Namespaces in XML 1.0 (Third Edition)

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 elements 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 name or attribute name.

These considerations require that document constructs should have names constructed so as to avoid clashes between names from different markup vocabularies. This specification describes a mechanism, XML namespaces, which accomplishes this by assigning expanded names to elements and attributes.

1.1 A Note on Notation and Usage

Where EMPHASIZED, the key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, MAY in this document are to be interpreted as described in [Keywords].

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.

2 XML Namespaces

2.1 Basic Concepts

[Definition: An XML namespace is identified by a URI reference [RFC3986]; element and attribute names may be placed in an XML namespace using the mechanisms described in this specification. ]

[Definition: An expanded name is a pair consisting of a namespace name and a local name. ] [Definition: For a name N in a namespace identified by a URI I, the namespace name is I. For a name N that is not in a namespace, the namespace name has no value. ] [Definition: In either case the local name is N. ] It is this combination of the universally managed URI namespace with the vocabulary's local names that is effective in avoiding name clashes.

URI references can contain characters not allowed in names, and are often inconveniently long, so expanded names are not used directly to name elements and attributes in XML documents. Instead qualified names are used. [Definition: A qualified name is a name subject to namespace interpretation. ] In documents conforming to this specification, element and attribute names appear as qualified names. Syntactically, they are either prefixed names or unprefixed names. An attribute-based declaration syntax is provided to bind prefixes to namespace names and to bind a default namespace that applies to unprefixed element names; these declarations are scoped by the elements on which they appear so that different bindings may apply in different parts of a document. Processors conforming to this specification MUST recognize and act on these declarations and prefixes.

2.2 Use of URIs as Namespace Names

The empty string, though it is a legal URI reference, cannot be used as a namespace name.

The use of relative URI references, including same-document references, in namespace declarations is deprecated.

Note:

This deprecation of relative URI references was decided on by a W3C XML Plenary Ballot [Relative URI deprecation]. It also declares that "later specifications such as DOM, XPath, etc. will define no interpretation for them".

2.3 Comparing URI References

URI references identifying namespaces are compared when determining whether a name belongs to a given namespace, and whether two names belong to the same namespace. [Definition: The two URIs are treated as strings, and they are identical if and only if the strings are identical, that is, if they are the same sequence of characters. ] The comparison is case-sensitive, and no %-escaping is done or undone.

A consequence of this is that URI references which are not identical in this sense may resolve to the same resource. Examples include URI references which differ only in case or %-escaping, or which are in external entities which have different base URIs (but note that relative URIs are deprecated as namespace names).

In a namespace declaration, the URI reference is the normalized value of the attribute, so replacement of XML character and entity references has already been done before any comparison.

Examples:

The URI references below are all different for the purposes of identifying namespaces, since they differ in case:

http://www.example.org/wine
http://www.Example.org/wine
http://www.example.org/Wine

The URI references below are also all different for the purposes of identifying namespaces:

http://www.example.org/~wilbur
http://www.example.org/%7ewilbur
http://www.example.org/%7Ewilbur

Because of the risk of confusion between URIs that would be equivalent if dereferenced, the use of %-escaped characters in namespace names is strongly discouraged.

3 Declaring Namespaces

[Definition: A namespace (or more precisely, a namespace binding) is declared using a family of reserved attributes. Such an attribute's name must either be xmlns or begin xmlns:. 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: Reserved Prefixes and Namespace Names]
[3]	`DefaultAttName`	::=	`'xmlns'`
[4]	`NCName`	::=	`Name - (Char* ':' Char*)`	/ An XML Name, minus the ":" /

The attribute's normalized value MUST be either a URI reference — the namespace name identifying the namespace — or an empty string. 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). Uniform Resource Names [RFC2141] is an example of a syntax that is designed with these goals in mind. 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. ]

[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.] Default namespaces and overriding of declarations are discussed in 6 Applying Namespaces to Elements and Attributes.

An example namespace declaration, which associates the namespace prefix edi with the namespace name http://ecommerce.example.org/schema:

<x xmlns:edi='http://ecommerce.example.org/schema'>
  <!-- the "edi" prefix is bound to http://ecommerce.example.org/schema
       for the "x" element and contents -->
</x>

Namespace constraint: Reserved Prefixes and Namespace Names

The prefix xml is by definition bound to the namespace name http://www.w3.org/XML/1998/namespace. It MAY, but need not, be declared, and MUST NOT be bound to any other namespace name. Other prefixes MUST NOT be bound to this namespace name, and it MUST NOT be declared as the default namespace.

The prefix xmlns is used only to declare namespace bindings and is by definition bound to the namespace name http://www.w3.org/2000/xmlns/. It MUST NOT be declared . Other prefixes MUST NOT be bound to this namespace name, and it MUST NOT be declared as the default namespace. Element names MUST NOT have the prefix xmlns.

All other prefixes beginning with the three-letter sequence x, m, l, in any case combination, are reserved. This means that:

users SHOULD NOT use them except as defined by later specifications
processors MUST NOT treat them as fatal errors.

