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Member-Confidential!
Charter of the XML Schema Working Group
[June 2004]
David Ezell, chair
C. M. Sperberg-McQueen and Henry Thompson, Team contacts
$Revision: 1.52 $ of $Date: 2004/06/16 18:55:46 $ by $Author: plehegar $
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.
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.
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.
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.
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.
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.
The XML Schema Working Group shall communicate among its
participants using the
w3c-xml-schema-wg
mailing list and with the other XML
Working Groups through the
w3c-xml-plenary@w3.org
mailing list. Both lists are archived.
The XML Schema Working Group communicates with the public through the
www-xml-schema-comments and
xmlschema-dev
mailing lists, both of which
are public.
The proceedings of this working group are Member-confidential.
In support of public accountability, the Working Group will
periodically make public a summary of all technical decisions made since
the last public summary, and the rationales for these decisions.
Face-to-face meetings will be held as needed; it is expected that
such meetings will be held about every two to three months.
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.
Chair, W3C XML Schema Working Group: David Ezell
W3C Team contacts: C. M. Sperberg-McQueen and
Henry Thompson
Last Modified: $Date: 2004/06/16 18:55:46 $