W3C

Resource Description Framework (RDF)

Editors' Working Draft 20 June 2002

This version:
http://www.ninebynine.org/wip/RDF-basics/Current/Overview.htm
Latest version:
http://www.ninebynine.org/wip/RDF-basics/Current/Overview.htm
Previous version:
(None)
Editors:
Graham Klyne (Clearswift and Nine by Nine)
Jeremy Carroll (Hewlett Packard Labs)
Series editor:
Brian McBride (Hewlett Packard Labs)

Copyright ©2002 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.

Abstract

[[[tbd]]]

Status of this Document

This is a W3C RDF Core Working Group Working Draft produced as part of the W3C Semantic Web Activity (Activity Statement).

This document is being released for review by W3C Members and other interested parties to encourage feedback and comments, especially with regard to how the changes affect existing implementations and content.

This is a public W3C Working Draft and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite as other than "work in progress". A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.

Comments on this document are invited and should be sent to the public mailing list www-rdf-comments@w3.org. An archive of comments is available at http://lists.w3.org/Archives/Public/www-rdf-comments/.

Table of contents

Appendices

1. Introduction

[[[tbd -- about this document]]]

2. RDF overview

RDF is a data format for representing metadata about Web resources, and other information. It uses well established ideas from the Knowledge Representation branch of Artificial Intelligence, with recognizable relationships to Conceptual Graphs, Frames, logic-based and relational database knowedge representation forms [Sowa,CG,Luger,Hayes,Gray]. (John Sowa [Sowa] argues compellingly that these are pretty much equivalent for the purposes of knowledge representation.)

RDF builds on XML, which provides a syntactic framework for representing documents and other information. It has a simple graph-based data model and formal semantics with a rigorously defined notion of entailment, which in turn provides a basis for well founded deductions in RDF data.

The real value of RDF comes not so much from any single application, but from the possibilities for sharing data between applications. The value of information thus increases as it becomes accessible to more and more applications across the entire Internet.

2.1 Motivation

The development of RDF has been motivated by the following uses, among others:

2.2 Design goals

The design of RDF is intended to meet the following goals:

2.2.1 A simple data model

RDF has a simple data model that is easy for applications to process and manipulate. The data model is independent of any specific serialization syntax.

NOTE: the term "model" used here in "data model" has a completely different sense to its use in the term "model theory". See the RDF model theory specification [RDF-MODEL] or a textbook on logical semantics (e.g. [HUNTER,DAVIS]) for more information about what logicians call "model theory".

2.2.2 Formal semantics and well-founded inference

RDF has a formal semantics which provides a sound basis for reasoning about the meaning of an RDF expression. In particular, it supports rigorously defined notions of entailment which provide a basis for defining reliable rules of inference in RDF data.

2.2.3 Extensible URI-based vocabulary

The vocabulary is fully extensible, being based on URIs with optional fragment identifiers (URIrefs). URIrefs are used for naming all kinds of things in RDF data. The only other kind of label that appears in RDF data is a literal string.

2.2.4 XML-based syntax

RDF has an XML-based serialization form which, if used appropriately, allows a wide range of "ordinary" XML data to be interpreted as RDF [STRIPEDRDF].

2.2.5 Use XML schema datatypes

RDF can be used with XML schema datatypes [XML-SCHEMA-DATATYPES], thus assisting the exchange of information between RDF and other XML applications.

[[[Add comment here if goal not fully achieved]]]

2.2.6 Anyone can say anything about anything

To allow operation at Internet scale, RDF is an open-world framework that allows anyone to say anything about anything. In general, it is not assumed that all information about any topic is available. A consequence of this is that RDF cannot prevent anyone from making nonsensical or inconsistent assertions, and applications that build upon RDF must find ways to deal with conflicting sources of information. (This is where RDF departs from the XML approach to data representation, which is generally quite prescriptive and aims to present an application with information that is well-formed and complete for the application's needs.)

