number of places to
"the conventional use of reification". As noted earlier, care is needed when using
the RDF reification vocabulary because it is easy to imagine that
the vocabulary defines some things that are not actually defined.
While there are applications that successfully use reification, they do so by
following some conventions, and making some assumptions, that are
in addition to the actual meaning that RDF defines for the reification
vocabulary, and the actual facilities that RDF provides to support it.
For one thing, it is important to
note that in the conventional use of reification, the subject of
the reification triples is assumed to identify
particular instance
of a triple in a particular RDF document, rather than some
arbitrary triple having the same subject, predicate, and
object. This particular convention is used because
reification is intended for expressing properties such
as dates of composition and source information, as in the
examples given already, and these properties need to be applied to specific
instances of triples. There could be several triples that have the same
subject, predicate, and object and, although a graph is
defined as a
set
of triples, several instances with the same
triple structure might occur in different documents. Thus,
to fully support this convention,
there needs to be some means of associating the subject of the reification
triples with
an individual triple in some document
However, RDF provides no way to do this.
For instance, in the examples above, there is no explicit information
in either the triples or the RDF/XML that actually indicates that
the original statement describing the tent's weight is the resource
exproducts:triple12345
, the resource that is the subject of
the four reification statements and the statement
that John Smith created it.
This can be seen by looking at the drawn graph shown in
Figure 17
The original statement is certainly part of this graph, but
as far as the information in the graph is concerned,
exproducts:triple12345
is a separate resource, rather
than identifying that part of the graph.
RDF does not provide a built-in way of indicating how a URIref
like
exproducts:triple12345
is associated
with a particular statement or graph, any more than it provides
a built-in way of indicating how a URIref like
exproducts:item10245
is associated with an actual tent.
Associating specific URIrefs with specific resources (statements
in this case) must be done using mechanisms outside of RDF.
Using
rdf:ID
as shown in
Example 20
generates the reification
automatically, and provides a convenient way of indicating the
URIref to be used as the subject of the statements in the reification.
Moreover, it provides a partial "hook" relating the triples
in the reification with the piece of RDF/XML syntax that caused
them to be created, since the value
triple12345
of
the
rdf:ID
attribute is used to generate the URIref
of the subject of the reification triples. However, this
relationship is once again outside RDF, since there is nothing
in the resulting triples that explicitly says that the original
triple had the URIref
exproducts:triple12345
(RDF
does not assume there is any relationship between a URIref
and any RDF/XML that it might have been used or abbreviated in).
The lack of a built-in means for assigning URIrefs to
statements does not mean that "provenance" information
of this kind cannot
be expressed in RDF, just that it cannot be done using only the
meaning RDF associates with the reification vocabulary. For
example, if an RDF document (say, a Web page) has a URI,
statements could be made about the resource identified by that URI
and, based on some application-dependent understanding of how
those statements should be interpreted, an application could act as if those
statements "distribute" over (apply equally to) all the
statements in the document. Also, if some mechanism exists
(outside of RDF) to assign URIs to individual RDF statements,
then statements could certainly be made about those individual
statements, using their URIs to identify them. However, in these cases,
it would also not be strictly necessary to use the reification vocabulary
in the conventional way.
To see this, assuming the original statement:
exproducts:item10245 exterms:weight "2.4"^^xsd:decimal .
had a URIref of
exproducts:triple12345
, the statement could be
attributed to John Smith simply by the statement:
exproducts:triple12345 dc:creator exstaff:85740 .
with no use of the reification vocabulary (although the
description of
exproducts:triple12345
as having
rdf:type
rdf:Statement
might also be helpful).
In addition, the reification vocabulary could be used
directly according to the convention described
above, along with an application-dependent understanding as to
how to associate specific triples with their reifications.
However, other applications receiving this RDF would not
necessarily share this application-dependent understanding, and
thus would not necessarily interpret the graphs
appropriately.
It is also important to note that the interpretation of reification described here is not the same as "quotation", as found in some languages. Instead, the reification describes the relationship between a particular instance of a triple and the resources the triple refers to. The reification can be read intuitively as saying "this RDF triple talks about these things", rather than (as in quotation) "this RDF triple has this form." For instance, in the reification example used in this section, the triple:
exproducts:triple12345 rdf:subject exproducts:item10245 .
describing the
rdf:subject
of the original statement says that the subject of the statement is the resource (the tent) identified by the URIref
exproducts:item10245
. It does
not
say that the subject of the statement is the URIref itself (i.e., a string beginning with certain characters), as quotation would do.
4.4 More on Structured
Values: rdf:value
Section 2.3
noted
that the RDF model intrinsically supports only
binary
relations; that is, a statement specifies a
relation between two resources. For example, the statement:
exstaff:85740 exterms:manager exstaff:62345 .
states that the relation
exterms:manager
holds between two
employees (presumably one manages the other).
However, in some cases it is necessary to represent
information involving higher arity relations (relations between
more than two resources) in RDF.
Section 2.3
discussed one example of
this, where
the problem was to represent the relationship between John
Smith and his address information, and the value of John's
address was a structured value of his street, city, state, and
postal code. Writing this as a relation shows that this
address is a 5-ary relation of the form:
address(exstaff:85740, "1501 Grant
Avenue", "Bedford", "Massachusetts", "01730")
Section 2.3
noted that
this kind of structured information can be represented
in RDF by considering the aggregate thing be described
(here, the group of components representing
John's address) as a separate resource, and then making
separate statements about that new resource, as in the
triples:
exstaff:85740 exterms:address _:johnaddress .
_:johnaddress exterms:street "1501 Grant Avenue" .
_:johnaddress exterms:city "Bedford" .
_:johnaddress exterms:state "Massachusetts" .
_:johnaddress exterms:postalCode "01730" .
(where
_:johnaddress
is the blank node identifier
of the blank node representing John's address.)
This is a general way to represent any n-ary relation in
RDF: select one of the participants (John in this case) to
serve as the subject of the original relation (
address
in this case), then specify an intermediate resource to
represent the rest of the relation (either with or without
assigning it a URI), then give that new resource properties
representing the remaining components of the relation.
In the case of John's address, none of the individual parts
of the structured value could be considered the "main" value of
the
exterms:address
property; all of the parts
contribute equally to the value. However, in some cases one of
the parts of the structured value is often thought of as the
"main" value, with the other parts of the relation providing
additional contextual or other information that qualifies the
main value. For instance, in
Example 9
in
Section 3.2
, the weight of a
particular tent was given as the
decimal value
2.4
using a typed literal
i.e.,
exproduct:item10245 exterms:weight "2.4"^^xsd:decimal .
In fact, a more complete description of the weight would
have been
2.4 kilograms
rather than just the decimal value
2.4
. To state
this, the value of the
exterms:weight
property would
need to have two components, the typed literal for the decimal value and an
indication of the unit of measure (kilograms). In this
situation the decimal value could be considered the "main"
value of the
exterms:weight
property, because
frequently the value would be recorded simply as the typed literal
(as in the triple above), relying on an
understanding of the context to fill in the unstated units
information.
In the RDF model a qualified property value of this kind can be
considered as simply another kind of structured value. To
represent this, a separate resource could be used to represent the
structured value as a whole (the weight, in this case), and to
serve as the object of the original statement.
That resource could then be given properties representing the individual parts of
the structured value. In this case, there should be a property for the
typed literal representing the decimal value, and a property for
the unit. RDF provides a predefined
rdf:value
property to describe
the main value (if there is one) of a structured value. So in
this case, the typed literal could be given as the value of the
rdf:value
property, and the resource
exunits:kilograms
as the value of an
exterms:units
property (assuming the resource
exunits:kilograms
is defined as part of example.org's
vocabulary). The resulting
triples would be:
exproduct:item10245 exterms:weight _:weight10245 .
_:weight10245 rdf:value "2.4"^^xsd:decimal .
_:weight10245 exterms:units exunits:kilograms .
which can be expressed using the RDF/XML shown in
Example 21
Example 21: RDF/XML
using
rdf:value
]>
Example 21
also illustrates a second use of
the
rdf:parseType
attribute introduced in
Section 4.2
in this case,
rdf:parseType="Resource"
. An
rdf:parseType="Resource"
attribute is used to indicate
that the contents of an element are to be interpreted as the description
of a new (blank node) resource, without actually having to write a
nested
rdf:Description
element. In this case, the
rdf:parseType="Resource"
attribute used in the
exterms:weight
property element indicates that a
blank node is to be created as the value of the
exterms:weight
property, and that the enclosed elements (
rdf:value
and
exterms:units
) describe properties of that blank node.
Further details on
rdf:parseType="Resource"
are given
in
[RDF-SYNTAX]
The same approach can be used to represent quantities using
any units of measure, as well as values taken from different
classification schemes or rating systems, by using the
rdf:value
property to give the main value, and using
additional properties to identify the classification scheme or
other information that further describes the value.
There is no need to use
rdf:value
for these purposes
(e.g., a user-defined property name, such as
exterms:amount
, could have been used instead of
rdf:value
in
Example 21
), and RDF does not
associate any special meaning with
rdf:value
rdf:value
is simply provided as a convenience for use in these
commonly-occurring situations.
However, even though much existing data in databases and on the Web
(and in later Primer examples) takes the form of simple values for properties
such as weights, costs, etc., the principle that such simple values are often
insufficient to adequately describe these values is an important one. In a global
environment such as the Web, it is generally
not
safe to make the
assumption that anyone accessing a property value will understand the units
being used (or other contextually-dependent information that may be involved).
For example, a U.S. site might give a weight value in pounds, but someone accessing
that data from outside the U.S. might assume that weights are given in kilograms.
The correct interpretation of data in the Web environment may require that
additional information (such as units information) be explicitly recorded.
This can be done in many ways, such as using
rdf:value
, building
units into property names (e.g.,
exterms:weightInKg
), defining
specialized datatypes that include units information (e.g.,
extypes:kilograms
),
or adding additional user-defined properties to specify this information
(e.g.,
exterms:unitOfWeight
), either in descriptions of individual
items or products, in descriptions of sets of data (e.g., all the data in a
catalog or on a site), or in schemas (see
Section 5
).
4.5 XML Literals
Sometimes the value of a property needs to be a fragment of XML,
or text that might contain XML markup. For example, a publisher might
maintain RDF metadata that includes the titles of books and articles.
While such titles are often just simple strings of characters, this is
not always the case. For instance, the titles of books on mathematics
may contain mathematical formulas that could be represented using
MathML
[MATHML]
. Titles might also include
markup for other reasons, such as for Ruby annotations
[RUBY]
, or for bidirectional rendering or
special glyph variants (see, e.g.,
[CHARMOD]
).
RDF/XML provides a special notation to make it easy to write
literals of this kind. This is done using a third value of the
rdf:parseType
attribute. Giving an element the attribute
rdf:parseType="Literal"
indicates that the contents of
the element are to be interpreted as an XML fragment.
Example 22
illustrates the use of
rdf:parseType="Literal"
Example 22: RDF/XML
for an XML Literal
xml:base="http://www.example.com/books">
The <br /> Element Considered Harmful.
The RDF/XML in
Example 22
describes a
graph containing a single triple with subject
ex:book12345
, and predicate
dc:title
The
rdf:parseType="Literal"
attribute in the RDF/XML
indicates that all the XML within the
element is an XML fragment that is the value of the
dc:title
property.
In the graph, this value is a typed literal, whose datatype,
rdf:XMLLiteral
, is defined in
[RDF-CONCEPTS]
specifically to
represent fragments of XML (including character sequences that may
or may not include XML markup). The XML fragment is canonicalized according
to the XML Exclusive Canonicalization recommendation
[XML-XC14N]
. This causes declarations
of used namespaces to be added to the fragment,
the uniform escaping or unescaping of characters,
the expansion of empty-element tags,
and other transformations. (For these reasons, and the fact that the
triples notation itself requires further escaping, the
actual typed literal is not shown here. RDF/XML provides the
rdf:parseType="Literal"
attribute so that RDF users will
not
have to deal directly with these transformations. Those
interested in the details should consult
[RDF-CONCEPTS]
and
[RDF-SYNTAX]
.)
Contextual attributes, such as
xml:lang
and
xml:base
are not inherited from the RDF/XML document, and, if required, must,
as shown in the example, be explicitly specified in the XML fragment.
This example illustrates that care must be taken in designing RDF data.
It might appear at first glance that titles are simple strings best
represented as plain literals, and only later might it be discovered
that some titles contain markup. In cases where the value of a property
may sometimes contain markup and sometimes not, either
rdf:parseType="Literal"
should be used throughout,
or software must handle both plain literals and literals of type
rdf:XMLLiteral
as values of the property.
5. Defining RDF
Vocabularies: RDF Schema
RDF provides a way to express simple statements about
resources, using named properties and values. However, RDF user
communities also need the ability
to define the
vocabularies
(terms) they intend to use in those statements, specifically,
to indicate that they are
describing specific kinds or classes of resources, and will use
specific properties in describing those resources. For example,
the company example.com from the examples in
Section 3.2
would want to describe
classes such as
exterms:Tent
, and use properties such as
exterms:model
exterms:weightInKg
, and
exterms:packedSize
to describe them (QNames with
various "example" namespace prefixes are used as the names of classes and
properties here as a reminder that in RDF these names are
actually
URI references
, as discussed in
Section 2.1
). Similarly, people
interested in describing bibliographic resources would want to
describe classes such as
ex2:Book
or
ex2:MagazineArticle
, and use properties such as
ex2:author
ex2:title
, and
ex2:subject
to describe them. Other applications might need to describe
classes such as
ex3:Person
and
ex3:Company
, and
properties such as
ex3:age
ex3:jobTitle
ex3:stockSymbol
, and
ex3:numberOfEmployees
RDF
itself provides no means for defining such application-specific
classes and properties.
