This document is a report of the results of a meeting of a group of W3C
Members involved in XML and
RDF to advance the general
understanding of a unified approach to the expression of Web data models.
This document is one response to the Web data architecture discussed
in "Web Architecture:
Describing and Exchanging Data".
This document is a W3C Note made available by the World Wide Web Consortium.
Public comments may be directed to the
RDF comments list
or to the
XML Schema comments list
Publication of a W3C Note does not imply endorsement by the entire W3C
Membership. A list of current W3C technical reports and publications,
including Working Drafts and Notes, can be found at http://www.w3.org/TR.
This section represents the status of this document at the time this
version was published. It will become outdated if and when a new
version is published. The latest
is maintained at the W3C.
- Observations and Recommendations
A group consisting of W3C Member representatives and W3C staff
involved in the
on August 26 and 27 to discuss the architectural
relationship between the schema work being undertaken within these two
activities. The goals of this meeting were to articulate a vision of
this relationship for the Web community, to feed input into the
and other W3C activities in support of this
vision, and to resolve issues raised in the Member review of the
Proposed Recommendation concerning overlap with XML work.
The group discussed a wide range of relevant issues which have in
common the goal of supporting the exchange of data on the Web:
has defined a transfer syntax for tree-structured documents;
Many data-oriented applications are being defined which build
their own data structures on
top of an XML document layer, effectively using XML documents as a
transfer mechanism for structured data;
is a W3C recommendation which already employs this layered
approach. RDF is a member of the Entity-Relationship modelling
family in which data structured as directed labelled graphs
can be exchanged via XML documents using a specific XML grammar;
It is a goal to facilitate the use of RDF mechanisms to access
the information contained in a broad range of XML documents,
including those that were not initially structured according
to the RDF 1.0 layering.
As the expressive facilities available in the XML family, which
includes RDF, become richer we need an understanding of how these
facilities relate to similar mechanisms available within other
formalisms that have adopted XML as their transfer syntax.
3. Observations and Recommendations
This group reached consensus on the following observations and recommendations:
The XML data model is the
XML Information Set
being specified by the XML Information Set
Other data models exist, both
generic and application-specific.
RDF is an example of one such generic data model.
The XML Schema and RDF Schema
languages are separate languages based on different data
models and do not need to be merged into a single comprehensive
An XML Schema schema document will be able to hold declarations for
validating instance documents.
It should also be able to hold declarations for mapping from instance
document XML infosets to application-oriented data structures.
For evolvability and interoperability, the
XML Schema specification should provide an extension mechanism
the augmentation of XML Schema schemas with additional material.
At a minimum, XML Schema should permit elements from other
namespaces to be included in schema documents. This extension mechanism
should also permit individual extensions to be marked 'mandatory',
meaning that a document instance cannot be deemed 'schema valid'
if the processing required by a marked extension cannot be performed.
The extension mechanism should be appropriate for use to incorporate
declarations ("mapping declarations") to aid the construction
of application-oriented data structures (e.g. ones implementing
the RDF model) as part of the schema-validation and XML infoset
construction process. This facility should not be exclusive to
RDF, but should also be useable to guide the construction of data
structures conforming to other data models, e.g.
Such mapping declarations should ideally also be useable by other
schema processors to map in the other direction, i.e. from
application-oriented data structures to XML infosets.
Many schema languages and query languages are or could be layered on top
of the XML foundation. RDF Schema is one such language. It is
appropriate that some mechanisms will apply to both layers and some will
only apply to one or the other.
XML Schema does not need to be the sole provider of support for
layering application data structures on XML.
XSLT, with a
proposed extension mechanism,
could be used for specifying mappings from XML document
instances to application data structures - including RDF graphs.
The reversibility of mappings specified with XSLT or similar
transformation languages is an issue.
A new simplified XML transfer syntax for RDF and an API for
accessing RDF data models should be produced. The
RDF 1.0 transfer
syntax remains a W3C Recommendation and applications are free to
continue to use it. It is not a requirement that XML Schema be able
to validate conformance to the full grammar of RDF 1.0 syntax
(e.g. equivalence of elements and attributes).
XML Schema type hierarchies and RDF type hierarchies are not the same
and need not be unified; in particular, it is too soon to tell if
RDF schemas can leverage XML Schema archetypes. However the atomic
data types, notably URIref, should be shared and work needs to be
done to support this.
The attendees understand that the XML Schema Working Group is presently
addressing some of these topics in the context of its existing
We trust that the consensus developed at this
meeting will help the XML Schema Working Group prioritize features for XML
Schema 1.0 and will also help the W3C Director while considering the next
steps for RDF Schema.
David Beech, Oracle Corp.
$Date: 1999/10/08 18:21:06 $
Gabe Beged-Dov, Rogue Wave Software
Tim Berners-Lee, W3C
Dan Brickley, University of Bristol
Allen Brown, Microsoft
Peter Chen, Bootstrap Alliance
David Cleary, Progress Software
Ron Daniel, DATAFUSION
Andrew Eisenberg, Progress Software
David Epstein, IBM
George Feinberg, Object Design
R.V. Guha (unable to attend but endorsing this communiqué)
Ora Lassila, Nokia
Eve Maler, ArborText, Inc.
Ashok Malhotra, IBM
Murray Maloney, Commerce One
Noah Mendelsohn, Lotus
Eric Miller, OCLC
Wei Song, SISU
Ralph Swick, W3C
Henry Thompson, University of Edinburgh
Ralph R. Swick
Henry S. Thompson,
1999-09-13T20:35 correct Daniel affiliation
1999-09-15T14:34 add a signature
1999-09-15T14:40 add document id to the document itself
1999-09-20T13:19 update Status to direct comments to public archives
1999-10-07T17:25 add abstract, table of contents, update status