xml:id Version 1.0

1 Introduction

[XML 1.0] and [XML 1.1] provide a mechanism for annotating elements with unique identifiers. This mechanism consists of declaring the type of an attribute as "ID", after which the parser will validate that

the ID value matches the allowed lexical form,
the value is unique within the XML document, and that
each element has at most a single unique identifier

Declarations in either the internal or external subset of an XML document can declare attributes to be of type ID. However, some specifications, notably [SOAP], forbid an internal subset, and processing the external subset is optional for conformant XML processors, leaving no guarantee that all consumers of the XML document will be able to successfully recognize the identifiers.

Identifiers can be declared through external mechanisms as well. Of particular interest is [XML Schemas] which provides a type "xs:ID" with the same uniqueness and validity constraints as XML. However, there are no guarantees that consumers will have the "correct" schema available, nor that they will process it if they do.

A mechanism allowing unique element identifiers to be recognized by all conformant XML processors, whether they validate or not, is desirable in making XML sub-resource linking robust.

An additional problem is that DTD-based and XML Schema-based identifiers are exposed through different conceptual mechanisms - the [attribute type] infoset property, and the [type definition] family of properties respectively. A uniform mechanism for recognizing identifiers is desirable.

This specification provides such a mechanism: it describes the semantics of xml:id attributes. This specification has been designed to be a separate layer in processing and to be compatible with existing validation technologies. Implementors are encouraged to support xml:id processing and to make ID assignment the default behavior of their processors.

2 Terminology

[Definition: The key words must, must not, required, shall, shall not, should, should not, recommended, may, and optional in this specification are to be interpreted as described in [IETF RFC 2119].]

[Definition: An xml:id processor is a software module that works in conjunction with an XML processor to provide access to the IDs in an XML document.]

[Definition: An xml:id error is a non-fatal error that occurs when an xml:id processor finds that a document has violated the constraints of this specification.]

Validation is the process of comparing an XML document (or part of an XML document) against a grammar or set of rules to determine if the actual structure of the document satisfies the constraints of the grammar or the rules. Some validation technologies also perform type assignment, determining not only if the document satisfies the specified constraints but also determining, for example, which (elements and/or) attributes are of type “ID”.

Although often performed together, validation and type assignment are not the same process. A non-validating XML 1.0 processor, for example, can perform type assignment using only declarations from the internal subset, without ever having any information about the structural validity of the document.

[Definition: The process of ID assignment associates an ID with an element.] This is often achieved by promoting the type of an untyped element or attribute value to a value of type “ID”, but that is not the only possible mechanism.

3 Syntax

Per [Namespaces in XML] (and [Namespaces in XML 1.1]), prefixes beginning “xml” are reserved for use by XML and XML-related specifications. This specification licenses the use of the attribute “xml:id” for use as a common syntax for identifiers in XML with the semantics specified herein.

Authors of XML documents are encouraged to name their ID attributes "xml:id" to increase the interoperability of these identifiers on the Web.

In namespace-aware XML processors, the "xml" prefix is bound to the namespace name http://www.w3.org/XML/1998/namespace as described in Namespaces in XML [Namespaces in XML] (and [Namespaces in XML 1.1]). Note that xml:id can be still used by non-namespace-aware XML processors.

4 Processing `xml:id` Attributes

Each xml:id attribute is processed in the following way:

The attributeʼs value is normalized according to the rules for attribute-value normalization on attributes of type ID. For xml:id processors operating on an infoset or some other output from an XML parser, the value will already be normalized, but unless the parser normalized it as a value of type ID it will still be necessary for the processor to trim leading and trailing space (#x20) characters and replace sequences of space characters by a single space.
ID assignment is performed.

An xml:id processor must assure that the following constraints hold for all xml:id attributes:

The normalized value of the attribute is a valid NCName.
The declared type of the attribute, if it has one, is “ID”. All declarations for xml:id attributes must specify “ID” as the type of the attribute.

An xml:id processor should assure that the following constraints hold:

The values of all xml:id attributes and all attributes of type “ID” within a document are unique.

An xml:id error occurs for any xml:id attribute that does not satisfy the constraints.

The xml:id processor performs ID assignment on all xml:id attributes, even those that do not satisfy the enumerated constraints.

An xml:id processor should update the [references] infoset property, and/or other appropriate referential properties to reflect the results of ID assignment.

Many validation technologies impose the constraint that an XML element can have at most one attribute of type ID. That constraint is not imposed by xml:id processing.

5 Informing the Application

ID assignment may be performed when xml:id attributes are processed. If ID assignment occurs, then the xml:id processor must report the assigned xml:id attributes to the application. How this is reported is implementation dependent.

For applications that operate conceptually on the Infoset, an xml:id processor can use the [attribute type] Infoset property:
The xml:id processor may report the results of ID assignment in a DTD compatible manner by setting the [attribute type] infoset property of the attribute to ID.
For applications that operate conceptually on the PSVI, an xml:id processor can use the [type definition] family of PSVI properties:
The xml:id processor may report the results of ID assignment in an XML Schema compatible manner by setting the PSVI [type definition] property of the attribute to xs:ID.
For applications that operate on data models defined in other ways, the mechanisms are implementation dependent:
The xml:id processor may report the results of ID assignment in some other way.

The key requirement is that the application be made aware of the results of ID assignment.

6 Errors

A violation of the constraints in this specification results in an xml:id error. Such errors are not fatal, but must be reported by the xml:id processor to the application invoking it.