2.2.7 Universal expression of ground facts

Through its use of extensible URI-based vocabularies, RDF aims to provide for universal expression of ground facts; i.e. assertions of specific properties about specific named things.

RDF itself does not provide the machinery of inference, but provides the raw data upon which such machinery can operate. Other work is looking for ways to build more expressive expressions on the basic capabilities of the RDF core language.

2.2.8 A basis for binding agrements

RDF is intended to convey assertions that are meaningful to the extent that they may, in appropriate contexts, be used to express the terms of binding agreements.

This goal explored further in section 2.3 below.

2.3 Meaning of RDF documents

The RDF specification emphasizes the formal structure and meaning of RDF. But there is also a social dimension that is easily overlooked when dealing with such formal aspects.

2.3.1 Formal semantics

[[[adapt words from primer 7.1]]]

2.3.2 Social meaning

[[[adapt words from F2F]]]

2.3.3 Interaction between social and formal meaning

[[[adapt words from F2F]]]

2.3.4 Implications of asserting RDF

[[[adapt words from F2F]]]

2.4 RDF concepts

RDF uses the following key concepts:

2.3.1 Graph data model

The underlying structure of any RDF expression is a directed labelled graph (or multigraph), which consists of nodes and labelled directed arcs that link pairs of nodes. The formal semantics for RDF is defined in terms of this graph syntax. An RDF expression is sometimes called an RDF graph. The graph can conveniently be represented as a set of triples, where each triple contains two node labels and an arc label:

[[[picture of node --arc--> node]]]

Each arc corresponds to a startement that asserts a relationship between the nodes that it links. The meaning of an RDF graph is the conjunction (i.e. logical AND) of all the statements that it contains.

2.3.2 URI-based vocabulary

Nodes in an RDF graph are labelled with URIs with optional fragment identifiers (URIrefs), literal strings, or nothing at all. Arcs are labelled with URIrefs.

The label on a node indicates what that node is meant to represent. The label on an arc names the relationship that is asserted to hold between the nodes connected by that arc. Some URIrefs may indicate web resources, and a node thus labelled is presumed to denote that resource. Other URIrefs may represent abstract ideas or values rather than a retreivable Web resource. RDF thus leverages the universal naming space of URIs [I10].

2.3.3 XML serialization syntax

RDF has a specific serialization syntax based on XML. There are several ways in which a given RDF graph can be prepresented in XML: these various forms allow RDF to be represented in ways that are amenable to specific XML applications. In this way, XML application data can easily be designed to be accessible to generic RDF processors [XML-AS-RDF].

Other syntaxes for RDF graphs are possible (e.g. [NOTATION3]), but only the XML syntax is normatively specified and recommended for use to exchange information between Internet applications.

3. RDF specification

The RDF specification defines the following components:

Further, the RDF documentation suite contains:

3.1 Namespaces

[[[designate namespace URIs]]]

3.2 Graph syntax

[[[description, and formal spec (later, maybe ref as appendix)]]]

3.2.1 Graph syntax reserved names

[[[note of RDF names that cannot appear in graph syntax; e.g. rdf:Description]]]

3.3 Formal semantics

[[[rework this]]]

The RDF abstract graph syntax and model theory [RDF-SEMANTICS] are at the heart of the RDF specifications. This document specifies the essential elements of RDF abstract syntax, and the associated model theoretic semantics. The syntax is specified in a terms of a directed labelled graph and an equivalent representation of <subject,predciate,object> triples. Also given are entailment lemmas and their proofs. The entailment lemmas form the basis of RDF-based deduction.

Building on the core language and semantics, this specification also calls out the RDF reserved vocabulary (URIrefs) for RDF schema and RDF datatyping, also with model theoretic semantics, entailment lemmas and proofs.

This document contains a fair amount of formal mathematical content, necessary to meet some of the stated goals for RDF. Because RDF is such a simple language, the document actually serves as a quite accessible introduction to formal semantics. Developers whose sole concern is to write software that processes RDF may prefer to work from the XML syntax and RDF schema specifications, referring to this formal semantics specification to resolve occasional questions about validity of deductions.

