Namespaces in XML
World Wide Web Consortium 14-January-1999
- This version:
- Latest version:
- Previous version:
- Tim Bray (Textuality) mailto:firstname.lastname@example.org
- Dave Hollander (Hewlett-Packard Company) mailto:email@example.com
- Andrew Layman (Microsoft) mailto:firstname.lastname@example.org
© 1999 W3C (MIT, INRIA, Keio
), All Rights Reserved. W3C liability,
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 http://www.w3.org/XML/xml-names-19990114-errata.
Please report errors in this document to email@example.com.
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.
- 1. Motivation
- 1.1 A Note on
Notation and Usage
- 2. Declaring
- 3. Qualified
- 4. Using
- 5. Applying
Namespaces to Elements and Attributes
- 5.1 Namespace
- 5.2 Namespace
- 5.3 Uniqueness
- 6. Conformance
- A. The Internal
Structure of XML Namespaces (Non-Normative)
- A.1 The
Insufficiency of the Traditional Namespace
- A.2 XML
- A.3 Expanded
Element Types and Attribute Names
- A.4 Unique
Expanded Attribute Names
- B. Acknowledgements
- C. References
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
association of the namespace prefix with a URI reference; software which
supports this namespace proposal must recognize and act on these declarations
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
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
[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|
[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
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
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
<!-- the "edi" prefix is bound to http://ecommerce.org/schema
for the "x" element and contents -->
Namespace Constraint: Leading "XML"
Prefixes beginning with the
l, in any
case combination, are reserved for use by XML and XML-related specifications.
[Definition:] In XML documents
conforming to this specification, some names (constructs corresponding to the
Name) may be given as
qualified names, defined as follows:
the namespace prefix
part of the qualified name, and must be associated with a namespace URI
reference in a namespace
declaration. [Definition:] The
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.
In XML documents conforming to this specification, element types are given as
qualified names, as
An example of a qualified name serving as an element type:
<!-- the 'price' element's namespace is http://ecommerce.org/schema -->
Attributes are either namespace declarations
or their names are given as qualified names:
An example of a qualified name serving as an attribute name:
<!-- the 'taxClass' attribute's namespace is http://ecommerce.org/schema -->
<lineItem edi:taxClass="exempt">Baby food</lineItem>
Namespace Constraint: Prefix Declared
The namespace prefix, unless
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
http://www.w3.org/XML/1998/namespace. 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
Element names and attribute types are also given as qualified names when they
appear in declarations in the DTD:
|Qualified Names in Declarations|
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
<!-- all elements here are explicitly in the HTML namespace -->
Multiple namespace prefixes can be declared as attributes of a single
element, as shown in this example:
<!-- both namespace prefixes are available throughout -->
<bk:title>Cheaper by the Dozen</bk:title>
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.
<!-- elements are in the HTML namespace, in this case by default -->
<!-- unprefixed element types are from "books" -->
<title>Cheaper by the Dozen</title>
A larger example of namespace scoping:
<!-- initially, the default namespace is "books" -->
<title>Cheaper by the Dozen</title>
<!-- make HTML the default namespace for some commentary -->
This is a <i>funny</i> 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
<!-- the default namespace is now that of HTML -->
<!-- no default namespace inside table cells -->
<td><origin xmlns="">Bath, UK</origin></td>
<pro>Wonderful hop, light alcohol, good summer beer</pro>
<con>Fragile; excessive variance pub to pub</con>
In XML documents conforming to this specification, no tag may contain two
- have identical names, or
- have qualified names with the same local part and
which have been bound to namespace names that
For example, each of the
bad start-tags is illegal in the
<!-- http://www.w3.org is bound to n1 and n2 -->
<bad a="1" a="2" />
<bad n1:a="1" n2:a="2" />
However, each of the following is legal, the second because the default
namespace does not apply to attribute names:
<!-- http://www.w3.org is bound to n1 and is the default -->
<good a="1" b="2" />
<good a="1" n1:a="2" />
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
this specification's production for
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
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.
In the computing disciplines, the term "namespace" conventionally refers to a
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:
<author title="Mr" name="Vikram Seth" />
<book title="A Suitable Boy" price="$22.95" /></signing>
<author title="Dr" name="Oliver Sacks" />
<book title="The Island of the Color-Blind" price="$12.95" /></signing>
In this example, there are three occurrences of the name
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:
<NAME HTML:CLASS="largeSansSerif">Layman, A</NAME>
<SEAT CLASS="Y" HTML:CLASS="reallyImportant">33B</SEAT>
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.
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
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
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
and an optional
ns attribute which, if the element is qualified,
gives its namespace
[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
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:
<!-- 1 --> <section xmlns='urn:com:books-r-us'>
<!-- 2 --> <title>Book-Signing Event</title>
<!-- 3 --> <signing>
<!-- 4 --> <author title="Mr" name="Vikram Seth" />
<!-- 5 --> <book title="A Suitable Boy" price="$22.95" />
The names would expand as follows:
||<ExpEType type="section" ns="urn:com:books-r-us" /> |
||<ExpEType type="title" ns="urn:com:books-r-us" /> |
||<ExpEType type="signing" ns="urn:com:books-r-us" />|
||<ExpEType type="author" ns="urn:com:books-r-us" /> |
||<ExpAName name='title' eltype="author" elns="urn:com:books-r-us"
||<ExpAName name='name' eltype="author" elns="urn:com:books-r-us"
||<ExpEType type="book" ns="urn:com:books-r-us" />|
||<ExpAName name='title' eltype="book" elns="urn:com:books-r-us"
||<ExpAName name='price' eltype="book" elns="urn:com:books-r-us"
<!-- 1 --> <RESERVATION xmlns:HTML="http://www.w3.org/TR/REC-html40">
<!-- 2 --> <NAME HTML:CLASS="largeSansSerif">Layman, A</NAME>
<!-- 3 --> <SEAT CLASS="Y" HTML:CLASS="largeMonotype">33B</SEAT>
<!-- 4 --> <HTML:A HREF='/cgi-bin/ResStatus'>Check Status</HTML:A>
<!-- 5 --> <DEPARTURE>1997-05-24T07:55:00+1</DEPARTURE></RESERVATION>
||<ExpEType type="RESERVATION" />|
||<ExpEType type="NAME" />|
||<ExpAName name="CLASS" ns=http://www.w3.org/TR/REC-html40
||<ExpEType type="SEAT" />|
||<ExpAName name="CLASS" eltype="SEAT">|
||<ExpAName name="CLASS" ns="http://www.w3.org/TR/REC-html40"
||<ExpEType type="A" ns="http://www.w3.org/TR/REC-html40" />|
||<ExpAName name="HREF" eltype="A"
||<ExpEType type="DEPARTURE" />|
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.
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.
- IETF (Internet Engineering Task Force) RFC 2141: URN Syntax, ed.
R. Moats. May 1997.
- IETF (Internet Engineering Task Force) RFC 2396: Uniform Resource
Identifiers (URI): Generic Syntax, eds. T. Berners-Lee, R. Fielding, L.
Masinter. August 1998.
- Extensible Markup Language (XML) 1.0, eds. Tim Bray, Jean Paoli,
and C. M. Sperberg-McQueen. 10 February 1998. Available at http://www.w3.org/TR/REC-xml.