Instead, such classes and properties are described as an RDF
vocabulary, using extensions to RDF provided by the
RDF Vocabulary
Description Language 1.0: RDF Schema
[RDF-VOCABULARY]
, referred
to here as
RDF Schema
RDF Schema does not provide a vocabulary of
application-specific classes like
exterms:Tent
ex2:Book
, or
ex3:Person
, and properties like
exterms:weightInKg
ex2:author
or
ex3:JobTitle
. Instead, it provides the facilities needed
to
describe
such classes and properties,
and to indicate which classes and properties are
expected to be used together (for example, to say that the
property
ex3:jobTitle
will be used in describing a
ex3:Person
). In other words, RDF Schema provides a
type system
for RDF. The RDF Schema type system is
similar in some respects to the type systems of object-oriented
programming languages such as Java. For example, RDF Schema
allows resources to be defined as instances of one or more
classes
. In addition, it allows classes to be organized
in a hierarchical fashion; for example a class
ex:Dog
might be defined as a subclass of
ex:Mammal
which is a
subclass of
ex:Animal
, meaning that any resource which
is in class
ex:Dog
is also implicitly in class
ex:Animal
as well. However, RDF classes and properties are in
some respects very different from programming language types. RDF
class and property descriptions do not create a straightjacket
into which information must be forced, but instead provide
additional information about the RDF resources they describe.
This information can be used in a variety of ways, which will be
discussed in
Section
5.3
The RDF Schema facilities are themselves provided in the form of
an RDF vocabulary; that is, as a specialized set of predefined RDF
resources with their own special meanings. The resources in the
RDF Schema vocabulary have URIrefs with the prefix
(conventionally
associated with the QName prefix
rdfs:
).
Vocabulary descriptions (schemas) written in the RDF Schema language
are legal RDF graphs. Hence, RDF software that is not written
to also process
the additional RDF Schema vocabulary can still interpret a schema as a legal
RDF graph consisting of various resources and properties,
but will not "understand" the additional built-in
meanings of the RDF Schema terms. To understand these additional
meanings, RDF software must
be written to process an extended language that includes not
only the
rdf:
vocabulary, but also the
rdfs:
vocabulary, together with their built-in meanings.
This point will be illustrated in the next section.
The following sections will illustrate RDF Schema's
basic resources and properties.
5.1 Describing
Classes
A basic step in any kind of description process is
identifying the various kinds of things to be described. RDF
Schema refers to these "kinds of things" as
classes
. A
class
in RDF Schema corresponds to the generic concept
of a
Type
or
Category
, somewhat like the
notion of a class in object-oriented programming languages such
as Java. RDF classes can be used to represent almost any
category of thing, such as Web pages, people, document types,
databases or abstract concepts. Classes are described using the
RDF Schema resources
rdfs:Class
and
rdfs:Resource
, and the properties
rdf:type
and
rdfs:subClassOf
For example, suppose an organization
example.org
wanted to use RDF to provide
information about different kinds of motor vehicles. In RDF
Schema,
example.org
would first need a class to represent the category
of things that are motor vehicles. The resources that belong to
a class are called its
instances
. In this case,
example.org
intends for the instances of this class to be resources that are
motor vehicles.
In RDF Schema, a
class
is any resource having an
rdf:type
property whose value is the
resource
rdfs:Class
. So the motor vehicle class would
be described by assigning the class a URIref, say
ex:MotorVehicle
using
ex:
to stand for the URIref
, which is used
as the prefix for URIrefs from example.org's vocabulary
and describing that resource with an
rdf:type
property whose value is the
resource
rdfs:Class
. That is,
example.org
would write the RDF
statement:
ex:MotorVehicle rdf:type rdfs:Class .
As indicated in
Section 3.2
the property
rdf:type
is used to indicate that a
resource is an instance of a class. So, having described
ex:MotorVehicle
as a class, resource
exthings:companyCar
would be described as a motor vehicle by
the RDF statement:
exthings:companyCar rdf:type ex:MotorVehicle .
(This statement uses a common convention that class names
are written with an initial uppercase letter, while property
and instance names are written with an initial lowercase
letter. However, this convention is not required in RDF Schema.
The statement also assumes that
example.org
has decided to define separate vocabularies for classes of things, and
instances of things.)
The resource
rdfs:Class
itself has an
rdf:type
of
rdfs:Class
. A resource may be an
instance of more than one class.
After describing class
ex:MotorVehicle
example.org
might
want to describe additional classes representing various
specialized kinds of motor vehicle, e.g., passenger vehicles,
vans, minivans, and so on. These classes can be described in the
same way as class
ex:MotorVehicle
, by
assigning a URIref for each new class, and writing RDF
statements describing these resources as classes, e.g.,
writing:
ex:Van rdf:type rdfs:Class .
ex:Truck rdf:type rdfs:Class .
and so on. However, these statements by themselves only describe
the individual classes.
example.org
may also want to
indicate their special
relationship to class
ex:MotorVehicle
, i.e., that they
are specialized
kinds
of MotorVehicle.
This kind of specialization relationship between
two classes is described using the predefined
rdfs:subClassOf
property to relate the two classes.
For example, to state that
ex:Van
is a
specialized kind of
ex:MotorVehicle
example.org
would write the RDF statement:
ex:Van rdfs:subClassOf ex:MotorVehicle .
The meaning of this
rdfs:subClassOf
relationship is
that any instance of class
ex:Van
is also an instance
of class
ex:MotorVehicle
So if resource
exthings:companyVan
is an instance of
ex:Van
then, based on the declared
rdfs:subClassOf
relationship,
RDF software written to understand the RDF Schema vocabulary
can
infer
the additional information
that
exthings:companyVan
is also an instance of
ex:MotorVehicle
This example of
exthings:companyVan
illustrates the point made earlier about RDF Schema defining
an extended language. RDF itself does not define the special
meaning of terms from the RDF Schema vocabulary such as
rdfs:subClassOf
So if an RDF schema defines this
rdfs:subClassOf
relationship
between
ex:Van
and
ex:MotorVehicle
RDF software not written to understand the RDF Schema terms
would recognize this as a triple,
with predicate
rdfs:subClassOf
, but it would not
understand the special significance of
rdfs:subClassOf
and not be able to
draw the additional inference that
exthings:companyVan
is also an instance of
ex:MotorVehicle
The
rdfs:subClassOf
property is
transitive
. This means, for example, that given
the RDF statements:
ex:Van rdfs:subClassOf ex:MotorVehicle .
ex:MiniVan rdfs:subClassOf ex:Van .
RDF Schema defines
ex:MiniVan
as also being a subclass of
ex:MotorVehicle
. As a result, RDF Schema defines resources that are
instances of class
ex:MiniVan
as also being
instances of class
ex:MotorVehicle
(as well as being instances of
class
ex:Van
). A class may be a subclass of more than
one class (for example,
ex:MiniVan
may be a subclass
of both
ex:Van
and
ex:PassengerVehicle
).
RDF Schema defines all classes as subclasses of class
rdfs:Resource
(since the instances belonging to all
classes are resources).
Figure 18
shows the full class
hierarchy being discussed in these examples.
Figure 18: A Vehicle Class
Hierarchy
(To simplify the figure, the
rdf:type
properties
relating each of the classes to
rdfs:Class
are omitted in
Figure 18
. In fact, RDF Schema defines
both the subjects and objects of statements that use the
rdfs:subClassOf
property to be resources of type
rdfs:Class
, so this information could be inferred.
However, in actually writing schemas, it is good practice to
explicitly provide this information.)
This schema could also be described by the triples:
ex:MotorVehicle rdf:type rdfs:Class .
ex:PassengerVehicle rdf:type rdfs:Class .
ex:Van rdf:type rdfs:Class .
ex:Truck rdf:type rdfs:Class .
ex:MiniVan rdf:type rdfs:Class .
ex:PassengerVehicle rdfs:subClassOf ex:MotorVehicle .
ex:Van rdfs:subClassOf ex:MotorVehicle .
ex:Truck rdfs:subClassOf ex:MotorVehicle .
ex:MiniVan rdfs:subClassOf ex:Van .
ex:MiniVan rdfs:subClassOf ex:PassengerVehicle .
Example 23
shows how this schema
could be written in RDF/XML.
Example 23: The Vehicle
Class Hierarchy in RDF/XML
]>
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://example.org/schemas/vehicles">
As discussed in
Section 3.2
in connection with
Example 13
RDF/XML provides an abbreviation for describing
resources having an
rdf:type
property (
typed nodes
).
Since RDF Schema classes are RDF resources,
this abbreviation can be applied to the description of classes.
Using this abbreviation, the schema could also be described
as shown in
Example 24
Example 24: The Vehicle
Class Hierarchy Using the Typed Node Abbreviation
]>
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://example.org/schemas/vehicles">
Similar typed node abbreviations will be used throughout the rest of
this section.
The RDF/XML in
Example 23
and
Example 24
introduces names, such as
MotorVehicle
, for the resources (classes) that it
describes using
rdf:ID
, to give the effect of
"assigning" URIrefs relative to the schema document as
described in
Section 3.2
rdf:ID
is useful here because
it both abbreviates the URIrefs, and also provides an additional
check that the value of the
rdf:ID
attribute is
unique against the current base URI (usually the document URI).
This helps pick up repeated
rdf:ID
values when defining
the names of classes and properties in RDF schemas.
Relative
URIrefs based on these names can then be used in other class
definitions within the same schema (e.g., as
#MotorVehicle
is used in the description of the other
classes). The full URIref of this class, assuming that the
schema itself was the resource
, would be
(shown in
Figure 18
). As noted in
Section 3.2
, to ensure that the
references to these schema classes would be consistently
maintained even if the schema were relocated or copied (or to
simply assign a base URIref for the schema classes without
assuming they are all published at a single location), the
class descriptions could also include an explicit
xml:base="http://example.org/schemas/vehicles"
declaration.
Use of an explicit
xml:base
declaration is considered good practice,
and one is provided in both examples.
To refer to these classes in RDF instance data (e.g., data
describing individual vehicles of these classes) located
elsewhere,
example.org
would need to identify
the classes
either by writing absolute URIrefs,
by using relative URIrefs together with an appropriate
xml:base
declaration, or by using QNames
together with an appropriate namespace declaration that allows the
QNames to be expanded to the proper URIrefs.
For example, the resource
exthings:companyCar
could be described as an instance of the class
ex:MotorVehicle
described in the schema of
Example 24
by
the RDF/XML shown in
Example 25
Example 25: An Instance
of
ex:MotorVehicle
xmlns:ex="http://example.org/schemas/vehicles#"
xml:base="http://example.org/things">
Note that the QName
ex:MotorVehicle
, when expanded using the
namespace declaration
xmlns:ex="http://example.org/schemas/vehicles#"
becomes the full URIref
which is the correct URIref for the
MotorVehicle
class
as shown in
Figure 18
. The
xml:base
declaration
xml:base="http://example.org/things"
is provided
to allow the
rdf:ID="companyCar"
to expand to the proper
exthings:companyCar
URIref (since a QName cannot be used as the
value of the
rdf:ID
attribute).
5.2 Describing
Properties
In addition to describing the specific
classes
of
things they want to describe, user communities also need to be
able to describe specific
properties
that characterize
those classes of things (such as
rearSeatLegRoom
to
describe a passenger vehicle). In RDF Schema, properties are
described using the RDF class
rdf:Property
and the RDF Schema properties
rdfs:domain
rdfs:range
, and
rdfs:subPropertyOf
All properties in RDF are described as instances of class
rdf:Property
. So a new property, such as
exterms:weightInKg
, is described by assigning the
property a URIref, and describing that resource with an
rdf:type
property whose value is the resource
rdf:Property
, for example, by writing the RDF
statement:
exterms:weightInKg rdf:type rdf:Property .
RDF Schema also provides vocabulary for describing how
properties and classes are intended to be used together in RDF
data. The most important information of this kind is supplied
by using the RDF Schema properties
rdfs:range
and
rdfs:domain
to further describe application-specific
properties.
The
rdfs:range
property is used to indicate that
the values of a particular property are instances of a
designated class. For example, if
example.org
wanted to indicate that
the property
ex:author
had values that are instances
of class
ex:Person
, it would write the RDF
statements:
ex:Person rdf:type rdfs:Class .
ex:author rdf:type rdf:Property .
ex:author rdfs:range ex:Person .
These statements indicate that
ex:Person
is a
class,
ex:author
is a property, and that RDF
statements using the
ex:author
property have instances
of
ex:Person
as objects.
