Charter of the XML Schema Working Group

April 2002

C. M. Sperberg-McQueen
and Dave Hollander, chairs
C. M. Sperberg-McQueen, staff contact
$Revision: 3.6 $ of $Date: 2002/08/21 16:33:54 $ by $Author: liam $

The XML Schema Working Group is a Working Group of the W3C and follows the working group process described in the W3C Process Document. Except as outlined elsewhere in this charter, the Working Group follows the Common Procedures for XML Working Groups.

This is the charter for this Working Group. It extends and supersedes the Working Group's previous charter.

1. Scope
2. Deliverables and Schedule
3. Relationship with Other Activities
4. WG Membership
5. Communication
6. Confidentiality
7. Face-to-Face Meetings
8. W3C staff resource commitment

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.

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.

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

• to provide continuing maintenance and support for the XML Schema 1.0 specification, in the form of an Errata document containing corrections of identified errors and clarifications of points which experience shows are obscure to careful readers, and to complete the document XML Schema: Formal Description, which provides a formal treatment of schema-based type assignment and validation in terms of a Gentzen calculus
• to prepare a version 1.1 of the XML Schema Recommendation, which corrects known errors and may make modest improvements to the language, while retaining compatibility with version 1.0
• to clarify various related problem areas, and do technical work leading possibly to Recommendations, possibly to Notes, and possibly to the determination that no publication is necessary; these related problem areas include:
  • the formalization of XML Schema in a manner suitable for use by XML Query and others
  • the definition of a free-standing specification describing how to name or refer to arbitrary components in an XML Schema; in existing discussions these are sometimes referred to as normalized universal names or NUNs
  • the application-programming interface (API) for the post-schema-validation (PSV) information set; this work is undertaken in response to comments on the Last-Call draft of XML Schema, and will be performed in consultation with the DOM WG. It is expected to be an extension of their generic interface to content models and related information
  • useful ways of handling the functionality provided in XML 1.0 DTDs by entities (with special reference to the use of entities to provide names for special characters)
  • the systematic development of test cases for XML Schema
  • 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 WGs; XML Schema participates in order to provide expertise and help avert problems.)
  • the study of various proposals for new architectural approaches to schema definition and schema validation, with an eye to their potential inclusion in future versions of XML Schema; these proposals include but are not limited to:
    • an even sharper separation between the level of abstract schema components and the XML transfer-syntax level than is present in XML Schema 1.0, possibly including separation of the abstract and concrete levels into separate specifications
    • possible use of multiple transfer syntaxes which 'compile' to the same set of abstract components
    • multiple inheritance
    • abstract simple types
    • various proposals postponed from the work on XML Schema 1.0
  • 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.
• interoperability with other schema languages such as RELAX-NG and Schematron
• 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

The XML Schema WG 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 WG 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.

Deliverables and Schedule

The XML Schema WG has three main deliverables:

• A formal description of XML Schema.
• Errata list for XML Schema version 1.0
• XML Schema version 1.1; this will be a maintenance release of the spec. It may differ from version 1.0 in incorporating:
  • fixes for errors in version 1.0
  • clearer (and if possible briefer) exposition
  • improvements to the XML transfer syntax, to improve usability based on reports from users and implementors
  • minor improvements to the specification of the abstract component level (if changes can be made without inconvenience to users of existing schemas or developers of existing implementations)

In addition, there are several other deliverables:

• collections of XML Schema use cases and test cases
• list of XML Schema test suites and report on coverage
• possible documents stemming from the various ‘related problem areas’ mentioned above:
  • application-programming interface (API) for the post-schema-validation (PSV) information set
  • entities (especially for special characters)
  • test cases for XML Schema
  • sample library of common types
  • work on schema annotation

Expected schedule:

For details of dates and locations see the WG home page.

A schedule of expected milestones in the development of requirements documents, use cases, working drafts, etc., will be maintained and linked to from the XML Schema WG home page.

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

The expected duration of the working group under this charter is through September 2003.