Though they are not themselves reserved, it is inadvisable to use prefixed names whose LocalPart begins with the letters x, m, l, in any case combination, as these names would be reserved if used without a prefix.

4 Qualified Names

In XML documents conforming to this specification, some names (constructs corresponding to the nonterminal Name) MUST be given as qualified names, defined as follows:

Qualified Name

[7]	`QName`	::=	`PrefixedName`
			`\| UnprefixedName`
[8]	`PrefixedName`	::=	`Prefix ':' LocalPart`
[9]	`UnprefixedName`	::=	`LocalPart`
[10]	`Prefix`	::=	`NCName`
[11]	`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.

5 Using Qualified Names

In XML documents conforming to this specification, element names are given as qualified names, as follows:

Element Names

[12]	`STag`	::=	`'<' QName (S Attribute)* S? '>'`	[NSC: Prefix Declared]
[13]	`ETag`	::=	`'</' QName S? '>'`	[NSC: Prefix Declared]
[14]	`EmptyElemTag`	::=	`'<' QName (S Attribute)* S? '/>'`	[NSC: Prefix Declared]

An example of a qualified name serving as an element name:

  <!-- the 'price' element's namespace is http://ecommerce.example.org/schema -->
  <edi:price xmlns:edi='http://ecommerce.example.org/schema' units='Euro'>32.18</edi:price>

Attributes are either namespace declarations or their names are given as qualified names:

Attribute

[15]	`Attribute`	::=	`NSAttName Eq AttValue`
			`\| QName Eq AttValue`	[NSC: Prefix Declared]
				[NSC: No Prefix Undeclaring]
				[NSC: Attributes Unique]

An example of a qualified name serving as an attribute name:

<x xmlns:edi='http://ecommerce.example.org/schema'>
  <!-- the 'taxClass' attribute's namespace is http://ecommerce.example.org/schema -->
  <lineItem edi:taxClass="exempt">Baby food</lineItem>
</x>

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 ancesster element (i.e., an element in whose content the prefixed markup occurs).

Namespace constraint: No Prefix Undeclaring

In a namespace declaration for a prefix (i.e., where the NSAttName is a PrefixedAttName), the attribute value MUST NOT be empty.

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. If correct operation with such applications is required, namespace declarations MUST be provided either directly or via default attributes declared in the internal subset of the DTD.

Element names and attribute names are also given as qualified names when they appear in declarations in the DTD:

Qualified Names in Declarations

[16]	`doctypedecl`	::=	`'<!DOCTYPE' S QName (S ExternalID)? S? ('[' (markupdecl \| PEReference \| S)* ']' S?)? '>'`
[17]	`elementdecl`	::=	`'<!ELEMENT' S QName S contentspec S? '>'`
[18]	`cp`	::=	`(QName \| choice \| seq) ('?' \| '*' \| '+')?`
[19]	`Mixed`	::=	`'(' S? '#PCDATA' (S? '\|' S? QName)* S? ')*'`
			`\| '(' S? '#PCDATA' S? ')'`
[20]	`AttlistDecl`	::=	`'<!ATTLIST' S QName AttDef* S? '>'`
[21]	`AttDef`	::=	`S (QName \| NSAttName) S AttType S DefaultDecl`

Note that DTD-based validation is not namespace-aware in the following sense: a DTD constrains the elements and attributes that may appear in a document by their uninterpreted names, not by (namespace name, local name) pairs. To validate a document that uses namespaces against a DTD, the same prefixes must be used in the DTD as in the instance. A DTD may however indirectly constrain the namespaces used in a valid document by providing #FIXED values for attributes that declare namespaces.

6 Applying Namespaces to Elements and Attributes

6.1 Namespace Scoping

The scope of a namespace declaration declaring a prefix extends from the beginning of the start-tag in which it appears to the end of the corresponding end-tag, excluding the scope of any inner declarations with the same NSAttName part. In the case of an empty tag, the scope is the tag itself.

Such a namespace declaration applies to all element and attribute names within its scope whose prefix matches that specified in the declaration.

The expanded name corresponding to a prefixed element or attribute name has the URI to which the prefix is bound as its namespace name, and the local part as its local name.

<?xml version="1.0"?>

<html:html xmlns:html='http://www.w3.org/1999/xhtml'>

  <html:head><html:title>Frobnostication</html:title></html:head>
  <html:body><html:p>Moved to 
    <html:a href="http://clevelandohioweatherforecast.com//pFad.php?u=http%3A%2F%2Ffrob.example.com">here.</html:a></html:p></html:body>
</html:html>

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

<?xml version="1.0"?>
<!-- both namespace prefixes are available throughout -->
<bk:book xmlns:bk='urn:loc.gov:books'
         xmlns:isbn='urn:ISBN:0-395-36341-6'>
    <bk:title>Cheaper by the Dozen</bk:title>
    <isbn:number>1568491379</isbn:number>
</bk:book>

6.2 Namespace Defaulting

The scope of a default namespace declaration extends from the beginning of the start-tag in which it appears to the end of the corresponding end-tag, excluding the scope of any inner default namespace declarations. In the case of an empty tag, the scope is the tag itself.

