XML Schema Working Group Charter

1. Scope

The XML 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. 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.

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.

Goals

The goals of the work group in the period described by this charter are five-fold:

to prepare a version 1.1 of the XML Schema Recommendation, which corrects known errors and may make modest improvements to the language. Function or syntax incompatible with XML Schema 1.0 will be adopted only if the resulting improvements compellingly justify the loss of interoperability with existing systems and documentation. Some substantive changes may be 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. Request for substantive changes may also come from other groups.
to provide continuing maintenance and support for XML Schema 1.0 and future versions of that specification. The Working Group has published and will maintain an Errata document 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 and future versions are also expected. We also plan to complete the document XML Schema: Formal Description, which provides a more formal treatment of schema-based type assignment and validation
to complete the document XML Schema: Component Designators
as needed and feasible, to clarify related problem areas, and do technical work leading possibly to Recommendations, possibly to Notes, and possibly to the determination that no publication is necessary, on technical questions connected with the design, implementation, use, and uptake of the XML Schema specification; 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

Additional areas of work

In addition to work directly related to XML Schema 1.1, technical areas relevant to the work under this charter include:

the systematic development of test cases for XML Schema
the framework and interoperability rules which should govern evolution of the XML Schema language
interoperability of schemas, data governed by them, and software, as user vocabularies evolve
provision of a sample library of common types, for illustration of the construction of type libraries using XML Schema and, as appropriate, for reference from schemas for W3C specifications
definition of operators on XML-Schema types, in collaboration with XML Query and XSL. (This work is currently driven by the other two Working Groups; XML Schema participates in order to provide expertise and help avert problems.)
use cases and requirements for a possible XML Schema 2.0
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.
the formalization of XML Schema in a manner suitable for use by XML Query and others
interoperability with other schema languages such as RELAX-NG and Schematron

Parallelism of work on 1.1 and preliminary discussion of 2.0

The XML Schema Working Group has repeatedly experienced tension between the desire to explore some proposal for a relatively major change in the design of the XML Schema language, and the desire to have a non-proprietary specification available for use as soon as possible. In order to allow adequate time for consideration of possible major changes in approach, the Working Group may (time permitting) do technical work on proposals intended for future versions of XML Schema (such as those listed above) in parallel with its work on XML Schema 1.1.

2. Deliverables, Milestones, and Schedule

Deliverables

The XML Schema Working Group has three main deliverables:

XML Schema version 1.1; this will be a maintenance release of the spec. It may differ from version 1.0 in incorporating:
- changes necessary to align the specification with the data model and type logic of XPath 2.0, XQuery 1.0, XSLT 2.0 and related specifications; these will include
  - definition of duration types with total orders
  - addition of an anyAtomic type
  - revision of the treatment of time zones in date/time types
  - revision of the float and double types to distinguish positive and negative zero values
  - reformulation of the definition of complex type restriction to remove anomalies
- changes necessary to provide better support for versioning of schemas
- fixes for errors in version 1.0
- clearer (and if possible briefer) exposition
- simplification and regularization of the abstract component level, where changes can be made without inconvenience to users of existing schemas or developers of existing implementations
- the definition of a limited-precision decimal type
Errata list for XML Schema version 1.0, with new editions of the 1.0 specification as needed
Schema Component Designators; this specification describes a syntax for referring to schema components

In addition, there are several other deliverables:

preliminary work on XML Schema 2.0: use cases, usage scenarios, and requirements; it is not expected that XML Schema 2.0 will be completed within this charter period
a collection of test cases for XML Schema, together with a list of other XML Schema test suites and a report on coverage
possible documents stemming from the various ‘related problem areas’ mentioned above:
- a formal description of XML Schema
- sample library of common types
- work on schema annotation
- working papers on other topics

Milestones and schedule

Listed here are the major expected milestones for the Working Group's deliverables; for a summary of the current target dates for these milestones, see the XML Schema Working Group home page.

