Namespaces in XML 1.1

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.

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

[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.] The attribute value may be an empty string.

[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.] The attribute value may an empty string. 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 http://ecommerce.org/schema:

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

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

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

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

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

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

Attribute

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

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

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

5 Applying Namespaces to Elements and Attributes

<?xml version="1.0"?>
<!-- all elements here are explicitly in the HTML namespace -->
<html:html xmlns:html='http://www.w3.org/TR/REC-html40'>
  <html:head><html:title>Frobnostication</html:title></html:head>
  <html:body><html:p>Moved to 
    <html:a href='http://frob.com'>here.</html:a></html:p></html:body>
</html:html>

<?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>

<?xml version="1.0"?>
<x xmlns:n1="http://www.w3.org">
    <n1:a/>               <!-- legal; the prefix n1 is bound to http://www.w3.org -->
    <x xmlns:n1="">
        <n1:a/>           <!-- illegal; the prefix n1 is not bound here -->
	<x xmlns:n1="http://www.w3.org">
            <n1:a/>       <!-- legal; the prefix n1 is bound again -->
        </x>
    </x>
</x>

<?xml version="1.0"?>
<!-- elements are in the HTML namespace, in this case by default -->
<html xmlns='http://www.w3.org/TR/REC-html40'>
  <head><title>Frobnostication</title></head>
  <body><p>Moved to 
    <a href='http://frob.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>

<?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='urn:w3-org-ns:HTML'>
          This is a <i>funny</i> book!
      </p>
    </notes>
</book>

<?xml version='1.0'?>
<Beers>
  <!-- the default namespace is now that of HTML -->
  <table xmlns='http://www.w3.org/TR/REC-html40'>
   <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><origin xmlns="">Bath, UK</origin></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>

<!-- 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>

<!-- 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>

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.