Relationship with Other Activities

XML has become a strategic technology in W3C and elsewhere. The deliverable of the XML Schema WG 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 WGs may change during the development process, which means the interdependency of the Schema work with these WGs must be managed actively:

XML Query WG

The XML Query WG 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 WG has expressed particular interest in the work on a formal description of XML Schema, and in the definition of the post-schema-validation infoset (PSVI). The XML Schema WG will 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.

XSL WG

In collaboration with the XML Query WG, the XSL WG expects to make future versions of XPath and XSLT support schema-related information, in particular types. The XML Schema WG will 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.

DOM WG

The DOM WG may wish to provide access to details of the data types and structural relationships (e.g. inheritance) among element and attribute types for schema-validated documents. We expect to work together with the DOM both on their definition of a generic interface to DTD- and schema-related information, and on our possible definition of an API to XML-Schema information and the PSVI.

HTML WG

It is a goal of the XML Schema work to ensure that the structural schema language we define will suffice for the requirements of current and future versions of the HTML specification.

XML Core WG

The XML Schema work will define 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 will use namespaces to combine fragmentary structural specifications; it will also define schema-validation for documents using namespaces. (In short: the XML Schema work will be name-space-aware.)

XForms WG

The XForms WG is responsible for a datatype-aware replacement for HTML forms; we will discuss points of mutual interest in order to ensure the alignment of the two specs and the utility of XML Schema for the work of XForms.

Internationalization WG

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 WG and the Internationalization WG 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 WG

The RDF Core WG is preparing a new version of RDF Schema, which will use and build upon XML Schema datatypes. The RDF Core WG is also chartered to "provide an account of the relationship between RDF and the XML family of technologies (particularly Schemas ...)".

Web Services Description and XML Protocol WGs

The XML Protocol WG and the Web Services Description WG in the Web Services Activity use XML Schema extensively. The XML Schema WG will ensure that the schema language is sufficient to meet the requirements of deliverables from these Web Services WGs. The XML Schema WG will also discuss points of mutual interest and will review deliverables from the Web Servicesb WGs and provide them with feedback.

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

WAI Protocols & Formats WG

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

XML Linking

The XML Linking and Schema WGs will consult with each other on the question of declaring element types and attributes as links or addresses.

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.

WG membership

WG members

Participation is expected to consume one to two days per week of each WG member'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 WG 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 member of the WG will be expected to show evidence to the chairs, 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.

Communication

The XML Schema WG (principals and alternates) shall communicate among its members using the w3c-xml-schema-wg mailing list and with the members of other XML WGs through the w3c-xml-plenary@w3.org mailing list. The WG mailing list is archived; the archive is visible to the public.

It shall communicate with the general public only through its Chair.

Confidentiality

Members must treat all Member-only documents as confidential within W3C and use reasonable efforts to maintain this confidentiality and not to release this information to the general public or press.

excerpt from section 1.1.3 - Member Confidentiality of the W3C Process

The proceedings of this working group are member-confidential, subject to exceptions made by the chair.

In particular, the XML Schema WG mailing list archive and the Group home page are accessible by members only. (N.B. published documents may of course be discussed publicly.)

W3C, and all W3C Working Groups, are accountable to the Web community as a whole for the quality of W3C technical work. In support of this public accountability, this working group will periodically (e.g. monthly) make public a summary of all technical decisions made since the last public summary, and the rationales for these decisions.

Face-to-Face Meetings

Face-to-face meetings will be held as needed; it is expected that such meetings will be held about every two to three months.

W3C staff resource commitment

To be successful, we expect the XML Schema WG to have 15 or more active principal members for its duration.

The W3C shall be represented by one principal and (optionally) one alternate WG member. The W3C staff resource commitment is no more than the minimum level of participation required of WG members in good standing by the W3C Process Document.

The W3C staff contact for this WG is, initially, C. M. Sperberg-McQueen. It is expected that this WG would consume about 60% of the contact's time including administrative logistics.

Communications resources for press and media relations and speaking appearances or meeting planning resources are amortized across the working groups in the XML Activity, and concentrated in the XML Coordination Group.