3.4 XML serialization syntax

The RDF XML Syntax document [RDF-SYNTAX] defines the XML serialized forms for RDF graphs. The XML syntax is described in terms of the XML infoset [XML-INFOSET], and its correspondence to RDF graph triples.

3.5 RDF vocabulary definitions

The RDF Schema document [RDF-VOCABULARY] introduces and describes the use of RDF schema and datatypes vocabularies used to describe the classes and types of things described by some RDF vocabulary. The essential information in this document is covered formally in the model theory, and this specification provides a less formal account of these features of RDF.

3.5.1 RDF defined vocabulary terms

[[[List vocabulary terms defined by RDF spec, with reference to definition]]]

3.6 RDF datatyping

[[[TBD -- if datatyping is finalized]]]

3.7 Test cases

The RDF Test Cases document [RDF-TESTS] provides specific examples of XML syntax and the corresponding RDF graph triples. To achieve this, it introduces a particular syntax for RDF graph triples, a very much simplified variant of Notation 3 [NOTATION3], which used to describe RDF graphs in a very direct and intuitive fashion. The test cases themselves are also published in machine-readable form at referenced Web locations, so developers may use these as the basis for some automated testing of RDF software.

The test cases document also contains a number of entailment tests, which indicate entailments that applications are licensed by the RDF specification to use as the basis of deductions in RDF data. Many of these entailments relate to inferences that can be drawn from RDF schema and RDF datatyping information.

The test cases are not a complete specification of RDF, and are not intended to take precedence over the main specification documents. However, they are intended to illustrate the intent of the working group with respect to the design of RDF, and developers may find these helpful should the specification wording be unclear on any point of detail.

3.8 Primer

The RDF Primer [RDF-PRIMER] serves two purposes:

4. Additional technical considerations

xxx

4.1 Internationalization

xxx

4.2 Character normalization

xxx

4.3 Conformance

[[[anything to say?]]]

xxx

4.4 RDF in HTML

xxx

5. Acknowledgments

[[[Acknowledgements from original RDFM&S]]]

The editors acknowledge valuable contributions of the following reviewers:

6. References

6.1 Normative References

[XML]
Extensible Markup Language (XML) 1.0, Second Edition, T. Bray, J. Paoli, C.M. Sperberg-McQueen and E. Maler, Editors. World Wide Web Consortium. 6 October 2000. This version is http://www.w3.org/TR/2000/REC-xml-20001006. latest version of XML is available at http://www.w3.org/TR/REC-xml.
[XML-NS]
Namespaces in XML, T. Bray, D. Hollander and A. Layman, Editors. World Wide Web Consortium. 14 January 1999. This version is http://www.w3.org/TR/1999/REC-xml-names-19990114. The latest version of Namespaces in XML is available at http://www.w3.org/TR/REC-xml-names.
[URIS]
RFC 2396 - Uniform Resource Identifiers (URI): Generic Syntax, T. Berners-Lee, R. Fielding and L. Masinter, IETF, August 1998. This document is http://www.isi.edu/in-notes/rfc2396.txt.
[RDF-SYNTAX]
(RDF syntax...)
[RDF-SEMANTICS]
RDF Model Theory, P. Hayes, Editor. Work in progress. World Wide Web Consortium, 14 February 2002. This version of the RDF Model Theory is http://www.w3.org/TR/2002/WD-rdf-mt-20020214. The latest version of the RDF Model Theory is at http://www.w3.org/TR/rdf-mt/.
[RDF-VOCABULARY]
(RDF vocabulary and schema...)
[RDF-TESTS]
RDF Test Cases, A. Barstow and D. Beckett, Editors. Work in progress. World Wide Web Consortium, 15 November 2001. This version of the RDF Test Cases is http://www.w3.org/TR/2001/WD-rdf-testcases-20011115/. The latest version of the RDF Test Cases is at http://www.w3.org/TR/rdf-testcases.
[KEYWORDS]
RFC 2119 - Key words for use in RFCs to Indicate Requirement Levels, S. Bradner, IETF. March 1997. This document is http://www.ietf.org/rfc/rfc2119.txt.
[RFC-3023]
RFC 3032 - XML Media Types, M. Murata, S. St.Laurent, D.Kohn, IETF. January 2001. This document is http://www.ietf.org/rfc/rfc3023.txt.