A property, say
ex:hasMother
, can have zero, one,
or more than one range property. If
ex:hasMother
has
no range property, then nothing is said about the values
of the
ex:hasMother
property. If
ex:hasMother
has one range property, say one specifying
ex:Person
as the range, this says that the values of the
ex:hasMother
property are instances of class
ex:Person
. If
ex:hasMother
has more than one
range property, say one specifying
ex:Person
as its
range, and another specifying
ex:Female
as its range,
this says that the values of the
ex:hasMother
property
are resources that are instances of
all
of the classes
specified as the ranges, i.e., that any value of
ex:hasMother
is
both
ex:Female
and
ex:Person
This last point may not be obvious. However, stating
that the property
ex:hasMother
has the two ranges
ex:Female
and
ex:Person
involves making
two separate statements:
ex:hasMother rdfs:range ex:Female .
ex:hasMother rdfs:range ex:Person .
For any given statement using this property, say:
exstaff:frank ex:hasMother exstaff:frances .
in order for
both
the
rdfs:range
statements to be
correct, it must be the case that
exstaff:frances
is
both
an instance of
ex:Female
and
of
ex:Person
The
rdfs:range
property can also be used to
indicate that the value of a property is given by a typed
literal, as discussed in
Section
2.4
. For example, if
example.org
wanted to indicate that the
property
ex:age
had values from the XML Schema
datatype
xsd:integer
, it would write the RDF
statements:
ex:age rdf:type rdf:Property .
ex:age rdfs:range xsd:integer .
The datatype
xsd:integer
is identified by its
URIref (the full URIref being
). This URIref
can be used without explicitly stating in the schema that
it identifies a datatype. However, it is often useful to
explicitly state that a given URIref identifies a datatype.
This can be done using the RDF Schema class
rdfs:Datatype
. To state that
xsd:integer
is a
datatype,
example.org
would write the RDF statement:
xsd:integer rdf:type rdfs:Datatype .
This statement says that
xsd:integer
is the URIref
of a datatype (which is assumed to conform to the requirements
for RDF datatypes described in
[RDF-CONCEPTS]
). Such a statement
does
not
constitute a
definition
of a
datatype, e.g., in the sense that
example.org
is defining a new
datatype. There is no way to define datatypes in RDF Schema. As noted
in
Section 2.4
, datatypes are
defined externally to RDF (and to RDF Schema), and
referred to
in RDF statements
by their URIrefs. This statement simply serves to document the
existence of the datatype, and indicate explicitly that it is
being used in this schema.
The
rdfs:domain
property is used to indicate that a
particular property applies to a designated class. For example,
if
example.org
wanted to indicate that the property
ex:author
applies to instances of class
ex:Book
, it would write
the RDF statements:
ex:Book rdf:type rdfs:Class .
ex:author rdf:type rdf:Property .
ex:author rdfs:domain ex:Book .
These statements indicate that
ex:Book
is a
class,
ex:author
is a property, and that RDF
statements using the
ex:author
property have instances
of
ex:Book
as subjects.
A given property, say
exterms:weight
, may have
zero, one, or more than one domain property. If
exterms:weight
has no domain property, then
nothing is said about the resources that
exterms:weight
properties may be used with (any resource could have a
exterms:weight
property). If
exterms:weight
has one domain property, say one specifying
ex:Book
as
the domain, this says that the
exterms:weight
property
applies to instances of class
ex:Book
. If
exterms:weight
has more than one domain property, say
one specifying
ex:Book
as the domain and another one
specifying
ex:MotorVehicle
as the domain, this says
that any resource that has a
exterms:weight
property
is an instance of
all
of the classes specified as the
domains, i.e., that any resource that has a
exterms:weight
property is both a
ex:Book
and
ex:MotorVehicle
(illustrating the need
for care in specifying domains and ranges).
As in the case of
rdfs:range
, this last point may
not be obvious. However, stating
that the property
exterms:weight
has the two domains
ex:Book
and
ex:MotorVehicle
involves making
two separate statements:
exterms:weight rdfs:domain ex:Book .
exterms:weight rdfs:domain ex:MotorVehicle .
For any given statement using this property, say:
exthings:companyCar exterms:weight "2500"^^xsd:integer .
in order for
both
the
rdfs:domain
statements to be
correct, it must be the case that
exthings:companyCar
is
both
an instance of
ex:Book
and
of
ex:MotorVehicle
The use of these range and domain
descriptions can be illustrated by extending the vehicle schema, adding two
properties
ex:registeredTo
and
ex:rearSeatLegRoom
, a new class
ex:Person
and explicitly describing the datatype
xsd:integer
as
a datatype. The
ex:registeredTo
property applies to
any
ex:MotorVehicle
and its value is a
ex:Person
. For the sake of this example,
ex:rearSeatLegRoom
applies only to instances of class
ex:PassengerVehicle
. The value is an
xsd:integer
giving the number of centimeters of rear
seat legroom. These descriptions are shown in
Example 26
Example 26: Some
Property Descriptions for the Vehicle Schema
Note that an
element is not used in
Example 26
, because
it is
assumed this RDF/XML is being added to the vehicle schema
described in
Example 24
. This same
assumption also allows the use of relative URIrefs like
#MotorVehicle
to refer to other classes from that
schema.
RDF Schema provides a way to specialize
properties
as well as classes. This specialization
relationship between two properties is described using the predefined
rdfs:subPropertyOf
property. For example, if
ex:primaryDriver
and
ex:driver
are both
properties,
example.org
could describe these properties,
and the fact that
ex:primaryDriver
is a specialization of
ex:driver
, by writing the RDF statements:
ex:driver rdf:type rdf:Property .
ex:primaryDriver rdf:type rdf:Property .
ex:primaryDriver rdfs:subPropertyOf ex:driver .
The meaning of this
rdfs:subPropertyOf
relationship
is that if an instance
exstaff:fred
is an
ex:primaryDriver
of the instance
ex:companyVan
then
RDF Schema defines
exstaff:fred
as also being
an
ex:driver
of
ex:companyVan
The RDF/XML describing these properties
(assuming again that it is being added to the vehicle schema
described in
Example 24
is shown in
Example
27
Example 27: More
Properties for the Vehicle Schema
A property may be a subproperty of zero, one or more
properties. All RDF Schema
rdfs:range
and
rdfs:domain
properties that apply to an RDF property
also apply to each of its subproperties. So, in the above example,
RDF Schema defines
ex:primaryDriver
as also having an
rdfs:domain
of
ex:MotorVehicle
because of its subproperty
relationship to
ex:driver
Example 28
shows the RDF/XML for the
full vehicle schema, containing all the descriptions
given so far:
Example 28: The Full
Vehicle Schema
]>
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://example.org/schemas/vehicles">
Having shown how to describe classes and properties
using RDF Schema, instances using those classes and properties
can now be illustrated. For example,
Example 29
describes an instance of the
ex:PassengerVehicle
class described in
Example 28
, together
with some hypothetical values for its properties.
Example 29: An Instance
of
ex:PassengerVehicle
]>
xml:base="http://example.org/things">
This example assumes that the instance is described in a
separate document from the schema. Since the
schema has an
xml:base
of
, the
namespace declaration
xmlns:ex="http://example.org/schemas/vehicles#"
is provided to allow
QNames such as
ex:registeredTo
in the instance data to
be properly expanded to the URIrefs of the classes and
properties described in that schema. An
xml:base
declaration is also provided for this
instance, to allow
rdf:ID="johnSmithsCar"
to expand to the proper URIref
independently of the location of the actual document.
Note that an
ex:registeredTo
property can be used in
describing this instance of
ex:PassengerVehicle
because
ex:PassengerVehicle
is a subclass of
ex:MotorVehicle
. Note also that a typed literal is used
for the value of the
ex:rearSetLegRoom
property in this
instance, rather than a plain literal (i.e., rather than stating the value
as
).
Because the schema describes the range of this property as an
xsd:integer
, the value of the property should be a typed
literal of that datatype in order to match the range description
(i.e., the range declaration does not
automatically "assign" a
datatype to a plain literal, and so a typed literal of the
appropriate datatype must be explicitly provided).
Additional information, either in the schema, or in additional
instance data, could also be provided to explicitly specify the
units
of the
ex:rearSetLegRoom
property
(centimeters), as discussed in
Section 4.4
5.3
Interpreting RDF Schema Declarations
As noted earlier, the RDF Schema type system is similar in
some respects to the type systems of object-oriented
programming languages such as Java. However, RDF differs from
most programming language type systems in several important
respects.
One important difference is that instead of describing a
class as having a collection of specific properties, an RDF
schema describes properties as applying to specific classes of
resources, using
domain
and
range
properties.
For example, a typical object-oriented programming language
might define a class
Book
with an attribute called
author
having values of type
Person
. A
corresponding RDF schema would describe a class
ex:Book
, and, in a separate description, a property
ex:author
having a domain of
ex:Book
and a
range of
ex:Person
The difference between these approaches may seem to be only
syntactic, but in fact there is an important difference. In the
programming language class description, the attribute
author
is part of the description of class
Book
, and applies
only
to instances of class
Book
. Another class (say,
softwareModule
might also have an attribute called
author
, but this
would be considered a
different
attribute. In other
words, the
scope
of an attribute description in most
programming languages is restricted to the class or type in
which it is defined. In RDF, on the other hand, property
descriptions are, by default,
independent
of class
definitions, and have, by default,
global
scope
(although they may optionally be declared to apply only to
certain classes using domain specifications).
As a result, an RDF schema could describe a property
exterms:weight
without a domain being specified. This
property could then be used to describe instances of any class
that might be considered to have a weight. One benefit of the
RDF property-based approach is that it becomes easier to extend
the use of property definitions to situations that might not
have been anticipated in the original description. At the same time,
this is a "benefit" which must be used with care, to insure
that properties are not mis-applied in inappropriate
situations.
Another result of the global scope of RDF property descriptions
is that it is not possible in an RDF schema to define a specific property as
having locally-different ranges depending on the class of the resource
it is applied to.
For example, in defining the property
ex:hasParent
, it
would be desirable to be able to say that if the property is used
to describe a resource of class
ex:Human
, then the range
of the property is also a resource of class
ex:Human
while if the property is used
to describe a resource of class
ex:Tiger
, then the range
of the property is also a resource of class
ex:Tiger
This kind of definition is not possible in RDF Schema. Instead, any
range defined for an RDF property applies to
all
uses of the
property, and so ranges should be defined with care. However, while
such locally-different ranges cannot be defined in RDF Schema,
they can be defined in some of the richer schema languages discussed in
Section 5.5
Another important difference is that RDF Schema descriptions
are not necessarily
prescriptive
in the way
programming language type declarations typically are. For
example, if a programming language declares a class
Book
with an
author
attribute having values
of type
Person
, this is usually interpreted as a group
of
constraints
. The language will not allow the
creation of an instance of
Book
without an
author
attribute, and it will not allow an instance of
Book
with an
author
attribute that does not
have a
Person
as its value. Moreover, if
author
is the
only
attribute defined for
class
Book
, the language will not allow an instance of
Book
with some other attribute.
RDF Schema, on the other hand, provides schema information
as additional
descriptions
of resources, but does not
prescribe how these descriptions should be used by an
application. For example, suppose an RDF schema states that an
ex:author
property has an
rdfs:range
of class
ex:Person
. This is simply an RDF statement that RDF
statements containing
ex:author
properties have
instances of
ex:Person
as objects.
This schema-supplied information might be used in different
ways. One application might interpret this statement as
specifying part of a template for RDF data it is creating, and
use it to ensure that any
ex:author
property has a
value of the indicated (
ex:Person
) class. That is,
this application interprets the schema description as a
constraint
in the same way that a programming language
might. However, another application might interpret this
statement as providing additional information about data it is
receiving, information which may not be provided explicitly in
the original data. For example, this second application might
receive some RDF data that includes an
ex:author
property whose value is a resource of unspecified class, and
use this schema-provided statement to conclude that the
resource must be an instance of class
ex:Person
. A
third application might receive some RDF data that includes an
ex:author
property whose value is a resource of class
ex:Corporation
, and use this schema information as the
basis of a warning that "there may be an inconsistency here,
but on the other hand there may not be". Somewhere else there
may be a declaration that resolves the apparent inconsistency
(e.g., a declaration to the effect that "a Corporation is a
(legal) Person").
Moreover, depending on how the application interprets the
property descriptions, a description of an instance might be
considered valid either
without
some of the
schema-specified properties (e.g., there might be an instance
of
ex:Book
without an
ex:author
property,
even if
ex:author
is described as having a domain of
ex:Book
), or with
additional
properties (there
might be an instance of
ex:Book
with an
ex:technicalEditor
property, even though the schema
describing class
ex:Book
does not describe such
a property).
In other words, statements in an RDF schema are always
descriptions
. They may also be
prescriptive
(introduce constraints), but only if the application
interpreting those statements wants to treat them that way. All
RDF Schema does is provide a way of stating this additional
information. Whether this information conflicts with explicitly
specified instance data is up to the application to determine
and act upon.
5.4 Other Schema
Information
RDF Schema provides a number of other built-in properties, which
can be used to provide documentation and other information
about an RDF schema or about instances. For example the
rdfs:comment
property can be used to provide a
human-readable description of a resource. The
rdfs:label
property can be used to provide a more
human-readable version of a resource's name. The
rdfs:seeAlso
property can be used to indicate a
resource that might provide additional information about the
subject resource. The
rdfs:isDefinedBy
property is a
subproperty of
rdfs:seeAlso
, and can be used to
indicate a resource that (in a sense not specified by RDF;
e.g., the resource may not be an RDF schema) "defines" the
subject resource.
