Namespaces in XML
REC-xml-names-19990114
Namespaces in XML
World Wide Web Consortium
14-January-1999
This version:
Latest version:
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Editors:
Tim Bray
(Textuality)

Dave Hollander
(Hewlett-Packard Company)

Andrew Layman
(Microsoft)

W3C
MIT
INRIA
Keio
), All Rights Reserved. W3C
liability,
trademark
document use
and
software licensing
rules apply.
Status of this document
This document has been reviewed by W3C Members and
other interested parties and has been endorsed by the
Director as a W3C Recommendation. It is a stable
document and may be used as reference material or cited
as a normative reference from another document. 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.
The list of known errors in this specification is
available at
Please report errors in this document to
xml-names-editor@w3.org
Abstract
XML namespaces provide a simple method for qualifying
element and attribute
names used in Extensible Markup Language documents by associating them
with namespaces identified by URI references.
Table of Contents
1.
Motivation and Summary
1.1
A Note on Notation and Usage
2.
Declaring Namespaces
3.
Qualified Names
4.
Using Qualified Names
5.
Applying Namespaces to Elements and Attributes
5.1
Namespace Scoping
5.2
Namespace Defaulting
5.3
Uniqueness of Attributes
6.
Conformance of Documents
Appendices
A.
The Internal Structure of XML Namespaces (Non-Normative)
A.1
The Insufficiency of the Traditional Namespace
A.2
XML Namespace Partitions
A.3
Expanded Element Types and Attribute Names
A.4
Unique Expanded Attribute Names
B.
Acknowledgements (Non-Normative)
C.
References
1. Motivation and Summary
We envision applications of Extensible Markup Language (XML) where
a single XML document may
contain elements and attributes
(here referred to as a "markup vocabulary")
that are defined for and used by multiple software modules.
One motivation for this is modularity; if such a markup vocabulary exists
which is well-understood and for which there is useful software
available, it is better to re-use this markup rather than re-invent it.
Such documents, containing multiple markup vocabularies,
pose problems of recognition and collision. Software modules need to
be able to recognize the tags and attributes which they are designed
to process, even in the face
of "collisions" occurring when markup intended for some other software
package uses the same element type or attribute name.
These considerations require that document
constructs
should have universal names,
whose scope extends beyond their
containing document.
This specification describes a mechanism,
XML namespaces
, which accomplishes this.
[Definition:] An
XML namespace
is a collection of names, identified by a URI reference
[RFC2396]
which are used in XML documents as
element types
and
attribute names
XML namespaces
differ from the "namespaces" conventionally used in computing disciplines
in that the XML version has internal structure and is not, mathematically
speaking, a set.
These issues are discussed in
A. The Internal Structure of XML Namespaces
".
[Definition:] URI references which
identify namespaces are considered
identical
when they are
exactly the same character-for-character.
Note that URI references which are not identical in this sense may in fact be
functionally equivalent.
Examples include URI references which differ only in case, or which are in
external entities which have different effective base URIs.
Names from XML namespaces may appear as
qualified names
which contain a single colon, separating the name into a
namespace prefix
and a
local part
The prefix, which is mapped to a URI reference, selects a namespace.
The combination of
the universally managed URI namespace and the document's own namespace
produces identifiers that are universally unique.
Mechanisms are provided for prefix scoping and defaulting.
URI references can contain characters not allowed in names, so cannot
be used directly as namespace prefixes.
Therefore, the
namespace prefix serves as a proxy for a URI reference.
An attribute-based syntax described below is used
to
declare
the association of the
namespace prefix with a URI reference;
software which supports this namespace proposal must
recognize and act on these declarations and prefixes.
1.1 A Note on Notation and Usage
Note that many of the
nonterminals in the productions in
this specification are defined not here but in
the XML specification
[XML]
When nonterminals defined here have the same names as nonterminals
defined in the XML specification, the productions here
in all cases match a subset of the strings matched by the
corresponding ones there.
In this document's productions,
the
NSC
is a "Namespace Constraint",
one of the rules that documents conforming to this specification must follow.
Note that all Internet domain names used in
examples, with the exception of
w3.org
, are selected
at random and should not be taken as having any import.
2. Declaring Namespaces
[Definition:] A namespace is
declared
using
a family of reserved attributes.
Such an attribute's name must either
be
xmlns
or have
xmlns:
as a prefix.
These attributes, like any other XML attributes, may be provided
directly or by
default
Attribute Names for Namespace Declaration
[1]
NSAttName
::=
PrefixedAttName
DefaultAttName
[2]
PrefixedAttName
::=
'xmlns:'
NCName
NSC:
Leading "XML"
[3]
DefaultAttName
::=
'xmlns'
[4]
NCName
::=
Letter
| '_')
NCNameChar
)*
/*
An XML
Name
, minus the ":" */
[5]
NCNameChar
::=
Letter
Digit
| '.' | '-' | '_'
CombiningChar
Extender
[Definition:] The attribute's
value
a URI reference,
is the
namespace name
identifying the namespace.
The namespace name, to serve its
intended purpose, should have the characteristics of uniqueness and
persistence.
It is not a goal that it be directly usable for retrieval of a schema (if
any exists).
An example of a syntax that is designed with these goals in mind is
that for Uniform Resource Names
[RFC2141]
However, it should be noted that ordinary URLs can be managed in such a way as
to achieve these same goals.
[Definition:] If the
attribute name matches
PrefixedAttName
then the
NCName
gives the
namespace prefix
used to associate element and attribute names with the
namespace name
in the attribute value
in the scope of the element to which the declaration
is attached.
In such declarations, the namespace name may not be empty.
[Definition:] If the
attribute name matches
DefaultAttName
then the
namespace name
in the
attribute value is
that of the
default namespace
in the scope of the element to which the declaration
is attached.
In such a default declaration, the attribute value may be empty.
Default namespaces and overriding of declarations are discussed in
5. Applying Namespaces to Elements and Attributes
".
An example namespace declaration, which associates the
namespace prefix
edi
with the namespace name