6.2 Informational References

[RDF-PRIMER]
RDF Primer, F. Manola, E. Miller, Editors, World Wide Web Consortium W3C Working Draft, work in progress, 19 March 2002. This version of the RDF Primer is http://www.w3.org/TR/2002/WD-rdf-primer-20020319/. The latest version of the RDF Primer is at http://www.w3.org/TR/rdf-primer/.
[XML-INFOSET]
(XML infoset document...)
[XML-SCHEMA-PRIMER]
XML Schema Part 0: Primer - W3C Recommendation, World Wide Web Consortium, 2 May 2001.
[XML-SCHEMA-STRUCTURES]
XML Schema Part 1: Structures - W3C Recommendation, World Wide Web Consortium, 2 May 2001.
[XML-SCHEMA-DATATYPES]
XML Schema Part 2: Datatypes - W3C Recommendation, World Wide Web Consortium, 2 May 2001.
[SOWA]
John Sowa, Knowledge Representation, ...
[CG]
Conceptual Graphs (spec)
[LUGER]
Luger and Subblefield, Artificial Intelligence
[HAYES]
Pat Hayes... in defense of logic
[GRAY]
Peter Gray, Logic, Algebra and Databases,...
[HUNTER]
Geoffrey Hunter, Metalogic ...
[DAVIS]
Ruth E. Davis, Truth, Deduction and Computation ...
[CHARMOD]
Character Model for the World Wide Web 1.0, M. Dürst, F. Yergeau, R. Ishida, M. Wolf, A. Freytag, T Texin, Editors, World Wide Web Consortium Working Draft, work in progress, 20 February 2002. This version of the Character Model is http://www.w3.org/TR/2002/WD-charmod-20020220/. The latest version of the Character Model is at http://www.w3.org/TR/charmod/.
[STRIPEDRDF]
RDF: Understanding the Striped RDF/XML Syntax, D. Brickley, W3C, 2001. This document is http://www.w3.org/2001/10/stripes/.
[XML-AS-RDF]
(example of generic XML data that is RDF compatible - draft-klyne-xxx-rfc822-xml-xxx is one...)
[NOTATION3]
Tim Berners-Lee, DesignIssues note on N3, ...
[RDF-MS]
Resource Description Framework (RDF) Model and Syntax Specification, O. Lassila and R. Swick, Editors. World Wide Web Consortium. 22 February 1999. This version is http://www.w3.org/TR/1999/REC-rdf-syntax-19990222. The latest version of RDF M&S is available at http://www.w3.org/TR/REC-rdf-syntax.

Appendix W: Issues addressed in this document

[[[For reviewers' reference. This appendix will be removed on final publication.]]]

xxxx

xxxx

Appendix X: Outstanding issues for this document

[[[For reviewers' reference. This appendix will be removed on final publication.]]]

xxxx

xxxx

Appendix Y: Change log

[[[For reviewers' reference. This appendix will be removed on final publication.]]]

$Log: Overview.htm,v $
Revision 1.4  2002/06/24 13:29:48  graham
Update current/previous version links

Revision 1.3  2002/06/21 14:45:34  graham
Futher rearrangement of outline, to accommodate:
- list of RDF vocabulary terms
- RDF-in-HTML
- RDF namespaces
- Addressed issues appendix
- Note about pure syntax vocabulary (e.f. rdf:Description)
Renamed some section titles

Revision 1.2  2002/06/21 10:21:23  graham
Rearranged outline to accommodate material
from the primer on formal semantics

Revision 1.1  2002/06/20 20:47:03  graham
Initial version of document

RDF/XML Metadata