RDF Vocabulary
Description Language 1.0: RDF Schema
[RDF-VOCABULARY]
should be consulted for further discussion of these properties.
As with a number of the built-in RDF properties such as
rdf:value
, the uses described for these RDF Schema properties
are only their
intended
uses.
[RDF-SEMANTICS]
defines no special
meanings for these properties, and RDF Schema does
not define any constraints based on these intended uses.
For example, there is no constraint specified that
the object of a
rdfs:seeAlso
property
must
provide additional information about the subject of the
statement in which it appears.
5.5 Richer
Schema Languages
RDF Schema provides basic capabilities for describing RDF
vocabularies, but additional capabilities are also possible,
and can be useful. These capabilities may be provided through
further development of RDF Schema, or in other languages based
on RDF. Other
richer schema capabilities that have been identified as useful
(but that are not provided by RDF Schema) include:
cardinality constraints
on properties, e.g.,
that a Person has
exactly one
biological
father.
specifying that a given property (such as
ex:hasAncestor
) is
transitive
, e.g., that if A
ex:hasAncestor
B, and B
ex:hasAncestor
C, then A
ex:hasAncestor
C.
specifying that a given property is a unique identifier
(or
key
) for instances of a particular class.
specifying that two different classes (having different
URIrefs) actually represent the same class.
specifying that two different instances (having different
URIrefs) actually represent the same individual.
specifying constraints on the range or cardinality
of a property that depend on the class of resource
to which a property is applied, e.g., being able to
say that for a soccer team the
ex:hasPlayers
property has 11 values, while for a basketball team
the same property should have only 5 values.
the ability to describe new classes in terms of
combinations (e.g., unions and intersections) of other
classes, or to say that two classes are disjoint (i.e., that
no resource is an instance of both classes).
The additional capabilities mentioned above, in addition to
others, are the targets of
ontology
languages such as
DAML+OIL
[DAML+OIL]
and
OWL
[OWL]
. Both these languages are based on
RDF and RDF Schema (and both currently provide all the
additional capabilities mentioned above). The intent of such
languages is to provide additional machine-processable
semantics
for resources, that is, to make the machine
representations of resources more closely resemble their
intended real world counterparts. While such capabilities are
not necessarily needed to build useful applications using RDF
(see
Section 6
for a description of
a number of existing RDF applications), the development of such
languages is a very active subject of work as part of the
development of the
Semantic Web
6. Some RDF
Applications: RDF in the Field
The previous sections have described the general
capabilities of RDF and RDF Schema. While examples were used
in those sections to illustrate those capabilities, and some
of those examples may have suggested potential RDF applications,
those sections did not actually discuss any
real
applications.
This
section will describe some actual deployed RDF applications,
showing how RDF supports various real-world requirements to
represent and manipulate information about a wide variety of
things.
6.1 Dublin
Core Metadata Initiative
Metadata
is
data about data
. Specifically,
the term refers to data used to identify, describe, or locate
information resources, whether these resources are physical or
electronic. While structured metadata processed by computers is
relatively new, the basic concept of metadata has been used for
many years in helping manage and use large collections of
information. Library card catalogs are a familiar example of
such metadata.
The Dublin Core is a set of "elements" (properties) for
describing documents (and hence, for recording metadata). The
element set was originally developed at the March 1995 Metadata
Workshop in Dublin, Ohio. The Dublin Core has subsequently been
modified on the basis of later Dublin Core Metadata workshops,
and is currently maintained by the
Dublin Core Metadata
Initiative
. The goal of the Dublin Core is to provide a
minimal set of descriptive elements that facilitate the
description and the automated indexing of document-like
networked objects, in a manner similar to a library card
catalog. The Dublin Core metadata set is intended to be
suitable for use by resource discovery tools on the Internet,
such as the "Webcrawlers" employed by popular World Wide Web
search engines. In addition, the Dublin Core is meant to be
sufficiently simple to be understood and used by the wide range
of authors and casual publishers who contribute information to
the Internet. Dublin Core elements have become widely used in
documenting Internet resources (the Dublin
Core
creator
element has already been used in earlier examples).
The current
elements of the Dublin Core are defined in the
Dublin Core
Metadata Element Set, Version 1.1: Reference Description
[DC]
, and contain definitions for
the following properties:
Title
: A name given to the
resource.
Creator
: An entity primarily responsible
for making the content of the resource.
Subject
: The topic of the content of the
resource.
Description
: An account of the content
of the resource.
Publisher
: An entity responsible for
making the resource available
Contributor
: An entity responsible for
making contributions to the content of the resource.
Date
: A date associated with an event in
the life cycle of the resource.
Type
: The nature or genre of the content
of the resource.
Format
: The physical or digital
manifestation of the resource.
Identifier
: An unambiguous reference to
the resource within a given context.
Source
: A reference to a resource from
which the present resource is derived.
Language
: A language of the intellectual
content of the resource.
Relation
: A reference to a related
resource.
Coverage
: The extent or scope of the
content of the resource.
Rights
: Information about rights held in
and over the resource.
Information using the Dublin Core elements may be
represented in any suitable language (e.g., in HTML
meta
elements). However, RDF is an ideal representation for Dublin
Core information. The examples below represent the simple
description of a set of resources in RDF using the Dublin Core
vocabulary. Note that the specific Dublin Core RDF vocabulary
shown here is not intended to be authoritative. The Dublin Core
Reference Description
[DC]
is
the authoritative reference.
The first example,
Example 30
describes a Web site home page using Dublin Core
properties:
Example 30: A Web Page
Described using Dublin Core Properties
xmlns:dc="http://purl.org/dc/elements/1.1/">
with research interests in digital libraries and electronic
publishing.
Note that both RDF and the Dublin Core define an (XML)
element called "Description" (although the Dublin Core element
name is written in lowercase). Even if the initial letter were
identically uppercase, the XML namespace mechanism enables
these two elements to be distinguished (one is
rdf:Description
, and the other is
dc:description
). Also, as a matter of interest,
accessing
(the namespace URI used to identify the Dublin Core vocabulary in this
example)
in a Web browser (as of the current writing) will retrieve an
RDF Schema declaration for
[DC]
The second example,
Example 31
describes a published magazine:
Example 31: Describing A
Magazine Using Dublin Core
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:dcterms="http://purl.org/dc/terms/">
- May 1998
contributed stories, commentary, and briefings.
Example 31
uses (in the
third line from the bottom) the Dublin Core
qualifier
isPartOf
(from a separate
vocabulary
) to indicate that
this magazine is "part of" the previously-described Web
site.
The third example,
Example 32
describes a specific article in the magazine described in
Example 31
Example 32: Describing a
Magazine Article
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:dcterms="http://purl.org/dc/terms/">
infrastructure that enables the encoding, exchange and reuse of
structured metadata. rdf is an application of xml that imposes needed
structural constraints to provide unambiguous methods of expressing
semantics. rdf additionally provides a means for publishing both
human-readable and machine-processable vocabularies designed to
encourage the reuse and extension of metadata semantics among
disparate information communities. the structural constraints rdf
imposes to support the consistent encoding and exchange of
standardized metadata provides for the interchangeability of separate
packages of metadata defined by different resource description
communities.
access methods
Example 32
also uses the
qualifier
isPartOf
, this time to indicate that this
article is "part of" the previously-described magazine.
Computer languages and file formats do
not always make explicit provision for
embedding metadata
with the
data it describes. In many cases, the metadata has to be specified
as a separate resource and explicitly linked to the data
(this has been done for the RDF metadata that describes the Primer;
there is an explicit link to this metadata at the end of the Primer).
However, applications and languages are increasingly making explicit
provision for embedding metadata directly with the data. For example, the
W3C's Scalable Vector Graphics language
[SVG]
(another XML-based language)
provides an explicit
metadata
element for recording metadata
along with other SVG data.
Any XML-based metadata language can be used inside this element.
[SVG]
includes the example
shown in
Example 33
of
how to embed metadata describing an SVG document in the SVG document itself.
The example uses the Dublin Core vocabulary, and RDF/XML for recording
the metadata.
Example 33: Including Metadata
in an SVG Document
Adobe's
Extensible Metadata Platform (XMP)
is another example of technology that allows metadata
about a file to be embedded into the file itself.
XMP uses RDF/XML as the basis of its metadata representation.
A number of Adobe products already support XMP.
6.2 PRISM
PRISM:
Publishing Requirements for Industry Standard Metadata
[PRISM]
is a metadata specification
developed in the publishing industry. Magazine publishers and
their vendors formed the PRISM Working Group to
identify the industry's needs for metadata and define a
specification to meet them. Publishers want to use existing
content in many ways in order to get a greater return on the
investment made in creating it. Converting magazine articles to
HTML for posting on the Web is one example. Licensing it to
aggregators like
LexisNexis
is another. All of these are "first
uses" of the content; typically they all go live at the time
the magazine hits the stands. The publishers also want their
content to be "evergreen". It might be used in new issues, such
as in a retrospective article. It could be used by other
divisions in the company, such as in a book compiled from the
magazine's photos, recipes, etc. Another use is to license it
to outsiders, such as in a reprint of a product review, or in a
retrospective produced by a different publisher. This overall
goal requires a metadata approach that emphasizes
discovery
rights tracking
, and
end-to-end metadata
Discovery:
Discovery is a general term for finding
content which encompasses searching, browsing, content routing,
and other techniques. Discussions of discovery frequently
center on a consumer searching a public Web site. However,
discovering content is much broader than that. The audience may
consist of consumers, or it may consist of internal users such
as researchers, designers, photo editors, licensing agents,
etc. To assist discovery, PRISM provides properties to describe
the topics, formats, genre, origin, and contexts of a resource.
It also provides means for categorizing resources using
multiple subject description taxonomies.
Rights Tracking:
Magazines frequently contain
material licensed from others. Photos from a stock photo agency
are the most common type of licensed material, but articles,
sidebars, and all other types of content may be licensed.
Simply knowing if content was licensed for one-time use,
requires royalty payments, or is wholly-owned by the publisher
is a struggle. PRISM provides elements for basic tracking of
such rights. A separate vocabulary defined in the
PRISM specification supports description of places, times, and
industries where content may or may not be used.
End-to-end metadata:
Most published content already
has metadata created for it. Unfortunately, when content moves
between systems, the metadata is frequently discarded, only to
be re-created later in the production process at considerable
expense. PRISM aims to reduce this problem by providing a
specification that can be used in multiple stages in the
content production pipeline. An important feature of the PRISM
specification is its use of other existing specifications.
Rather than create an entirely new thing, the group decided to
use existing specifications as much as possible, and only
define new things where needed. For this reason, the PRISM
specification uses XML, RDF, Dublin Core, and well as various
ISO formats and vocabularies.
A PRISM description may be as simple as a few Dublin Core
properties with plain literal values.
Example 34
describes a photograph, giving
basic information on its title, photographer, format, etc.
Example 34: A PRISM
Description of a Photograph
xml:lang="en-US">
PRISM also augments the Dublin Core to allow more detailed
descriptions. The augmentations are defined
as three new
vocabularies
, generally cited using the prefixes
prism:
pcv:
, and
prl:
prism:
This prefix refers to the main PRISM
vocabulary, whose terms use the URI prefix
. Most
of the properties in this vocabulary are more specific versions of properties from
the Dublin Core. For example, more specific versions of
dc:date
are provided by properties like
prism:publicationTime
prism:releaseTime
prism:expirationTime
, etc.
pcv:
This prefix refers to the PRISM Controlled
Vocabulary (pcv)
vocabulary, whose terms use the URI prefix
Currently, common practice for describing the subject(s) of an
article is by supplying descriptive keywords. Unfortunately,
simple keywords do not make a great difference in retrieval
performance, due to the fact that different people will use
different keywords
[BATES96]
. Best
practice is to code the articles with subject terms from a
"controlled vocabulary". The vocabulary should provide as many
synonyms as possible for its terms in the vocabulary. This way
the controlled terms provide a meeting ground for the keywords
supplied by the searcher and the indexer. The pcv vocabulary
provides properties for specifying
terms in a vocabulary, the relations between terms, and
alternate names for the terms.
prl:
This prefix refers to the PRISM Rights
Language
vocabulary, whose terms use the URI prefix
. Digital
Rights Management is an area undergoing considerable upheaval.
There are a number of proposals for rights management
languages, but none are clearly favored throughout the
industry. Because there was no clear choice to recommend, the
PRISM Rights Language (PRL) was defined as an interim measure.
It provides properties which let people say if an item can or
cannot be "used", depending on conditions of time, geography,
and industry. This is believed to be an 80/20 trade-off which
will help publishers begin to save money when tracking rights.
It is not intended to be a general rights language, or allow
publishers to automatically enforce limits on consumer uses of
the content.
PRISM uses RDF because of its abilities for dealing with
descriptions of varying complexity. Currently, a great deal of
metadata uses simple character string (plain literal) values,
such as:
Over time the developers of PRISM expect uses of the PRISM
specification to become more sophisticated, moving from simple
literal values to more structured values. In fact, that range
of values is a situation being faced now. Some publishers
already use sophisticated controlled vocabularies, others are
barely using manually-supplied keywords. To illustrate this,
some examples of the different kinds of values that can be
given for the
dc:coverage
property are:
(i.e., using either a plain literal or a URIref to identify
the country) and
(using a structured value to provide both a URIref and names
in various languages).
