The W3C XML Schema Working Group has published a revised working draft of the XML Schema: Component Designators specification. W3C XML Schemas "express shared vocabularies and allow machines to carry out rules made by people; they provide a means for defining the structure, content and semantics of XML documents."

The Component Designators draft defines a scheme for identifying XML Schema components as described in the "XML Schema Abstract Data Model" of XML Schema Part 1: Structures. A schema component is the generic term for the building blocks that comprise the abstract data model of the schema. The revised Component Designators working draft introduces a three-layer model for schema component reference corresponding to a 3-layer model of schema processor conformance. The scheme divides the thirteen (13) kinds of schema components into: components in context, schema assembly, and schema reference. "Schema component designators thus on a related 3-layer model of schema component reference. Layer 1, the reference core, specifies the manner in which schema component paths relate to schema components; Layer 2 specifies the mapping of a set of schema documents to a schema; Layer 3 provides a set of conventions for the identifying such a set of schema documents on the Web."

The new draft is also reorganized, with extended and clarified use-cases and requirements; it now allows for non-canonical schema component paths. The draft specification has been produced by the W3C XML Schema Working Group as part of the W3C XML Activity and is on the W3C Recommendation track.

Bibliographic Information

XML Schema: Component Designators. W3C Working Draft 9-March-2004. Edited by Mary Holstege (Cerisent) and Asir S. Vedamuthu (webMethods). Produced by members of the W3C XML Schema Working Group. Version URL: http://www.w3.org/TR/2004/WD-xmlschema-ref-20040309/. Latest version URL: http://www.w3.org/TR/xmlschema-ref/. Previous version URL: http://www.w3.org/TR/2003/WD-xmlschema-ref-20030109/.

Overview of Schema Designators

"Section 4 of XML Schema Part 1: Structures specifies a 3-layer model of schema processor conformance. Layer 1 describes schema assessment in the context of a set of assembled schema components. Layer 2 describes assembly of schemas from schema documents. Layer 3 describes some conventions for accessing schema documents over the World Wide Web. Schema component designators rely on a related 3-layer model of schema component reference:

Layer 1: Reference Core. The schema component reference core defines reference to a schema component in the context of an assembled XML Schema. Schema Component Paths defines this relationship between an assemblage of schema components and component paths. For the purposes of component paths, a missing component cannot be used to construct a valid path, nor can a schema component be successfully referenced through a path that references a missing component.

Layer 2: Schema Assembly. For our purposes, layer 2 is about assembling a schema from a collection of schema documents. That set of schema documents may have been obtained in a number of ways, including:

1. from a single root schema document that imported or included other documents
2. from a set of schema elements embedded in or referred to in some start document (a WSDL document, for example)
3. from some kind of package format defining the collection
4. from the set of schema documents a processor deems necessary to perform schema assessment on a particular instance document

Layer 3: Schema URI. A schema designator is a single URI that represents a collection of schema documents forming a schema. Layer 3 notes conventions for obtaining a single URI to refer to a collection of schema documents forming a schema. We expect that it will be highly desirable for the community to evolve one convention for doing this to ensure consistency of global schema component designators. This specification declines to specify what that one way should be. The URI on the left hand side of the schema component designator should be a URI of an actual document, in some media type. That media type should be some XML derivative, so that the XPointer framework applies. For any of the cases outlined above, there is a single root XML document, whose URI can be taken as the starting point for assembling the schema and from there resolving the component path. In the simplest case, where there is one root schema document, the URI of that document suffices. It therefore follows that a distinct copy of that schema document at some other location will give rise to different absolute schema component designators..." [adapted from spec section 4]

About the W3C XML Schema Working Group

The W3C XML Schema Working Group is "currently working to develop a set of requirements for XML Schema 1.1, which is intended to be mostly compatible with XML Schema 1.0 and to have approximately the same scope. The Working Group has also done work on the post-schema-validation Infoset. There has also been work on a formal description of W3C XML Schema. The XML Schema Working Group has also held joint meetings with the XML Query Working Group and the XSL Working Group in order to ensure that the XPath 2.0 data model and type system is consistent with W3C XML Schema 1.1..."