XML Schema Working Group Charter

The mission of the XML Schema Working Group, part of the Extensible Markup Language (XML) Activity, is to maintain and revise the XML Schema specifications, addressing primitive data typing, structural constraints, and conformance.

Join the XML Schema Working Group.

End date	31 December 2007
Confidentiality	Proceedings are Member-only
Initial Chairs	David Ezell
Initial Team Contacts (FTE %: 45)	Michael Sperberg-McQueen
Usual Meeting Schedule	Telcons: Weekly Ftf: 5-6 per year

Scope

The XML 1.0 specification defines the concepts of well-formedness and validity; the former characteristic is very simple to check, while the latter requires more work to check, and allows the user to define more powerful constraints on document structure. XML validity requires that a document follow the constraints expressed in its document type definition, which provides the rough equivalent of a context-free grammar for a document type.

In some contexts, there is need for constraints tighter than, looser than, or simply different from those which can be expressed using document type definitions as defined in XML 1.0. There is also a widespread desire for an XML-based syntax for expressing constraints on document types, in order to allow tools for XML documents to be used on the constraint specifications.

Validation (and schema validation in particular) provide far more than mere authoring convenience. Recent research has shown that XML processing systems optimized for a specific set of validation constraints (i.e. an XML language) can out-perform standard generic processors by a wide margin.

Using a term borrowed from database terminology, the expression of constraints on a document type is called (in this document, at least) a schema. (Note that the term schema has two plurals, schemas and schemata; we use the former.)

The XML Schema working group will maintain and revise the XML Schema specification developed beginning in 1998 and published as a W3C Recommendation on 2 May 2001. The XML Schema specification addresses several topics relevant to users of XML:

primitive data typing: XML Schema defines a type system for integers, dates, and the like, based on experience with SQL, Java primitives, etc.
structural constraints: XML Schema provides a mechanism roughly analogous to DTDs for constraining document structure (order, occurrence of elements, attributes). XML Schema differs from DTDs in expressing these constraints in the form of an XML document, rather than in a special-purpose ad hoc notation. Unlike DTDs, which contain declarations both for the logical structure of elements and attributes, and for the physical structure of entities, XML Schema provides declarations only for constraining elements and attributes; entity declarations continue to use DTD syntax. In addition to DTD-like functionality, XML Schema:
- is integrated with namespaces;
- allows incomplete constraints on the content of an element type in the form of wildcards, so-called ‘black box’ validation, and so-called ‘white box’ validation;
- is integrated with primitive data types;
- defines inheritance for datatypes;
- provides various hooks for safe adaptation and reuse of schemas.
conformance: XML Schema defines the relation of schemas to XML document instances and specifies the obligations of schema-aware processors.

Technical areas relevant to the work under this charter include:

the systematic development, documentation, and publication of test cases for XML Schema
a co-constraint language for XML Schema.
mechanisms and guidelines for facilitiating XML Schema versioning.
use cases and requirements for a possible XML Schema 1.2
schema annotation and the specification of mappings which allow software to translate data from XML document instances which conform to a given schema into other formalisms or data models, such as application-specific data structures, generic data structures (e.g. relational tables), the entity-relationship model, and RDF. This work follows on from the discussions reported in the Cambridge Communiqué of 1999, with the aim of showing how the goals described there can be achieved.
interoperability of schemas, data governed by them, and software, as user vocabularies evolve (in cooperation with the XML Schema Patterns for Databinding WG).
the framework and interoperability rules which should govern evolution of the XML Schema language
interoperability with other schema languages such as RELAX-NG and Schematron
examination of specific use cases for XML Schema (such as composing RDF identifiers) and technical discussion of possible solutions.

