The Cambridge Communiqué

20 September, 1999
Document identifier
Published to W3C Membership, publicly visible. Public comments may be directed to the RDF comments list/archive or to the XML Schema comments list/archive.

1. Introduction

A group consisting of W3C Member representatives and W3C staff involved in the XML and RDF activities met 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 XML Schema Working Group and other W3C activities in support of this vision, and to resolve issues raised in the Member review of the RDF Schema Proposed Recommendation concerning overlap with XML work.

2. Background

The group discussed a wide range of relevant issues which have in common the goal of supporting the exchange of data on the Web:

  1. XML has defined a transfer syntax for tree-structured documents;
  2. 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;
  3. RDF 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;
  4. 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:

  1. The XML data model is the XML Information Set being specified by the XML Information Set Working Group. 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 language.
  2. 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.
  3. For evolvability and interoperability, the XML Schema specification should provide an extension mechanism allowing for 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.
  4. 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. UML.
  5. 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.
  6. 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.
  7. 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.
  8. 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).
  9. 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.

4. Conclusions

The attendees understand that the XML Schema Working Group is presently addressing some of these topics in the context of its existing Requirements Document. 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.

5. Signatories

David Beech, Oracle Corp.
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
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

$Date: 1999/09/20 13:19:23 $
Ralph R. Swick <swick@w3.org> and Henry S. Thompson, <ht@cogsci.ed.ac.uk>, rapporteurs

Revision History:
1999-09-13T19:54 published
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