Note also that there are properties whose meanings are
similar, or subsets of other properties. For example, the
geographic subject of a resource could be given with
or
Any of those properties might use the simple literal value,
or a more complex structured value. Such a range of
possibilities cannot be adequately described by DTDs, or even
by the newer XML Schemas. While there is a wide range of
syntactic variations to deal with, RDF's graph model has a
simple structure - a set of triples. Dealing with the metadata
in the triples domain makes it much easier for older software
to accommodate content with new extensions.
This section closes with two final examples.
Example 35
says that the image
.../Corfu.jpg
) cannot be used (
#none
) in the
tobacco industry (code 21 in SIC, the Standard Industrial
Classifications).
Example 35: A PRISM
Description of an Image
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc="http://purl.org/dc/elements/1.1/">
Example 36
says that the
photographer for the Corfu image was employee 3845, better
known as John Peterson. It also says that the geographic
coverage of the photo is Greece. It does so by providing, not
just a code from a controlled vocabulary, but a cached version
of the information for that term in the vocabulary.
Example 36: Additional
Information about the Image from Example 35
xmlns:dc="http://purl.org/dc/elements/1.1/"
xml:base="http://travel.example.com/">
6.3 XPackage
Many situations involve the need to maintain information
about structured groupings of resources and their associations
that are, or may be, used as a unit. The
XML Package
(XPackage) specification
[XPACKAGE]
provides a framework for
defining such groupings, called
packages
. XPackage
specifies a framework for describing the resources included in
such packages, the properties of those resources, their method
of inclusion, and their relationships with each other. XPackage
applications include specifying the style sheets used by a
document, declaring the images shared by multiple documents,
indicating the author and other metadata of a document,
describing how namespaces are used by XML resources, and
providing a manifest for bundling resources into a single
archive file.
The XPackage framework is based upon XML, RDF, and the
XML Linking Language
[XLINK]
, and provides multiple RDF
vocabularies: one for general packaging descriptions, and
several other vocabularies for providing supplemental
resource information useful to package processors.
One application of XPackage is the description of XHTML
documents and their supporting resources. An XHTML document
retrieved from a Web site may rely on other resources such as
style sheets and image files that also need to be retrieved.
However, the identities of these supporting resources may not
be obvious without processing the entire document. Other
information about the document, such as the name of its author,
may also not be available without processing the document.
XPackage allows such descriptive information to be stored in a
standard way in a package description document containing RDF.
The outer elements of a package description document describing
such an XHTML document might look like
Example 37
(with namespace declarations
removed for simplicity):
Example 37: Outer
Elements of an XPackage Package Description Document
(description of individual resources go here)
Resources (such as the XHTML document, style sheets, and
images) are described within this package description document
using standard RDF/XML syntax.
Each resource description element may include
RDF properties from various vocabularies (XPackage uses the
term "ontology" for what RDF calls a "vocabulary").
Besides the main packaging vocabulary, XPackage itself
specifies several supplemental vocabularies, including:
a vocabulary (using prefix
file:
) for describing files
(with properties such as
file:size
a vocabulary (using prefix
mime:
) for providing MIME information
(with properties such as
mime:contentType
a vocabulary (using prefix
unicode:
) for providing character usage information
(with properties such as
unicode:script
a vocabulary (using prefix
x:
) for describing XML-based resources
(with properties such as
x:namespace
and
x:style
In
Example 38
, the document's MIME content
type ("application/xhtml+xml") is defined using a standard
XPackage property from the XPackage MIME vocabulary,
mime:contentType
. Another property, the document's
author (in this case, "Garret Wilson"), is described using a
property from the Dublin Core vocabulary, defined outside
of XPackage, resulting in a
dc:creator
property.
Example 38: A
Description of an XHTML Document
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:mime="http://xpackage.org/namespaces/2003/mime#"
xmlns:x="http://xpackage.org/namespaces/2003/xml#"
xmlns:xlink="http://www.w3.org/1999/xlink">
The
xpackage:manifest
property indicates that both
the style sheet and image resources are necessary for
processing; those resources are described separately within the
package description document. The example style sheet resource
description in
Example 39
lists its
location within the package ("stylesheet.css") using the
general XPackage vocabulary
xpackage:location
property (which is compatible with
XLink), and shows through use of the XPackage MIME vocabulary
mime:contentType
property that it is a CSS
style sheet ("text/css").
Example 39: A Style Sheet
Resource Description
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:mime="http://xpackage.org/namespaces/2003/mime#"
xmlns:x="http://xpackage.org/namespaces/2003/xml#"
xmlns:xlink="http://www.w3.org/1999/xlink">
The full version of this example may be found in
[XPACKAGE]
6.4 RSS 1.0: RDF Site
Summary
People sometimes need to access a wide variety of information on
the Web on a day-to-day basis, such as schedules, to-do lists, news
headlines, search results, "What's New", etc. As the sources
and diversity of the information on the Web increases, it becomes
increasingly difficult to manage this information and integrate
it into a coherent whole.
RSS
1.0
("RDF Site Summary") is an RDF vocabulary that provides
a lightweight, yet powerful way of describing information for
timely, large-scale distribution and reuse. RSS 1.0 is also perhaps
the most widely deployed RDF application on the Web.
To give a simple example, the
W3C home page
is a primary point of contact
with the public and serves in part to disseminate information
about the deliverables of the Consortium.
An example of the W3C home page as of a certain date is shown
in
Figure 19
. The center column of
news items changes frequently. To support the timely
dissemination of this information, the W3C Team has implemented
an RDF Site Summary (
RSS
1.0
) news feed that makes the content in the center column
available to others to reuse as they will. News syndication
sites may merge the headlines into a summary of the day's
latest news, others may display the headlines as links as a
service to their readers, and, increasingly, individuals may
subscribe to this feed with a desktop application. These
desktop
RSS readers
allow their users to keep track of
potentially hundreds of sites, without having to visit each one
in their browser.
Figure 19: The W3C Home
Page
Numerous sites all over the Web provide RSS 1.0 feeds.
Example 40
is an example of the
W3C
feed
(from a different date):
Example 40: An Example
of the W3C RSS 1.0 Feed
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
http://www.w3.org/
advancement of User Agent Accessibility Guidelines 1.0 to
Proposed Recommendation. Comments are welcome through 14 November.
Written for developers of user agents, the guidelines lower
barriers to Web accessibility for people with disabilities
(visual, hearing, physical, cognitive, and neurological).
The companion Techniques Working Draft is updated. Read about
the Web Accessibility Initiative. (News archive)
http://www.w3.org/News/2002#item164
Working Group has released the first public Working Draft of
Authoring Challenges for Device Independence. The draft describes
the considerations that Web authors face in supporting access to
their sites from a variety of different devices. It is written
for authors, language developers, device experts and developers
of Web applications and authoring systems. Read about the Device
Independence Activity (News archive)
http://www.w3.org/News/2002#item168
released two Last Call Working Drafts and welcomes comments
on them through 27 November. CSS3 module: text is a set of
text formatting properties and addresses international contexts.
CSS3 module: Ruby is properties for ruby, a short run of text
alongside base text typically used in East Asia. CSS3 module:
The box model for the layout of textual documents in visual
media is also updated. Cascading Style Sheets (CSS) is a
language used to render structured documents like HTML and
XML on screen, on paper, and in speech. Visit the CSS home
page. (News archive)
http://www.w3.org/News/2002#item167
As
Example 40
shows, the format is
designed for content that can be packaged into easily
distinguishable sections. News sites, Web logs, sports scores,
stock quotes, and the like are all use-cases for RSS 1.0.
The RSS feed can be requested by any application able to
"speak" HTTP. More recently, however, RSS 1.0 applications are
splitting into three different categories:
On-line aggregators - Sites such as
Meerkat
and
NewsIsFree
shown side-by-side in
Figure 20
(each
mirroring W3C's column of news). These gather feeds from
thousands of sources, separate each of the
s out, and add them together again into
one large group. The whole group is then made searchable. In
this way, one can search for the latest news on, for example,
"Java" from perhaps thousands of sites, without having to
search them all.
Desktop Readers - Utilities such as
Amphetadesk
and
NetNewsWire
Lite
allow their users to subscribe to hundreds of feeds
from their desktop. Readers customarily refresh each feed
once an hour, allowing users to stay up to date.
Scripts - RSS's original purpose was to allow Webmasters
to include the content of another's site within their own.
RSS 1.0 is still used in this way, with many sites (
Slashdot
for example)
incorporating RSS feeds on their front page.
Figure 20: MeerKat and
NewsIsFree
RSS 1.0 is extensible by design. By importing additional RDF
vocabularies (or
modules
as they are known within the
RSS development community), the RSS 1.0 author can provide
large amounts of metadata and handling instructions to the
recipient of the file. Modules can, as with more general RDF
vocabularies, be written by anyone. Currently there are
3 official modules
and
19
proposed modules
readily recognized by the community at
large. These modules range from the complete
Dublin Core
module
to more specialized RSS-centric modules such as the
Aggregation
module
Care should be taken when discussing "RSS" in the scope of
RDF. There are currently two RSS specification strands. One
strand (RSS 0.91,0.92,0.93,0.94 and 2.0) does not use RDF. The
other strand (RSS 0.9 and 1.0) does.
6.5 CIM/XML
Electric utilities use power system models for a number of
different purposes. For example, simulations of power systems
are necessary for planning and security analysis. Power system
models are also used in actual operations, e.g., by the Energy
Management Systems (EMS) used in energy control centers. An
operational power system model can consist of thousands of
classes of information. In addition to using these models
in-house, utilities need to exchange system modeling
information, both in planning, and for operational purposes,
e.g., for coordinating transmission and ensuring reliable
operations. However, individual utilities use different
software for these purposes, and as a result the system models
are stored in different formats, making the exchange of these
models difficult.
In order to support the exchange of power system models,
utilities needed to agree on common definitions of power system
entities and relationships. To support this, the
Electric Power Research
Institute
(EPRI) a non-profit energy research consortium,
developed a Common
Information Model (CIM)
[CIM]
The CIM specifies common semantics
for power system resources, their attributes, and
relationships. In addition, to further support the ability to
electronically exchange CIM models, the power industry has
developed
CIM/XML
, a
language for expressing CIM models in XML. CIM/XML is an RDF
application, using RDF and RDF Schema to organize its XML
structures. The
North American
Electric Reliability Council
(NERC) (an industry-supported
organization formed to promote the reliability of electricity
delivery in North America) has adopted CIM/XML as the standard
for exchanging models between power transmission system
operators. The CIM/XML format is also going through an IEC
international standardization process. An excellent discussion
of CIM/XML can be found in
[DWZ01]
[NB: This power industry CIM should not be confused with the
CIM developed by the
Distributed
Management Task Force
for
representing management information for distributed software,
network, and enterprise environments. The DMTF CIM also has an
XML representation, but does not currently use RDF, although
independent research is underway in that direction.]
The CIM can represent all of the major objects of an
electric utility as object classes and attributes, as well as
their relationships. CIM uses these object classes and
attributes to support the integration of independently
developed applications between vendor specific EMS systems, or
between an EMS system and other systems that are concerned with
different aspects of power system operations, such as
generation or distribution management.
The CIM is specified as a set of class diagrams using the
Unified Modeling Language
(UML). The base class of the CIM is
the
PowerSystemResource
class, with other more
specialized classes such as
Substation
Switch
, and
Breaker
being defined as
subclasses. CIM/XML represents the CIM as an RDF Schema
vocabulary, and uses RDF/XML as the language for exchanging
specific system models.
Example 41
shows examples of CIM/XML class and property definitions:
Example 41: Examples of
CIM/XML Class and Property Definitions
individual element such as a switch or a set of elements
such as a substation. PowerSystemResources that are sets
could be members of other sets. For example a Switch is a
member of a Substation and a Substation could be a member
of a division of a Company"
carrying, and breaking currents under normal circuit conditions
and also making, carrying for a specified time, and breaking
currents under specified abnormal circuit conditions e.g. those
of short circuit. The typeName is the type of breaker, e.g.,
oil, air blast, vacuum, SF6."
CIM/XML uses only a subset of the complete RDF/XML syntax,
in order to simplify expressing the models. In addition,
CIM/XML implements some extensions to the RDF Schema vocabulary.
These extensions support the description of inverse
roles and multiplicity (cardinality) constraints describing how
many instances of a given property are allowed for a given
resource (allowable values for a
multiplicity declaration are zero-or-one, exactly-one,
zero-or-more, one-or-more). The properties in
Example 42
illustrate these
extensions (which are identified by a
cims:
QName prefix):
Example 42: Some CIM/XML
Extensions of RDF Schema
protection relays."
protection relays."
EPRI has conducted successful interoperability tests using
CIM/XML to exchange real-life, large-scale models (involving,
in the case of one test, data describing over 2000 substations)
between a variety of vendor products, and validating that these
models would be correctly interpreted by typical utility
applications. Although the CIM was originally intended for EMS
systems, it is also being extended to support power
distribution and other applications as well.