Milestones for XML Schema 1.1:

first public working draft
revision of 1.1 Requirements document
second public working draft
last-call working draft
Candidate Recommendation
Proposed Recommendation
Recommendation

Milestones for XML Schema 1.0 errata:

regular re-publication of errata lists with (non-normative) corrections
periodic re-publication of XML Schema 1.0, integrating corrections

Milestones for XML Schema: Component Designators:

revised working draft(s)
last-call working draft
Candidate Recommendation
Proposed Recommendation
Recommendation

Milestones for work on XML Schema 2.0:

first working draft(s) of use cases and requirements document(s)
stable use cases and requirements document(s)
possible working papers on high-level design and architecture principles for XML Schema 2.0
decision on whether to proceed with XML Schema 2.0
(if appropriate) first public working draft

Milestones for work on XML Schema: Formal Description:

revised working draft(s)
last-call working draft
Candidate Recommendation
Proposed Recommendation
Recommendation

Milestones for work on XML Schema test collections:

agreement on disclosure statement to be made by test contributors
creation of collection of donated tests
public release of test collection
period integration of new tests, new releases of test collection
creation of collection of test results
collection of information about other available test collections
report on coverage of test collection(s)

Note that public working drafts will be made available at least once every three months, in accordance with the W3C Process.

Expected schedule:

Q3 2004: first public working draft of XML Schema 1.1
Q3 2004: revised XML Schema 1.1 Requirements document
Q3 2004: public working draft of XML Schema: Component Designators
Q3 2004: first public release of new XML Schema test collection (periodic republication as new contributions warrant)
Q4 2004: public working draft of XML Schema 1.1
Q4 2004: last-call working draft of XML Schema: Component Designators
Q4 2004: Candidate Recommendation draft of XML Schema: Component Designators
Q1 2005: last-call working draft of XML Schema 1.1
Q1 2005: report on existing test collections, coverage
Q1 2005: Proposed Recommendation draft of XML Schema: Component Designators
Q1 2005: Candidate Recommendation draft of XML Schema 1.1
Q2 2005: XML Schema: Component Designators Recommendation
Q2 2005: first public working draft of XML Schema 2.0 use cases and requirements
Q3 2005: Proposed Recommendation draft of XML Schema 1.1
Q3 2005: XML Schema 1.1 Recommendation
Q4 2005: decision on whether to proceed with XML Schema 2.0 (and possible request for charter alteration)

For details of dates and locations of face to face meetings, see the Working Group home page.

The expiration date of this charter is 30 June 2006.

3. Relationship with Other Activities

XML has become a strategic technology in W3C and elsewhere. The deliverable 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; the requirements posed for the Schema work by these Working Groups may change 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, in particular types. 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.
HTML and XForms Working Groups: It is a goal of the XML Schema work to ensure that the schema language we define will suffice for the requirements of current and future versions of the HTML specification and XForms.
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.
RDF Core Working Group: The RDF Core Working Group is preparing a new version of RDF Schema, which will use and build upon XML Schema datatypes. The RDF Core Working Group is also chartered to "provide an account of the relationship between RDF and the XML family of technologies (particularly Schemas ...)". RDF has also expressed particular interest in the problem of schema annotation and will be asked to provide feedback on the work the XML Schema Working Group does in that area.
Web Services Working Groups: The XML Protocol Working Group and the Web Services Description Working Group in the Web Services Activity use XML Schema extensively. The Web Services Choreography Working Group is working in an area which may be expected to raise complex questions for type systems and schemas. 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, and, where appropriate, direct liaison with task forces such as the Semantic Web Best Practicies Working Group XML Schema Datatypes and Semantic Web Task Force.

4. Working Group participation

To be successful, we expect the XML Schema Working Group to have 15 or more active participants for its duration. Participation is expected to consume one to two days per week of each participant's time, though the time commitment for the chair and editors may be higher. The list of members and details about joining are on the group home page.

The level of participation expected for this Working Group suggests that representatives be experts in the development or exploitation of schemas for XML-based markup languages. In addition to the time commitment noted above, each participant of the Working Group will be expected to show evidence to the Chair, each six months, of active work in the general topic area. Such active cultivation may take the form of:

development of at least one non-trivial schema, using XML Schema 1.0 or a draft under development
development of at least a small piece of software which processes XML Schema documents, or participation in the development of larger software packages which support XML Schema.

Chair

The initial Chair of the XML Schema Working Group is David Ezell, National Association of Convenience Stores.

W3C Team resources

The initial W3C Team contacts for this Working Group are C. M. Sperberg-McQueen and Henry Thompson. It is expected that this Working Group would consume about 0.9 FTE, including administrative logistics.

Charter of the XML Schema Working Group

[June 2004]