7 Conformance

7.1 Conformance to xml:id

Conformance to xml:id for applications that rely on XML processors using validation technologies consists in the use of the xml:id construct as explained in 4 Processing xml:id Attributes and by conformance to both the constraints of this specification and the rules of the validation technology.

Conformance to xml:id for applications that rely on non-validating XML processors is defined by the recognition of xml:id attributes as explained in 4 Processing xml:id Attributes and by conformance to the constraints of this specification.

7.2 XML Information Set Conformance

This specification conforms to the [XML Information Set]. The following information items must be present in the input infosets to enable correct processing:

Element Information Items with [attributes] property.
Attribute Information Items with [namespace name], [local name] and [normalized value] properties.

In addition, the following properties might be present in the output infoset:

[attribute type] properties on Attribute Information Items.

A References

IETF RFC 2119: RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. Internet Engineering Task Force, 1997. (See http://www.ietf.org/rfc/rfc2119.txt.)
XML 1.0: Tim Bray, Jean Paoli, C.M. Sperberg-McQueen, et. al., editors. Extensible Markup Language (XML) 1.0 (Third Edition). World Wide Web Consortium, 2004. (See http://www.w3.org/TR/REC-xml/.)
XML 1.1: Tim Bray, Jean Paoli, C.M. Sperberg-McQueen, et. al., editors. Extensible Markup Language (XML) 1.1. World Wide Web Consortium, 2004. (See http://www.w3.org/TR/xml11/.)
XML Information Set: John Cowan and Richard Tobin, editors. XML Information Set (Second Edition). World Wide Web Consortium, 2004. (See http://www.w3.org/TR/xml-infoset/.)
Namespaces in XML: Tim Bray, Dave Hollander, and Andrew Layman, editors. Namespaces in XML. World Wide Web Consortium, 1999. (See http://www.w3.org/TR/REC-xml-names/.)
Namespaces in XML 1.1: Tim Bray, Dave Hollander, Andrew Layman, and Richard Tobin, editors. Namespaces in XML 1.1. World Wide Web Consortium, 2004. (See http://www.w3.org/TR/xml-names11/.)

B References (Non-Normative)

XML Schemas: Henry S. Thompson, David Beech, Murray Maloney, Noah Mendelsohn, editors. XML Schema Part 1: Structures. World Wide Web Consortium, 2001. (See http://www.w3.org/TR/xmlschema-1/.)
SOAP: Martin Gudgin, Marc Hadley, Noah Mendelsohn, et. al., editors. SOAP Version 1.2 Part 1: Messaging Framework. World Wide Web Consortium, 2003. (See http://www.w3.org/TR/soap12-part1/.)

C Impacts on Other Standards (Non-Normative)

XPath 1.0: The id() function only recognizes IDs declared in the DTD. If xml:id processing is performed before the document is provided to the XPath 1.0 processor, and if ID assignment is performed by setting the [attribute type] infoset property, XPath 1.0 will recognize xml:id attributes as IDs. An erratum to XPath 1.0 would be required to allow Schema-declared IDs to be included in the results of this function.
XPath 2.0: No change required. The id() function recognizes both DTD- and Schema-declared identifiers, and as such would also recognize xml:id attributes identified with a minimally conforming schema processor.
Fragment Identifiers and the XPointer Framework: No change required. Barename fragment identifiers and the element() scheme recognize both DTD- and Schema-declared identifiers, and as such would also recognize xml:id attributes identified with a minimally conforming schema processor.
DOM: To the extent that a DOM instance is constructed from an Infoset or PSVI, no change is required. The xml:id processorʼs ID assignment of xml:id attributes will be visible to the DOM construction process.
CSS: To the extent that a CSS operates on a tree constructed from an Infoset or PSVI, no change is required. The xml:id processorʼs ID assignment of xml:id attributes will be visible to the construction process.

D Validation Technologies (Non-Normative)

This appendix describes how xml:id processing interacts with selected validation technologies.

D.1 With DTD Validation

DTD authors are encouraged to use xml:id exclusively in their DTDs to indicate element identifiers.

The following (non-normative) DTD fragment illustrates a sample declaration for the xml:id attribute:

<!ATTLIST someElement
    xml:id     ID          #IMPLIED
>

DTD authors must not declare xml:id as something other than ID. DTD authors who declare attributes other than xml:id as type ID will not receive the full benefits of xml:id processing. Such attributes may or may not be recognized processors, including xml:id processors.

Validating consumers of documents using properly declared xml:id attributes can recognize IDs through the [attribute type] property.

D.2 With XML Schema Validation

XML Schema authors are encouraged to use xml:id exclusively in their schemas to indicate element identifiers.

The following (non-normative) XML Schema fragment illustrates a sample declaration for the xml:id attribute:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           targetNamespace="http://www.w3.org/XML/1998/namespace">

    <xs:attribute name="id" type="xs:ID"/>

</xs:schema>

XML Schema authors must not declare xml:id as something other than xs:ID (for instance xs:string). XML Schema authors who declare attributes other than xml:id as type xs:ID will not receive the full benefits of xml:id processing. Such attributes may or may not be recognized processors, including xml:id processors.

Consumers of documents validating the xml:id attributes against an appropriate schema can recognize IDs through the [type definition] family of PSVI properties.

Applications can recognize xml:id attributes as IDs by conceptually using a Minimally Conforming Schema Processor processing the schema above.

Note that the effects of a Minimally Conforming Schema Processor, processing the above schema, are approximated by simply looking for attributes named xml:id, ensuring the value of such attributes has the correct lexical form (NCName), and the value is unique within the document.