The
Object Management
Group
has adopted an object interface standard to access
CIM power system models called the Data Access Facility
[DAF]
. Like the CIM/XML language, the DAF
is based on the RDF model and shares the same CIM schema.
However, while CIM/XML enables a model to be exchanged as a
document, DAF enables an application to access the model as a
set of objects.
CIM/XML illustrates the useful role RDF can play in
supporting XML-based exchange of information that is naturally
expressed as entity-relationship or object-oriented classes,
attributes, and relationships (even when that information will
not necessarily be Web-accessible). In these cases, RDF
provides a basic structure for the XML in support of
identifying objects, and using them in structured
relationships. This connection is illustrated by a number of
applications using RDF/XML for information interchange, as well
as a number of projects investigating linkages between RDF (or
ontology languages such as OWL) and UML (and its XML
representations).
CIM/XML's need to extend RDF Schema to support cardinality
constraints and inverse relationships also illustrates the kinds
of requirements
that have led to the development of more powerful
RDF-based schema/ontology languages such as DAML+OIL and OWL
described in
Section 5.5
. Such
languages may be appropriate in supporting many similar
modeling applications in the future.
Finally, CIM/XML also illustrates an important fact for
those looking for additional examples of "RDF in the Field":
sometimes languages are described as "XML" languages, or
systems are described as using "XML", and the "XML" they are
actually using is RDF/XML, i.e., they are RDF applications.
Sometimes it is necessary to go fairly far into the description
of the language or system in order to find this out (in some
examples that have been found, RDF is never explicitly
mentioned at all, but sample data clearly shows it is RDF/XML).
Moreover, in applications such as CIM/XML, the RDF that is
created will not be readily found on the Web, since it is
intended for information exchange between software components
rather than for general access (although future scenarios could
be imagined in which more of this type of RDF would become
Web-accessible).
6.6 Gene Ontology
Consortium
Structured metadata using controlled vocabularies such as
SNOMED RT
(Systematized
Nomenclature of Medicine Reference Terminology) and
MeSH
(Medical Subject Headings) plays an important role in medicine,
enabling more efficient literature searches and aiding in the
distribution and exchange of medical knowledge
[COWAN]
. At the same time, the field of
medicine is rapidly changing, and with that comes the need to
develop additional vocabularies.
The objective of the
Gene Ontology (GO)
Consortium
[GO]
is to provide controlled vocabularies to
describe specific aspects of gene products. Collaborating
databases annotate their gene products (or genes) with GO
terms, providing references and indicating what kind of
evidence is available to support the annotations. The use of
common GO terms by these databases facilitates uniform queries
across them. The GO ontologies are structured to allow both
attribution and querying to be performed at different levels of
granularity. The GO vocabularies are dynamic, since knowledge
of gene and protein roles in cells is accumulating and
changing.
The three organizing principles of the GO are
molecular
function
biological process
, and
cellular component
A gene
product has one or more molecular functions and is used in one
or more biological processes; it may be, or may be associated
with, one or more cellular components. Definitions of the terms
within all three of these ontologies are contained in a single
(text) definition file. XML formatted
versions, containing all three ontology files and all available
definitions, are generated monthly.
Function, process and component are represented as directed
acyclic graphs (DAGs) or networks. A child term may be an
"instance" of its parent term (isa relationship) or a component
of its parent term (part-of relationship). A child term may
have more than one parent term and may have a different class
of relationship with its different parents. Synonyms and
cross-references to external databases are also represented in
the ontologies.
GO uses RDF/XML facilities to represent the relationships
between terms in the XML versions of
the ontologies, because of its flexibility in representing these
graph structures, as well as its widespread tool support.
At the same time, GO currently uses
non
-RDF nested XML structures
within the term descriptions, so the language used is not
pure RDF/XML.
Example 43
shows some sample GO
information from the
GO
documentation
Example 43: Sample GO
Information
Example 43
illustrates that
go:term
is the basic element. In some cases, the GO has
defined its own terms rather than using RDF Schema. For
example, term
GO:0016209
has the element
/>
. This tag represents the relationship
GO:0016209
isa
GO:0003674
", or, in English,
"Antioxidant is a molecular function." Another specialized
relationship is
go:part-of
. For example,
GO:0003674
has the element
/>
. This says that "Molecular function is part of the
Gene Ontology".
Every annotation must be attributed to a source, which may
be a literature reference, another database or a computational
analysis. The annotation must indicate what kind of evidence is
found in the cited source to support the association between
the gene product and the GO term. A simple controlled
vocabulary is used to record evidence. Examples include:
ISS means "inferred from sequence similarity [with
IDA means "inferred from direct assay"
TAS means "traceable author statement"
The
go:dbxref
element represents the term in an
external database, and
go:association
represents the
gene associations of each term.
go:association
can
have both
go:evidence
, which holds a
go:dbxref
to the evidence supporting the association,
and a
go:gene_product
, which contains the gene symbol
and
go:dbxref
These elements illustrate
that the GO XML syntax is not "pure" RDF/XML, since the
nesting of other elements within these elements does not
conform to the alternate node/predicate arc "stripes" described in
Sections 2.1 and 2.2 of
[RDF-SYNTAX]
The GO illustrates a number of interesting points. First, it
shows that the value of using XML for information exchange can
be enhanced by structuring that XML using RDF. This is
particularly true for data that has an overall graph or network
structure, rather than being a strict hierarchy. The GO is also
another example in which data using RDF will not necessarily appear
for direct use on the Web (although the files are
Web-accessible). It is also another example of data which is,
on the surface, described as "XML", but on closer examination
uses RDF/XML facilities (albeit not "pure" RDF/XML).
Finally, the GO illustrates the role RDF can
play as a basis for representing ontologies. This role will be
further enhanced once richer RDF-based languages for specifying
ontologies, such as the DAML+OIL or OWL languages discussed in
Section 5.5
, become more widely
used.
In fact, a
Gene Ontology
Next Generation
project is
currently developing a representation of the GO ontologies in
these richer languages.
6.7 Describing Device
Capabilities and User Preferences
In recent years a large number of new mobile devices for
browsing the Web have appeared. Many of these devices have
highly divergent capabilities including a wide range of input
and output capabilities as well as different levels of language
support. Mobile devices may also have widely differing network
connectivity capabilities. Users of these new devices expect a
usable presentation regardless of the device's capabilities or
the current network characteristics. Likewise, users want their
dynamically changing preferences (e.g. turn audio on/off) to be
considered when content or an application is presented. The
reality, however, is that device heterogeneity, and the lack of
a standard way for users to convey their preferences to the
server, may result in: content that cannot be stored on the
device, content that cannot be displayed, or content that
violates the desires of the user. Additionally, the resulting
content may take too long to convey over the network to the
client device.
A solution for addressing these problems is for a client to
encode its
delivery context
- the device's
capabilities, the user's preferences, the network
characteristics, etc. - in such a way that a server can use the
context to customize content for the device and user (see
[DIPRINC]
for a definition of delivery
context). The W3C's Composite Capabilities/Preferences Profile
(CC/PP) specification
[CC/PP]
helps to
address this problem by defining a generic framework for
describing a delivery context.
The CC/PP framework defines a relatively simple structure -
a two-level hierarchy of components and attribute/value pairs.
component
may be used to capture a part of a
delivery context (e.g. network characteristics, software
supported by a device, or the hardware characteristics of a
device). A component may contain one or more
attributes
. For example a component that encodes user
preferences may contain an attribute to specify whether or not
AudioOutput
is desired.
CC/PP defines its structure (the hierarchy described above)
using RDF Schema (see
[CC/PP]
for
details of the structure schema). A CC/PP
vocabulary
defines specific components and their attributes.
[CC/PP]
, however, does not define such
vocabularies. Instead, vocabularies are defined by other
organizations or applications (as described below).
[CC/PP]
also does not define a protocol
for transporting an instance of a CC/PP vocabulary.
An instance of a CC/PP vocabulary is called a
profile
. CC/PP attributes are encoded as RDF
properties in a profile.
Example 44
shows a profile fragment of user preferences for a user that
prefers an audio presentation:
Example 44: A CC/PP
Profile Fragment
There are several advantages to using RDF in this
application. First, a profile encoded via CC/PP may include
attributes that were defined in schemas created by different
organizations. RDF is a natural fit for these profiles because
no single organization is likely to create a
super
schema for the aggregated profile data. A second advantage of
RDF is that it facilitates (by virtue of its graph-based data
model) the insertion of arbitrary attributes (RDF properties)
into a profile. This is particularly useful for profiles that
include frequently changing data such as location
information.
The Open Mobile Alliance has defined the User Agent Profile
(UAProf)
[UAPROF]
- a CC/PP-based
framework that includes a vocabulary for describing device
capabilities, user agent capabilities, network characteristics,
etc., as well as a protocol for transporting a profile. UAProf
defines six components including:
HardwarePlatform
SoftwarePlatform
NetworkCharacteristics
and
BrowserUA
. It also defines several attributes for each
of its components although a component's attributes are not
fixed - they may be supplemented or overridden.
Example 45
shows a fragment of UAProf's
HardwarePlatform
component:
Example 45: A Fragment
of UAProf's HardwarePlatform Component
The UAProf protocol supports both
static
profiles
and
dynamic
profiles. A
static
profile is
accessed via a URI. This has several advantages: a client's
request to a server only contains a URI rather a potentially
verbose XML document (thus minimizing over the air traffic);
the client does not have to store and/or create the profile;
the implementation burden on a client is relatively
light-weight.
Dynamic
profiles are created on-the-fly
and consequently do not have an associated URI. They may
consist of a profile fragment containing a
difference
from a static profile, but they may also contain unique data
that is not included in the client's static profile. A request
may contain any number of static profiles and dynamic profiles.
However, the ordering of the profiles is important as later
profiles override earlier profiles in the request. See
[UAPROF]
for more information about
UAProf's protocol and its rules for resolving multiple
profiles.
Several other communities (i.e. 3GPP's TS 26.234
[3GPP]
and the WAP Forum's Multimedia
Messaging Service Client Transactions Specification
[MMS-CTR]
) have defined vocabularies based
on CC/PP. As a result, a profile may take advantage of the
distributed nature of RDF and include components defined from
various vocabularies.
Example 46
shows such a profile:
Example 46: A Profile
Using Several Vocabularies
xmlns:mms="http://www.wapforum.org/profiles/MMS/ccppschema-20010111#"
xmlns:pss="http://www.3gpp.org/profiles/PSS/ccppschema-YYYYMMDD#">
The definition of a delivery context and the data within a
context will continually evolve. Consequently, RDF's inherent
extensibility, and thus support for dynamically changing
vocabularies, make RDF a good framework for encoding a delivery
context.
7. Other Parts of the
RDF Specification
Section 1
indicated that the RDF
Specification consists of a number of documents (in addition to
this Primer):
RDF Concepts
and Abstract Syntax
[RDF-CONCEPTS]
RDF/XML
Syntax Specification
[RDF-SYNTAX]
RDF Vocabulary
Description Language 1.0: RDF Schema
[RDF-VOCABULARY]
RDF Semantics
[RDF-SEMANTICS]
RDF Test
Cases
[RDF-TESTS]
The Primer has already discussed the subjects of several of
these documents, basic RDF concepts (in
Section 2
), the RDF/XML syntax (in
Section 3
) and RDF Schema (in
Section 5
). This section briefly
describes the remaining documents (even though there have already
been numerous references to
[RDF-SEMANTICS]
as well), in order to explain their role
in the complete specification of RDF.
7.1 RDF
Semantics
As discussed in the preceding sections, RDF is intended to
be used to express statements about resources in the form of a
graph, using specific vocabularies (names of resources,
properties, classes, etc.). RDF is also intended to be the
foundation for more advanced languages, such as those discussed
in
Section 5.5
. In order to serve
these purposes, the "meaning" of an RDF graph must be defined
in a very precise manner.
Exactly what constitutes the "meaning" of an RDF graph in a
very general sense may depend on many factors, including
conventions within a user community to interpret user-defined
RDF classes and properties in specific ways,
comments in natural language, or links to other
content-bearing documents. As noted briefly in
Section 2.2
, much of
the meaning conveyed in these forms will not be directly
accessible to machine processing, although this meaning may be
used by human interpreters of the RDF information, or by
programmers writing software to perform various kinds of
processing on that RDF information. However, RDF statements
also have a
formal
meaning which determines, with
mathematical precision, the conclusions (or
entailments
) that machines can draw from a given RDF graph.
The
RDF Semantics
[RDF-SEMANTICS]
document defines this
formal meaning, using a technique called
model theory
for specifying the semantics of a formal language.
[RDF-SEMANTICS]
also defines the
semantic extensions to the RDF language represented by RDF Schema, and by
individual datatypes.
In other
words, the RDF model theory provides the formal underpinnings
for all RDF concepts. Based on the
semantics defined in the model theory, it is simple to
translate an RDF graph into a logical expression with
essentially the same meaning.
7.2 Test Cases
The
RDF Test
Cases
[RDF-TESTS]
supplement
the textual RDF specifications with test cases (examples)
corresponding to particular technical issues addressed by the
RDF Core Working Group. To help describe these examples, the
Test Cases document introduces a notation called
N-Triples
, which provides the basis for the triples
notation used throughout this Primer. The test cases are
published in machine-readable form at Web locations referenced
by the Test Cases document, so developers can use these as the
basis for automated testing of RDF software.