Goals

The goals of the work group in the period described by this charter are as follows:

to finish publication of version 1.1 of the XML Schema Recommendation, which corrects known errors and makes modest improvements to the language, and do preparatory work for a possible version 1.2. Changes in function or syntax incompatible with XML Schema 1.0 have been / will be made only if the resulting improvements compellingly justify the loss of interoperability with existing systems and documentation. Some substantive changes have been made in the interests of aligning version 1.1 with the needs of the XML Query 1.0, XPath 2.0, and XSLT 2.0 family of specifications and with XML 1.1. Requests for substantive changes may also come from other groups.
to maintain and support XML Schema 1.0, XML Schema 1.1, and future versions of each specification. The Working Group has published Errata in Bugzilla containing corrections of identified errors and clarifications of points which experience shows are obscure to careful readers. Corrected re-editions of XML Schema 1.0, XML Schema 1.1, and future versions are also expected.
to finalize and publish the document XML Schema: Component Designators
as needed and feasible, to clarify related problem areas, and do technical work leading possibly to Notes on technical questions connected with the design, implementation, use, and uptake of the XML Schema specifications; some problem areas of concern are listed below.
to cooperate with other W3C working groups and assist them in exploiting XML Schema for their work and adding schema-awareness to their specifications where appropriate; this will include work on the problem of schema-annotation and the problem of mapping from the XML vocabularies defined by a schema into target data models
to publish documents that provide outreach and education in support of the XML Schema specifications, possibly including Notes on “best practices” and “how-to” for specific problems, for instance “how to compose RDF identifiers.”
to work on and possibly publish a version of XML Schema 1.0 or XML Schema 1.1 syntax that is easier to author without any special tools.

Success Criteria

The Working Group expects to demonstrate at least 2 interoperable implementations of all required and optional features before requesting to advance to Proposed Recommendation.

Deliverables

A set of W3C Recommendations for:

The Working Group may decide to publish XML Schema 1.1 with only the datatypes section, and postpone any changes to XML Schema: Structures until a possible fuuture version 1.2.

The following document will become Working Group Notes:

XML Schema: Formal Description
Requirements for XML Schema 1.1 (WD)
How to Version XML Languages defined in XML Schema.

Other Deliverables

Errata list for XML Schema version 1.0 and XML Schema 1.1, with new editions of the 1.0 and 1.1 specification as needed
Interoperability test suite for the specifications, intended to assess the accuracy of the Candidate Recommendations, and to promote interoperability.

Milestones

Milestones
Specification	FPWD	LC	CR	PR	Rec
Note: The group will document significant changes from this initial schedule on the group home page.
XML Schema 1.1 Datatypes	N/A	N/A	April 2007	August 2007	October 2007
XML Schema 1.1 Structures	N/A	November 2006	April 2007	August 2007	October 2007

Timeline View Summary

August 2006: face-to-face meeting, hosted by Microsoft, WA, USA.
November 2006: XML Schema 1.1 Structures LC
April 2007: XML Schema 1.1 Datatypes and Structures CR
August 2007: XML Schema 1.1 Datatypes and Structures PR
October 2007: XML Schema 1.1 Datatypes and Structures REC

Dependencies

W3C Groups

XML has become a strategic technology in W3C and elsewhere. The deliverables of the XML Schema Working Group must satisfy the dependencies from other W3C Working Groups. Some dependencies to and from other W3C Working Groups will require close cooperation during the development process; history has shown that the requirements posed for the Schema work by these Working Groups may change substantially during the development process, which means the interdependency of the Schema work with these Working Groups must be managed actively:

XML Query Working Group: The XML Query Working Group expects to use schema-related information in the processing of queries; we will discuss points of mutual interest in an effort to ensure that XML Schema provides the information Query needs. In particular, the XML Query Working Group has expressed particular interest in the definition of the post-schema-validation infoset (PSVI). The XML Schema Working Group will continue to collaborate with XML Query and XSL in work on operators for XML Schema datatypes and (as appropriate) on the integration of support for the XML Schema type inventory into future versions of XPath. An important goal of XML Schema 1.1 will be to align its type system more fully with the needs of the XML Query language and related specifications. Work on XQuery 1.1, updates, and full-text search may raise schema-related problems, and the Working Groups will cooperate in resolving them.
XSL Working Group: In collaboration with the XML Query Working Group, the XSL Working Group expects to make future versions of XPath and XSLT support schema-related information, specifically type information. The XML Schema Working Group will continue to collaborate with XML Query and XSL in work on operators for XML Schema datatypes and (as appropriate) on the integration of support for the XML Schema type inventory into future versions of XPath. An important goal of XML Schema 1.1 will be to align its type system more fully with the needs of the XSLT and XPath 2.0 languages and related specifications.
XML Core Working Group: The XML Schema work defines methods for specifying constraints on XML documents; those constraints should apply to objects and properties identified as significant by the XML Information Set specification.The XML structural schema specification uses namespaces to combine fragmentary structural specifications; it also defines schema-validation for documents using namespaces. (In short: the XML Schema work is namespace-aware.)
Internationalization Working Group: Since XML Schema is expected to be an important tool for the definition of markup vocabularies, it may have follow-on effects on a larger number of people than create XML Schema documents directly. The XML Schema Working Group and the Internationalization Working Group will work together to clarify and resolve internationalization and localization issues in XML Schema, and will jointly ensure that it satisfies W3C goals for international access to the Web.
Web Services Working Groups: XML Protocol and Web Services Description Language specified in the Web Services Activity use XML Schema extensively. The newly formed "XML Schema Patterns for Databinding Working Group" is working to produce specifications to ease interoperability in Web Services where XML Schema is involved. The XML Schema Working Group will ensure that the schema language is sufficient to meet the requirements of deliverables from these Web Services Working Groups. The XML Schema Working Group will also discuss points of mutual interest and will review deliverables from the Web Services Working Groups and provide them with feedback.

Some other work groups should be mentioned here; there are no requirements for co-development of features with these Working Groups, but there are points of contact between their work and that of this Working Group, and thus logical dependency between their deliverables and those of this Working Group. Requirements from these Working Groups are expected to be well suited for communication via documents.

WAI Protocols & Formats Working Group: Reuse of common constructs greatly facilitates accessibility; the WAI PF Working Group will review work on structural schemas to be sure cost/benefit design decisions are informed of the benefits of accessibility.

Formal liaison between the XML Schema Working Group and other W3C Working Groups, including the other XML Working Groups and the WAI (Web Accessibility Initiative) group, as well as organizations outside of the W3C, shall normally be accomplished by the exchange of documents (requirements, reviews, etc.) transmitted through the XML Coordination Group.

When approved by the XML Coordination Group, liaison with other W3C Working Groups can be accomplished through direct exchange of documents or by joint task forces. It is expected that this be required for liaison with at least the XML Query, XSL, and Internationalization Working Groups.

External Groups

ISO/IEC JTC1 SC34: This group is responsible for ISO/IEC 19757 DSDL (Document Schema Definition Languages). In particular since Schema 1.0 datatypes are used by RelaxNG, and RelaxNG might want to use the 1.1 data types as well.

Participation

To be successful, the XML Schema Working Group is expected to have 6 or more active participants for its duration. Effective participation to XML Schema Working Group is expected to consume half work day per week for each participant; two days per week for editors. The XML Schema Working Group will allocate also the necessary resources for building Test Suites for each specification.

Participants are reminded of the Good Standing requirements of the W3C Process.

Communication

This group primarily conducts its work on the Member-only mailing list w3c-xml-schema-wg@w3.org (archive).

Information about the group (deliverables, participants, face-to-face meetings, teleconferences, etc.) is available from the XML Schema Working Group home page.

Patent Policy

This Working Group operates under the W3C Patent Policy (5 February 2004 Version). To promote the widest adoption of Web standards, W3C seeks to issue Recommendations that can be implemented, according to this policy, on a Royalty-Free basis.

For more information about disclosure obligations for this group, please see the W3C Patent Policy Implementation.

About this Charter

This charter for the XML Schema Working Group has been created according to section 6.2 of the Process Document. Except as specified in this charter, the Working Group follows the Common Procedures for XML Working Groups. In the event of a conflict between this document, the common procedures, or the provisions of any charter and the W3C Process, the W3C Process shall take precedence.

Please also see the previous charter for this group.

Michael Sperberg-McQueen, Team Contact

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