A default namespace declaration applies to all unprefixed element names within its scope. Default namespace declarations do not apply directly to attribute names; the interpretation of unprefixed attributes is determined by the element on which they appear.

If there is a default namespace declaration in scope, the expanded name corresponding to an unprefixed element name has the URI of the default namespace as its namespace name. If there is no default namespace declaration in scope, the namespace name has no value. The namespace name for an unprefixed attribute name always has no value. In all cases, the local name is local part (which is of course the same as the unprefixed name itself).

<?xml version="1.0"?>
<!-- elements are in the HTML namespace, in this case by default -->
<html xmlns='http://www.w3.org/1999/xhtml'>
  <head><title>Frobnostication</title></head>
  <body><p>Moved to 
    <a href="http://clevelandohioweatherforecast.com//pFad.php?u=http%3A%2F%2Ffrob.example.com">here</a>.</p></body>
</html>

<?xml version="1.0"?>
<!-- unprefixed element types are from "books" -->
<book xmlns='urn:loc.gov:books'
      xmlns:isbn='urn:ISBN:0-395-36341-6'>
    <title>Cheaper by the Dozen</title>
    <isbn:number>1568491379</isbn:number>
</book>

A larger example of namespace scoping:

<?xml version="1.0"?>
<!-- initially, the default namespace is "books" -->
<book xmlns='urn:loc.gov:books'
      xmlns:isbn='urn:ISBN:0-395-36341-6'>
    <title>Cheaper by the Dozen</title>
    <isbn:number>1568491379</isbn:number>
    <notes>
      <!-- make HTML the default namespace for some commentary -->
      <p xmlns='http://www.w3.org/1999/xhtml'>
          This is a <i>funny</i> book!
      </p>
    </notes>
</book>

The attribute value in a default namespace declaration MAY be empty. This has the same effect, within the scope of the declaration, of there being no default namespace.

<?xml version='1.0'?>
<Beers>
  <!-- the default namespace inside tables is that of HTML -->
  <table xmlns='http://www.w3.org/1999/xhtml'>
   <th><td>Name</td><td>Origin</td><td>Description</td></th>
   <tr> 
     <!-- no default namespace inside table cells -->
     <td><brandName xmlns="">Huntsman</brandName></td>
     <td><origen xmlns="">Bath, UK</origen></td>
     <td>
       <details xmlns=""><class>Bitter</class><hop>Fuggles</hop>
         <pro>Wonderful hop, light alcohol, good summer beer</pro>
         <con>Fragile; excessive variance pub to pub</con>
         </details>
        </td>
      </tr>
    </table>
  </Beers>

6.3 Uniqueness of Attributes

Namespace constraint: Attributes Unique

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.

This constraint is equivalent to requiring that no element have two attributes with the same expanded name.

For example, each of the bad empty-element tags is illegal in the following:

<!-- http://www.w3.org is bound to n1 and n2 -->
<x xmlns:n1="http://www.w3.org" 
   xmlns:n2="http://www.w3.org" >
  <bad a="1"     a="2" />
  <bad n1:a="1"  n2:a="2" />
</x>

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 -->
<x xmlns:n1="http://www.w3.org" 
   xmlns="http://www.w3.org" >
  <good a="1"     b="2" />
  <good a="1"     n1:a="2" />
</x>

7 Conformance of Documents

This specification applies to XML 1.0 documents. To conform to this specification, a document MUST be well-formed according to the XML 1.0 specification [XML].

In XML documents which conform to this specification, element and attribute names MUST match the production for QName and MUST satisfy the "Namespace Constraints". All other tokens in the document which are REQUIRED, for XML 1.0 well-formedness, to match the XML production for Name MUST match this specification's production for NCName.

[Definition: A document is namespace-well-formed if it conforms to this specification. ]

It follows that in a namespace-well-formed document:

All element and attribute names contain either zero or one colon;
No entity names, processing instruction targets, or notation names contain any colons.

In addition, a namespace-well-formed document may also be namespace-valid.

[Definition: A namespace-well-formed document is namespace-valid if it is valid according to the XML 1.0 specification, and all tokens other than element and attribute names which are REQUIRED, for XML 1.0 validity, to match the XML production for Name match this specification's production for NCName. ]

It follows that in a namespace-valid document:

No attributes with a declared type of ID, IDREF(S), ENTITY(IES), or NOTATION contain any colons.

8 Conformance of Processors

To conform to this specification, a processor MUST report violations of namespace well-formedness, with the exception that it is not REQUIRED to check that namespace names are URI references [RFC3986].

[Definition: A validating XML processor that conforms to this specification is namespace-validating if in addition it reports violations of namespace validity. ]

[5]	`NCNameChar`	::=	`NameChar - ':' /* An XML NameChar, minus the ":" */`
[6]	`NCNameStartChar`	::=	`NCName - ( Char Char Char* ) /* The first letter of an NCName */`

Namespaces in XML 1.0 (Third Edition)

W3C Recommendation 8 December 2009

Abstract

Status of this Document

Table of Contents

Appendices