The test cases are divided into a number of categories:
Positive and Negative Parser Tests: These test whether
RDF/XML parsers produce a correct N-Triples output graph from
legal RDF/XML input documents, or correctly report errors if
the input documents are not legal RDF/XML.
Positive and Negative Entailment Tests: These test
whether proper entailments (conclusions) are or are not drawn
from sets of specified RDF statements.
Datatype-aware Entailment Tests: These are positive or
negative entailment tests that involve the use of datatypes,
and hence require additional support for the specific
datatypes involved in the tests.
Miscellaneous Tests: These are tests that do not fall
into one of the other categories.
The test cases are not a complete specification of RDF, and
are not intended to take precedence over the other
specification documents. However, they are intended to
illustrate the intent of the RDF Core Working Group with
respect to the design of RDF, and developers may find these
test cases helpful should the wording of the specifications be
unclear on any point of detail.
8. References
8.1 Normative References
[RDF-CONCEPTS]
Resource
Description Framework (RDF): Concepts and Abstract
Syntax
, Klyne G., Carroll J. (Editors), W3C Recommendation, 10 February 2004.
This version
is http://www.w3.org/TR/2004/REC-rdf-primer-20040210/. The
latest version
is http://www.w3.org/TR/rdf-concepts/.
[RDF-MIME-TYPE]
MIME Media Types
, The Internet Assigned Numbers Authority (IANA). This document is http://www.iana.org/assignments/media-types/ . The
registration for
application/rdf+xml
is archived at http://www.w3.org/2001/sw/RDFCore/mediatype-registration .
[RDF-MS]
Resource
Description Framework (RDF) Model and Syntax
Specification
, Lassila O., Swick R. (Editors),
World Wide Web Consortium, 22 February 1999.
This version
is http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/. The
latest
version
is http://www.w3.org/TR/REC-rdf-syntax/.
[RDF-SEMANTICS]
RDF
Semantics
, Hayes P. (Editor), W3C Recommendation,
10 February 2004.
This
version
is
latest version
is http://www.w3.org/TR/rdf-mt/.
[RDF-SYNTAX]
RDF/XML Syntax Specification (Revised)
, Beckett
D. (Editor), W3C Recommendation, 10 February 2004.
This version
latest
version
is
[RDF-TESTS]
RDF
Test Cases
, Grant J., Beckett D. (Editors), W3C
Recommendation, 10 February 2004.
This version
is http://www.w3.org/TR/2004/REC-rdf-testcases-20040210/. The
latest
version
is http://www.w3.org/TR/rdf-testcases/.
[RDF-VOCABULARY]
RDF
Vocabulary Description Language 1.0: RDF Schema
Brickley D., Guha R.V. (Editors), W3C Recommendation, 10 February 2004.
This version
is http://www.w3.org/TR/2004/REC-rdf-schema-20040210/. The
latest version
is http://www.w3.org/TR/rdf-schema/.
[UNICODE]
The Unicode Standard, Version 3
, The Unicode
Consortium, Addison-Wesley, 2000. ISBN 0-201-61633-5, as updated
from time to time by the publication of new versions. (See
for the latest version and additional information on versions of
the standard and of the Unicode Character Database).
[URIS]
RFC 2396 -
Uniform Resource Identifiers (URI): Generic
Syntax
, Berners-Lee T., Fielding R., Masinter L.,
IETF, August 1998, http://www.isi.edu/in-notes/rfc2396.txt.
[XML]
Extensible
Markup Language (XML) 1.0, Second Edition
, Bray
T., Paoli J., Sperberg-McQueen C.M., Maler E. (Editors),
World Wide Web Consortium, 6 October 2000.
This
version
is http://www.w3.org/TR/2000/REC-xml-20001006.
The
latest version
is http://www.w3.org/TR/REC-xml.
[XML-BASE]
XML
Base
, Marsh J. (Editor), World Wide Web Consortium, 27 June 2001.
This
version
is
latest version
is
[XML-NS]
Namespaces
in XML
, Bray T., Hollander D., Layman A.
(Editors), World Wide Web Consortium, 14 January 1999.
This
version
is
latest version
is http://www.w3.org/TR/REC-xml-names/.
[XML-XC14N]
Exclusive
XML Canonicalization Version 1.0
, Boyer J., Eastlake D.E. 3rd,
Reagle J. (Authors/Editors), World Wide Web Consortium, 18 July 2002.
This
version
is http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/.
The
latest version
is
8.2 Informational
References
[3GPP]
3GPP
TS 26.234.
3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
Transparent end-to-end packet switched streaming service;
Protocols and codecs V5.2.0 (2002-09).
This document
is available at http://www.3gpp.org/specs/specs.htm via
directory
ftp://ftp.3gpp.org/specs/2002-09/Rel-5/26_series/.
[ADDRESS-SCHEMES]
Addressing
Schemes
, Connolly D., 2001.
This
document
is
[BATES96]
Indexing
and Access for Digital Libraries and the Internet: Human,
Database, and Domain Factors
, Bates M.J., 1996.
This
document
is
[BERNERS-LEE98]
What the
Semantic Web can represent
, Berners-Lee T., 1998.
This
document
is
[CC/PP]
Composite Capability/Preference Profiles (CC/PP): Structure
and Vocabularies
, Klyne G., Reynolds F., Woodrow
C., Ohto H., Hjelm J., Butler M., Tran, L., W3C Recommendation, 15 January 2004.
This version
is http://www.w3.org/TR/2004/REC-CCPP-struct-vocab-20040115/. The
latest
version
is http://www.w3.org/TR/CCPP-struct-vocab/.
[CG]
Conceptual Graphs
, Sowa J., ISO working
document ISO/JTC1/SC32/WG2 N 000, 2 April 2001 (work in
progress). Available at
[CHARMOD]
Character
Model for the World Wide Web 1.0
, Dürst M., Yergeau F.,
Ishida R., Wolf M., Freytag A., Texin T. (Editors), World Wide Web Consortium,
20 February 2002 (work in progress).
This version
is http://www.w3.org/TR/2002/WD-charmod-20020220/. The
latest version
is
[CIM]
Common Information Model (CIM): CIM 10 Version
EPRI, Palo Alto, CA: 2001, 1001976.
This document
is available at
[COWAN]
Metadata, Reuters Health Information, and Cross-Media
Publishing
, Cowan J., 2002. Presentation at
Seybold New York 2002 Enterprise Publishing Conference.
This document
is
An accompanying
transcript
is
[DAF]
Utility
Management System (UMS) Data Access Facility
, version 2.0,
Object Management Group, November 2002.
This
document
is available at
[DAML+OIL]
DAML+OIL
(March 2001) Reference Description
, Connolly D.,
van Harmelen F., Horrocks I., McGuinness D.L.,
Patel-Schneider P.F., Stein L.A., World Wide Web Consortium,
18 December 2001.
This
document
is http://www.w3.org/TR/daml+oil-reference.
[DC]
Dublin Core
Metadata Element Set, Version 1.1: Reference
Description
, 02 June 2003.
This
version
is http://dublincore.org/documents/2003/06/02/dces/.
The
latest
version
is http://dublincore.org/documents/dces/.
[DIPRINC]
Device
Independence Principles.
Gimson, R., Finkelstein,
S., Maes, S., Suryanarayana, L., World Wide Web Consortium,
18 September 2001 (work in progress).
This
version
is
latest version
is
[DWZ01]
XML for
CIM Model Exchange
, deVos A., Widergreen S.E.,
Zhu J., Proc. IEEE Conference on Power Industry Computer
Systems, Sydney, Australia, 2001.
This document
is
[GO]
Gene
Ontology: tool for the unification of biology
The Gene Ontology Consortium,
Nature Genetics
, Vol. 25: 25-29, May 2000.
Available at
[GRAY]
Logic, Algebra and Databases
, Gray P., Ellis
Horwood Ltd., 1984. ISBN 0-85312-709-3, 0-85312-803-0,
0-470-20103-7, 0-470-20259-9.
[HAYES]
In Defense of Logic
, Hayes P., Proceedings
from the International Joint Conference on Artificial
Intelligence, 1975, San Francisco. Morgan Kaufmann Inc.,
1977. Also in
Computation and Intelligence: Collected
Readings
, Luger G. (ed), AAAI press/MIT press, 1995.
ISBN 0-262-62101-0.
[KIF]
Knowledge Interchange Format
, Genesereth M.,
draft proposed American National Standard NCITS.T2/98-004.
Available at
[LBASE]
LBase:
Semantics for Languages of the Semantic Web
, Guha R. V., Hayes P.,
W3C Note, 10 October 2003.
This
version
is http://www.w3.org/TR/2003/NOTE-lbase-20031010/.
The
latest version
is http://www.w3.org/TR/lbase/.
[LUGER]
Artificial Intelligence: Structures and Strategies
for Complex Problem Solving
(3rd ed.), Luger G.,
Stubblefield W., Addison Wesley Longman, 1998. ISBN
0-805-31196-3.
[MATHML]
Mathematical
Markup Language (MathML) Version 2.0
, Carlisle D., Ion P.,
Miner R., Poppelier N. (Editors); Ausbrooks R., Buswell S., Dalmas S.,
Devitt S., Diaz A., Hunter R., Smith B., Soiffer N., Sutor R.,
Watt S. (Principal Authors), World Wide Web Consortium, 21 February 2001.
This
version
is http://www.w3.org/TR/2001/REC-MathML2-20010221/.
The
latest version
is
[MMS-CTR]
Multimedia
Messaging Service Client Transactions
Specification.
WAP-206-MMSCTR-20020115-a.
This document is available at
[NAMEADDRESS]
Naming and
Addressing: URIs, URLs, ...
, Connolly D., 2002.
This document
is
[OWL]
OWL Web
Ontology Language Reference
, Dean M.,
Schreiber G (Editors); van Harmelen F., Hendler J., Horrocks I.,
McGuinness D.L., Patel-Schneider P.F., Stein L.A. (Authors),
W3C Recommendation, 10 February 2004. The
latest version
is
[PRISM]
PRISM:
Publishing Requirements for Industry Standard
Metadata
, Version 1.1, 19 February 2002. The
latest version
of the PRISM specification is available at
[RDFISSUE]
RDF Issue
Tracking
, McBride B., 2002.
This
document
is http://www.w3.org/2000/03/rdf-tracking/.
[RDF-S]
Resource Description Framework (RDF) Schema Specification 1.0
, Brickley D., Guha, R.V. (Editors), World Wide Web Consortium.
27 March 2000.
This version
is http://www.w3.org/TR/2000/CR-rdf-schema-20000327/.
[RSS]
RDF Site
Summary (RSS) 1.0
, Beged-Dov G., Brickley D.,
Dornfest R., Davis I., Dodds L., Eisenzopf J., Galbraith D.,
Guha R.V., MacLeod K., Miller E., Swartz A., van der Vlist
E., 2000.
This
document
is http://purl.org/rss/1.0/spec.
[RUBY]
Ruby
Annotation
, Sawicki M., Suignard M., Ishikawa M., Dürst M.,
Texin T. (Editors), World Wide Web Consortium, 31 May 2001.
This version
is http://www.w3.org/TR/2001/REC-ruby-20010531/. The
latest version
is
[SOWA]
Knowledge Representation: Logical, Philosophical
and Computational Foundations
, Sowa J., Brookes/Cole,
2000. ISBN 0-534-94965-7.
[SVG]
Scalable
Vector Graphics (SVG) 1.1 Specification
, Ferraiolo J.,
Fujisawa J., Jackson D. (Editors), World Wide Web Consortium, 14
January 2003.
This
version
is http://www.w3.org/TR/2003/REC-SVG11-20030114/.
The
latest version
is http://www.w3.org/TR/SVG11/.
[UAPROF]
User
Agent Profile.
OMA-WAP-UAProf-v1_1. This document
is available at http://www.openmobilealliance.org/.
[WEBDATA]
Web
Architecture: Describing and Exchanging Data
Berners-Lee T., Connolly D., Swick R., World Wide Web Consortium, 7 June 1999.
This document
is
[XLINK]
XML
Linking Language (XLink) Version 1.0
, DeRose S.,
Maler E., Orchard D. (Editors), World Wide Web Consortium, 27
June 2001.
This
version
is http://www.w3.org/TR/2001/REC-xlink-20010627/.
The
latest version
is http://www.w3.org/TR/xlink/.
[XML-SCHEMA2]
XML
Schema Part 2: Datatypes
, Biron P., Malhotra A.
(Editors), World Wide Web Consortium. 2 May 2001.
This
version
is
latest version
is http://www.w3.org/TR/xmlschema-2/.
[XPACKAGE]
XML Package (XPackage) 1.0
, Wilson G., Editor's Working Draft,
6 March 2003.
This version
is
The
latest
version
is http://www.xpackage.org/specification/.
9.
Acknowledgments
This document has benefited from inputs from many members of
the
RDF Core Working
Group
. Specific thanks are due to Art Barstow, Dave Beckett,
Dan Brickley, Ron Daniel, Ben Hammersley, Martyn Horner, Graham
Klyne, Sean Palmer, Patrick Stickler, Aaron Swartz, Ralph Swick,
and Garret Wilson who, together with the many people who
commented on earlier versions of the Primer, provided valuable
contributions to this document.