Namespace Constraint:
Leading "XML"
Prefixes beginning with the three-letter sequence
, in any case combination, are reserved
for use by XML and XML-related specifications.
3. Qualified Names
[Definition:] In XML
documents conforming to this specification, some
names (constructs corresponding to the nonterminal
Name
may be given as
qualified names
, defined as follows:
Qualified Name
[6]
QName
::=
Prefix
':')?
LocalPart
[7]
Prefix
::=
NCName
[8]
LocalPart
::=
NCName
The
Prefix
provides the
namespace prefix
part of the qualified name, and must be associated with a namespace URI
reference in a
namespace declaration
[Definition:]
The
LocalPart
provides the
local part
of the qualified name.
Note that the prefix functions
only
as a placeholder for a
namespace name.
Applications should use the namespace name, not the prefix, in constructing
names whose scope extends beyond the
containing document.
4. Using Qualified Names
In XML documents conforming to this specification,
element types are given as
qualified names
, as
follows:
Element Types
[9]
STag
::=
'<'
QName
Attribute
)*
? '>'
NSC:
Prefix Declared
[10]
ETag
::=
'QName
? '>'
NSC:
Prefix Declared
[11]
EmptyElemTag
::=
'<'
QName
Attribute
)*
? '/>'
NSC:
Prefix Declared
An example of a qualified name serving as an element type:


32.18

Attributes are either
namespace
declarations
or their names are given as
qualified names
Attribute
[12]
Attribute
::=
NSAttName
Eq
AttValue
QName
Eq
AttValue
NSC:
Prefix Declared
An example of a qualified name serving as an attribute name:


Baby food

Namespace Constraint:
Prefix Declared
The namespace prefix, unless it is
xml
or
xmlns
, must have been
declared in a
namespace declaration
attribute in either the start-tag of the element where the prefix
is used or in an an ancestor element (i.e. an element in whose
content
the
prefixed markup occurs).
The prefix
xml
is by definition bound to the
namespace name
The prefix
xmlns
is used only for namespace bindings
and is not itself bound to any namespace name.
This constraint may lead to operational difficulties in the case where
the namespace declaration attribute is provided, not directly in the XML
document entity
, but
via a default attribute declared in an external entity.
Such declarations may not be read by software which is based on a
non-validating XML processor.
Many XML applications, presumably including namespace-sensitive ones, fail to
require validating processors.
For correct operation with such applications, namespace declarations must be
provided either directly or via default attributes declared in the
internal subset of the DTD
Element names and attribute types are also given as qualified names when
they appear in declarations in the
DTD
Qualified Names in Declarations
[13]
doctypedecl
::=
'QName
ExternalID
)?
? ('['
markupdecl
PEReference
)*
']'
?)? '>'
[14]
elementdecl
::=
'QName
contentspec
? '>'
[15]
cp
::=
QName
choice
seq
('?' | '*' | '+')?
[16]
Mixed
::=
'('
'#PCDATA'
'|'
QName
)*
')*'
| '('
? '#PCDATA'
? ')'
[17]
AttlistDecl
::=
'QName
AttDef
? '>'
[18]
AttDef
::=
QName
NSAttName
AttType
DefaultDecl
5. Applying Namespaces to Elements and Attributes
5.1 Namespace Scoping
The namespace declaration is considered to apply to the
element where it is specified and to all elements within the content
of that element, unless overridden by another namespace declaration
with the same
NSAttName
part:



Frobnostication
Moved to
here.

Multiple namespace prefixes can be declared as attributes of a single element,
as shown in this example:


xmlns:isbn='urn:ISBN:0-395-36341-6'>
Cheaper by the Dozen
1568491379

5.2 Namespace Defaulting
default namespace
is considered
to apply to the element where it is declared (if that element has no
namespace prefix
), and to all elements
with no prefix within the content of that
element.
If the URI reference in a default namespace declaration is empty, then
unprefixed
elements in the scope of the declaration are not considered to be in
any namespace.
Note that default namespaces do not apply directly to attributes.



Frobnostication

Moved to
here.





xmlns:isbn='urn:ISBN:0-395-36341-6'>
Cheaper by the Dozen
1568491379

A larger example of namespace scoping:


xmlns:isbn='urn:ISBN:0-395-36341-6'>
Cheaper by the Dozen
1568491379



This is a funny book!




The default namespace can be set to the empty string.
This has the same
effect, within the scope of the declaration, of there being no default
namespace.




NameOriginDescription


Huntsman
Bath, UK

BitterFuggles
Wonderful hop, light alcohol, good summer beer
Fragile; excessive variance pub to pub





5.3 Uniqueness of Attributes
In XML documents conforming
to this specification, no tag
may contain two attributes which:
have identical names, or
have qualified names with the same
local part
and with
prefixes
which have been bound
to
namespace names
that
are
identical
For example, each of the
bad
start-tags is illegal in the
following:

xmlns:n2="http://www.w3.org" >



However, each of the following is legal, the second because the default
namespace does not apply to attribute names:

xmlns="http://www.w3.org" >



6. Conformance of Documents
In XML documents which conform to this specification,
element types and attribute names must match the production for
QName
and
must satisfy the "Namespace Constraints".
An XML document conforms to this specification if all other
tokens in the document which are required, for XML conformance, to match the
XML production for
Name
, match
this specification's production for
NCName
The effect of conformance is that in such a document:
All element types and attribute names contain either zero
or one colon.
No entity names, PI targets, or notation names contain any colons.
Strictly speaking, attribute values declared to be of types
ID
IDREF(S)
ENTITY(IES)
and
NOTATION
are also
Names
and thus should be colon-free.
However, the declared type of attribute values is only available
to processors which read markup declarations, for example
validating processors
Thus, unless the use of a validating processor has been specified,
there can be no assurance that the contents of attribute values
have been checked for conformance to this specification.
Appendices
A. The Internal Structure of XML Namespaces (Non-Normative)
A.1 The Insufficiency of the Traditional Namespace
In the computing disciplines, the term "namespace" conventionally refers to
set
of names, i.e. a collection containing no duplicates.
However, treating the names used in XML markup
as such a namespace would greatly impair their usefulness.
The primary use of such names in XML documents is to enable identification
of logical
structures in documents by software modules such as query processors,
stylesheet-driven
rendering engines, and schema-driven validators.
Consider the following example:
Book-Signing Event







In this example, there are three occurrences of the name
title
within markup, and the name alone clearly provides insufficient information
to allow correct processing by a software module.
Another problematic area comes from the use of "global" attributes, as
illustrated by this example, a fragment of an XML document which is to
be displayed using a CSS stylesheet:

Layman, A
33B
1997-05-24T07:55:00+1
In this case, the
CLASS
attribute, which describes the fare basis
and takes values such as "J", "Y", and "C", is distinct at all semantic levels
from the
HTML:CLASS
attribute, which
is used to
simulate syntactic richness in HTML, as a means of overcoming the
limited element repertoire by subclassing.
XML 1.0 does not provide a built-in way to declare "global" attributes;
items such as the HTML
CLASS
attribute are global only in their
prose description and their interpretation by HTML applications.
However, such attributes, an important distinguishing feature of which is that
their names are unique, are commonly observed to occur in a variety of
applications.
A.2 XML Namespace Partitions
In order to support the goal of making both qualified and unqualified names
useful in meeting their intended purpose, we identify the names appearing in
an XML namespace as belonging to one of several disjoint traditional
(i.e. set-structured) namespaces, called namespace partitions.
The partitions are:
The All Element Types Partition
All element types in an XML namespace appear in this partition.
Each has a unique
local part
; the
combination of the namespace name and the local part uniquely identifies the
element type.
The Global Attribute Partition
This partition contains the names of all attributes which are defined,
in this namespace, to be global.
The only required characteristic of a global attribute is that its name be
unique in the global attribute partition.
This specification makes no
assertions as to the proper usage of such attributes.
The combination of the namespace name and the attribute name uniquely
identifies the global attribute.
The Per-Element-Type Partitions
Each type in the All Element Types Partition has an
associated namespace in which appear the names of the unqualified attributes
that are provided for that element.
This is a traditional namespace because the appearance of duplicate attribute
names on an element is forbidden by XML 1.0.
The combination of the attribute name with the element's type and namespace
name uniquely identifies each unqualified attribute.
In XML documents conforming to this specification, the names of
all qualified (prefixed) attributes are assigned to the global attribute
partition, and the names of all unqualified attributes are assigned to the
appropriate per-element-type partition.
A.3 Expanded Element Types and Attribute Names
For convenience in specifying rules and in making comparisons, we define an
expanded form, expressed here in XML element syntax, for each element type and
attribute name in an XML document.
[Definition:] An
expanded element
type
is expressed as an empty XML element of type
ExpEType
It has a required
type
attribute which
gives the type's
LocalPart
, and an optional
ns
attribute which, if the element is qualified, gives its
namespace name
[Definition:] An
expanded
attribute name
is expressed as an empty XML element of type
ExpAName
It has a required
name
attribute which gives the name.
If the attribute is global, it has a required
ns
attribute
which gives the
namespace name
otherwise, it has a required attribute
eltype
which
gives the type of the attached element, and an optional attribute
elns
which gives the namespace name, if known, of the
attached element.
Slight variations on the examples given above will illustrate
the working of expanded element types and attribute names.
The following two fragments are each followed by a table showing
the expansion of the names:
 
   Book-Signing Event
   
     
     


The names would expand as follows:
Line
Name
Expanded
section

title

signing

author

title

name

book

title

price

 
 Layman, A
 33B
 Check Status
 1997-05-24T07:55:00+1
RESERVATION

NAME

HTML:CLASS

SEAT

CLASS

HTML:CLASS

HTML:A

HREF

DEPARTURE

A.4 Unique Expanded Attribute Names
The constraint expressed by "
5.3 Uniqueness of Attributes
" above
may straightforwardly be implemented by requiring that
no element have
two attributes whose expanded names are equivalent, i.e. have the same
attribute-value pairs.
B. Acknowledgements (Non-Normative)
This work reflects input from a very large number of people,
including especially the members of the World Wide
Web Consortium XML Working Group and Special Interest Group
and the participants in the W3C Metadata Activity.
The contributions of Charles Frankston of Microsoft
were particularly valuable.
C. References
RFC2141
IETF (Internet Engineering Task Force)
RFC 2141: URN Syntax
, ed. R. Moats. May 1997.
RFC2396
IETF (Internet Engineering Task Force)
RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax
, eds.
T. Berners-Lee, R. Fielding, L. Masinter.
August 1998.
XML
Extensible Markup Language
(XML) 1.0
, eds. Tim Bray, Jean
Paoli, and C. M.
Sperberg-McQueen.
10 February 1998. Available at