In addition, this document contains a significant contribution
from Pat Hayes, Sergey Melnik, and Patrick Stickler, who led the
development of the RDF datatype facilities described in the RDF
family of specifications.
Frank Manola also thanks
The MITRE Corporation
, Frank's employer
during most of the preparation of this document, for its support of
his RDF Core Working Group activities under a MITRE Sponsored Research
grant.
Appendix
A: More on Uniform Resource Identifiers (URIs)
Note: This section is intended to provide a
brief introduction to URIs. The definitive specification
of URIs is
RFC 2396
[URIS]
, which should be consulted for further
details. Additional discussion of URIs
can also be found in
Naming
and Addressing: URIs, URLs, ...
[NAMEADDRESS]
As discussed in
Section 2.1
, the Web
provides a general form of identifier, called the
Uniform Resource
Identifier
(URI), for identifying (naming) resources on the
Web. Unlike URLs, URIs are not limited to identifying things that
have network locations, or use other computer access mechanisms.
A number of different
URI schemes
(URI forms) have been
already been developed, and are being used, for various purposes.
Examples include:
http:
(Hypertext Transfer Protocol, for Web
pages)
mailto:
(email addresses), e.g.,
mailto:em@w3.org
ftp:
(File Transfer Protocol)
urn:
(Uniform Resource Names, intended to be
persistent location-independent resource identifiers), e.g.,
urn:isbn:0-520-02356-0
(for a book)
A list of existing URI schemes can be found in
Addressing
Schemes
[ADDRESS-SCHEMES]
and it is a good idea to consider adapting one of the existing
schemes for any specialized identification purposes,
rather than trying to invent a new one.
No one person or organization controls who makes URIs or how
they can be used. While some URI schemes, such as URL's
http:
, depend on centralized systems such as DNS, other
schemes, such as
freenet:
, are completely decentralized.
This means that, as with any other kind of name, no one needs
special authority or permission to create a URI for something.
Also, anyone can create URIs to refer to things they do not own, just as in
ordinary language anyone can use whatever name they like for things
they do not own.
As also noted in
Section 2.1
RDF uses
URI references
[URIS]
to
name subjects, predicates, and objects in RDF statements. A URI
reference (or
URIref
) is a URI, together with an
optional
fragment identifier
at the end. For example,
the URI reference
consists of
the URI
and (separated
by the "#" character) the fragment identifier
Section2
RDF URIrefs can contain
Unicode
[UNICODE]
characters (see
[RDF-CONCEPTS]
), allowing many languages to be reflected in URIrefs.
URIrefs may be either
absolute
or
relative
An
absolute
URIref refers to a resource independently of
the context in which the URIref appears, e.g., the URIref
. A
relative
URIref is a shorthand form of an absolute URIref, where some
prefix of the URIref is missing, and information from the context
in which the URIref appears is required to fill in the missing
information. For example, the relative URIref
otherpage.html
, when appearing in a resource
, would be filled out
to the absolute URIref
. A URIref without
a URI part is considered a reference to the current document (the
document in which it appears). So, an empty URIref within a
document is considered equivalent to the URIref of the document
itself. A URIref consisting of just a fragment identifier is
considered equivalent to the URIref of the document in which it
appears, with the fragment identifier appended to it. For
example, within
, if
#section2
appeared as a URIref, it would be considered
equivalent to the absolute URIref
[RDF-CONCEPTS]
notes that RDF
graphs (the abstract models) do not use relative URIrefs, i.e.,
the subjects, predicates, and objects (and datatypes in typed
literals) in RDF statements must always be identified
independently of any context. However, a specific concrete RDF
syntax, such as RDF/XML, may allow relative URIrefs to be used as
a shorthand for absolute URIrefs in certain situations. RDF/XML
does permit such use of relative URIrefs, and some of the RDF/XML
examples in this Primer illustrate such uses.
[RDF-SYNTAX]
should be
consulted for further details.
Both RDF and Web browsers use URIrefs to identify things.
However, RDF and browsers interpret URIrefs in slightly different
ways. This is because RDF uses URIrefs
only
to identify
things, while browsers also use URIrefs to
retrieve
things. Often there is no effective difference, but in some cases
the difference can be significant. One obvious difference is that when
a URIref is used in a browser, there is the expectation that it
identifies a resource that can actually be retrieved: that
something is actually "at" the location identified by the URI.
However, in RDF a URIref may be used to identify something, such
as a person, that
cannot
be retrieved on the Web. People
sometimes use RDF together with a convention that, when a URIref
is used to identify an RDF resource, a page containing
descriptive information about that resource will be placed on the
Web "at" that URI, so that the URIref can be used in a browser to
retrieve that information. This can be a useful convention in
some circumstances, although it creates a difficulty in
distinguishing the identity of the original resource from the
identity of the Web page describing it (a subject discussed
further in
Section 2.3
).
However, this convention is not an explicit part of the
definition of RDF, and RDF itself does not assume that a URIref
identifies something that can be retrieved.
Another difference is in the way URIrefs with fragment
identifiers are handled. Fragment identifiers are often seen in
the URLs that identify HTML documents, where they serve to
identify a specific place within the document identified by the
URL. In normal HTML usage, where URI references are used to
retrieve the indicated resources, the two URIrefs:
are related (they both refer to the same document, the second
one identifying a location within the first one). However, as
noted already, RDF uses URI references purely to
identify
resources, not to retrieve them, and RDF
assumes no particular relationship between these two URIrefs. As
far as RDF is concerned, they are syntactically different URI
references, and hence may refer to unrelated things. This
does not mean that the HTML-defined containment relationship might
not exist, just that RDF does not assume that a relationship
exists based only on the fact that the URI parts of the URI
references are the same.
Carrying this point further, RDF
does not assume that there is any relationship between URI references
that share a common leading string, whether there is a fragment
identifier or not. For example, as far as RDF is concerned, the
two URIrefs:
have no particular relationship even though both of them start
with the string
. To RDF, they
are simply different resources, because their URIrefs are different.
(They may in fact be two files located in the same directory, but
RDF does not assume this or any other relationship exists.)
Appendix B:
More on the Extensible Markup Language (XML)
Note: This section is intended to provide a
brief introduction to XML. The definitive specification
of XML is
[XML]
, which should be consulted for
further details.
The
Extensible
Markup Language
[XML]
was designed to
allow anyone to design their own document format and then write a
document in that format. Like HTML documents (Web pages), XML
documents contain text. This text consists primarily of plain
text content, and markup in the form of
tags
. This
markup allows a processing program to interpret the various
pieces of content (called
elements
).
Both XML content and (with certain exceptions) tags can contain Unicode
[UNICODE]
characters, allowing information from many languages to be directly represented.
In HTML, the set of
permissible tags, and their interpretation, is defined by the
HTML specification. However, XML allows users to define their own
markup languages (tags and the structures in which they can
appear) adapted to their own specific requirements
(the RDF/XML language described in
Section
is one such XML markup language). For example,
the following is a simple passage marked up using an XML-based
markup language:
just got a new pet
Elements delimited by tags (
, etc.) are introduced to reflect a
particular structure associated with the passage. The tags allow
a program written with an understanding of these particular
elements, and the way they are structured, to properly interpret
the passage. For example, one of the elements in this example is
. This consists of the
start-tag
, the element
content
, and a matching
end-tag
. This
animal
element, together
with the
person
element, are nested as part of the
content of the
sentence
element. The nesting is possibly
clearer (and closer to some of the more "structured" XML
contained in the rest of this Primer) if the sentence is
written:
just got a new pet
In some cases, an element may have no content. This can be
written either by enclosing no content within the pair of
delimiting start- and end-tags, as in
, or by using a shorthand
form of tag called an
empty-element tag
, as in
In some cases, a start-tag (or empty-element tag) may contain
qualifying information other than the tag name, in the form of
attributes
. For example, the start-tag of the
element contains the attribute
webid="http://example.com/#johnsmith"
(presumably
identifying the specific person referred to). An attribute
consists of a name, an equal sign, and a value (enclosed in
quotes).
This particular markup language uses the words "sentence,"
"person," and "animal" as tag names in an attempt to convey some
of the meaning of the elements; and they
would
convey
meaning to an English-speaking person reading it, or to a program
specifically written to interpret this vocabulary. However, there
is no built-in meaning here. For example, to non-English
speakers, or to a program not written to understand this markup,
the element
may mean absolutely nothing.
Take the following passage, for example:
just got a new pet
To a machine, this passage has exactly the same structure as
the previous example. However, it is no longer clear to an
English-speaker what is being said, because the tags are no
longer English words. Moreover, others may have used the same
words as tags in their own markup languages, but with completely
different intended meanings. For example, "sentence" in another
markup language might refer to the amount of time that a
convicted criminal must serve in a penal institution. So
additional mechanisms must be provided to help keep XML
vocabulary straight.
To prevent confusion, it is necessary to uniquely identify
markup elements. This is done in XML using
XML Namespaces
[XML-NS]
. A
namespace
is just
a way of identifying a part of the Web (space) which acts as a
qualifier for a specific set of names. A namespace is created for
an XML markup language by creating a URI for it. By qualifying
tag names with the URIs of their namespaces, anyone can create
their own tags and properly distinguish them from tags with
identical spellings created by others. A convention that is
sometimes followed is to
create a Web page to describe the markup language (and the
intended meaning of the tags) and use the URL of that Web page as
the URI for its namespace. However, this is just a convention, and
neither XML nor RDF assumes that a namespace URI identifies a
retrievable Web resource.
The following example illustrates the
use of an XML namespace.
just got a new pet
In this example, the attribute
xmlns:user="http://example.com/xml/documents/"
declares a
namespace for use in this piece of XML. It maps the
prefix
user
to the namespace URI
. The XML content can
then use
qualified names
(or
QNames
) like
user:person
as tags. A QName contains a prefix that
identifies a namespace, followed by a colon, and then a
local
name
for an XML tag or attribute name. By using namespace
URIs to distinguish specific groups of names, and qualifying tags
with the URIs of the namespaces they come from, as in this
example, there is no need to worry about tag names conflicting. Two
tags having the same spelling are considered the same only if
they also have the same namespace URIs.
Every XML document is required to be
well-formed
This means the XML document must satisfy a number of syntactic
conditions, for example, that every start-tag must have a matching
end-tag, and that elements must be properly nested within other elements
(elements may not overlap). The complete set of well-formedness
conditions is defined in
[XML]
In addition, an XML document may optionally include an XML
document type declaration
to define additional constraints
on the structure of the document, and to support the use of
predefined units of text within the document.
The document type declaration (introduced with
DOCTYPE
contains or points to declarations that define a grammar for the
document. This grammar is known as a
document type definition
or
DTD
The declarations in a DTD specify such things as
which XML elements and attributes
may appear in XML documents corresponding to the DTD, the relationships
of these elements and attributes (e.g., which elements can be nested within
which other elements, or which attributes may appear with which elements),
and whether elements or attributes are required or optional.
The document type declaration can point to a set of declarations located
outside the document (called the
external subset
, which can
be used to allow common declarations to be shared among multiple
documents), can include
the declarations directly in the document (called the
internal subset
), or can have both internal and external
DTD subsets.
The complete DTD for a document consists of both subsets taken together.
A simple example of an XML document with a document type declaration
is shown in
Example 47
Example 47: An XML Document
With a Document Type Declaration
In this case, the document has only an external DTD subset, and the
system identifier
provides its location (a URIref).
An XML document
is
valid
if it has an associated document type declaration
and the document complies with the constraints defined by the document
type declaration.
An RDF/XML document is only required to be well-formed XML; it is not
intended to be validated against an XML DTD (or an XML Schema), and
[RDF-SYNTAX]
does not specify a normative
DTD that could be used for validating arbitrary RDF/XML (an appendix of
[RDF-SYNTAX]
does provide a non-normative
example schema for RDF/XML). As a result, more detailed
discussion of XML DTD grammars is beyond the scope of this Primer.
Further information on XML DTDs and XML validation can be found in
[XML]
, and the numerous books on XML.
However, there is one use of XML document type declarations
that
is
relevant to RDF/XML, and that is their use in defining XML
entities
. An XML entity
declaration essentially associates a name with a string of characters.
When the entity name is used elsewhere within an XML document, XML
processors replace the entity name with the corresponding string.
This provides a way to abbreviate long strings such as URIrefs, and can
help make XML documents containing such strings more readable.
Using a document type declaration just to declare
XML entities is allowed, and can be useful, even when (as in
RDF/XML) the documents are not intended to be validated.
In RDF/XML documents, entities are generally declared within the
document itself, i.e., using only an internal DTD subset (one reason
for this is that RDF/XML is not intended to be validated, and non-validating
XML processors are not required to process external DTD subsets).
For example, providing the document type declaration
shown in
Example 48
at the beginning of an RDF/XML document allows the URIrefs in that document
for the
rdf
rdfs
, and
xsd
namespaces to be abbreviated as
&rdf;
&rdfs;
, and
&xsd;
respectively,
as shown in the example.
Example 48: Some XML Entity
Declarations
]>
xmlns:rdfs = "&rdfs;"
xmlns:xsd = "&xsd;">
...RDF statements...
Appendix C: Changes
Only minor editorial and typographic changes have been made since
the
Proposed Recommendation version
. Older changes are detailed in its
change log