Resource Description Framework (RDF)

Editors' Working Draft 24 June 2002

This version:

http://www.ninebynine.org/wip/RDF-basics/2002-06-24/Overview.htm

Latest version:

http://www.ninebynine.org/wip/RDF-basics/Current/Overview.htm

Previous version:

http://www.ninebynine.org/wip/RDF-basics/2002-06-21/Overview.htm

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.

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Abstract

[[[tbd]]]

Status of this Document

The following text refers to the intended status of this document: it has not yet been approved by the RDF core working group an has no status other than editors' working draft at this time.

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/.

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1. Introduction

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

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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:

• Web metadata: providing information about web resources and the systems that use them (e.g. content rating, capability descriptions, privacy preferences, etc.)
• Applications that require open rather than constrained information formats (e.g. scheduling activities, describing organizational processes, annotation of Web resources, etc.)
• To do for machine processable information (application data) what the World Wide Web has done for hypertext: to allow data to be processed outside the particular environment in which it was created, in a fashion that can work at Internet scale.
• Interworking between applications: combining data from several applications to arrive at new information.
• Automated processing of Web information by software agents: the Web is moving from having just human-readable information to being a world-wide network of cooperating processes. RDF provides a world-wide lingua franca for these processes.

2.2 Design goals

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

• A simple data model
• Formal semantics and well-founded inference
• Extensible URI-based vocabulary
• XML-based syntax
• Use XML schema datatypes
• Anyone can say anything about anything
• A basis for legally binding agrements
• Universal expression of ground facts

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-SEMANTICS] 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-SCHEMA2], 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 is 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

[[[These words adapted from Primer 7.1]]]

RDF is a language designed to support the Semantic Web, in much the same way that HTML is the language that supports the original Web. The Semantic Web aims for data to be shared and processed by automated tools as well as by people. To serve this purpose, certain meanings of RDF statements must be defined in a very precise manner; this is provided by the RDF Model Theory [RDF-SEMANTICS].

Model-theoretic semantics assumes that a language refers to a 'world', and describes the minimal conditions that a world must satisfy in order to assign an appropriate meaning for every expression in the language. A particular world is called an interpretation, so that model theory might be better called 'interpretation theory'. The idea is to provide an abstract, mathematical account of the properties that any such interpretation must have, making as few assumptions as possible about its actual nature or intrinsic structure. The RDF model theory is couched in the language of set theory simply because that is the normal language of mathematics - for example, the model theory assumes that names denote things in a set IR called the 'universe' - but the use of set- theoretic language is not supposed to imply that the things in the universe are set-theoretic in nature.

The chief utility of such a semantic theory is not to suggest any particular processing model, or to provide any deep analysis of the nature of the things being described by the language (in our case, the nature of resources), but rather to provide a technical tool to analyze the semantic properties of proposed operations on the language; in particular, to provide a way to determine when they preserve meaning.

The RDF model theory treats RDF as a simple assertional language, in which each triple makes a distinct assertion, and the meaning of any triple is not changed by adding other triples. Based on the semantics defined in the model theory, it is simple to translate an RDF graph into a logical expression with essentially the same meaning.

2.3.2 Social meaning

[[[Adapted words from DanBri/PatHayes]]]

2.3.3 Interaction between social and formal meaning

2.3.4 Implications of asserting RDF

When an RDF graph is asserted in the web, its publisher is saying something about their view of the world. (The mechanism for deciding whether or not a graph is asserted is not defined here, but it is presumed that the publisher's intent will be clear in some way -- social convention or logical deduction.)

When a user invokes an application, there is also a social and technical context of invocation that determines some set of RDF assertions that will be assumed to be true: the application itself, and any RDF files that are passed to it. Garbage-in, garbage-out applies: if the initial assumed facts are wrong or meaningless, the results will have little value. No specfic mechanisms for deciding or evaluating the validity of any such assertions are defined here.

An assertion tells us something about "the world" and human affairs, through the normal model theoretic possible-world constraint mechanisms. Some of the truths that are asserted may be logical truths that can be evaluated using logical machinery. Others may be assumed truths that cannot be evaluated logically, but can be determined by human interpretation. So when we assert an RDF graph, one is stating a constraint on the real world, saying that both the logically testable and humanly interpretable truths in the graph are indeed true in that world.

In accordance with appropriately sanctioned logical entailment, it is intended that inferences may be used to deduce new RDF statements with the same force of assertion as the explicitly statements from which they are derived.

Noting that there is no single human opinion about the truth of some statements, the graph may further contain commentary for human interpreters to indicate the realm of human interpretation that should be applied. This means a graph may contain "defining information" that is opaque to logical reasoners. This information may be used by human interpreters of RDF informaton, or programmers writing software to perform specialized forms of deduction in the Semantic Web.

2.4 RDF concepts

RDF uses the following key concepts:

• Graph data model
• URI-based vocabulary
• XML serialization syntax

2.4.1 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:

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.

2.4.2 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 [URIS].

2.4.3 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:

• URI vocabulary collections
• Graph syntax
• Formal semantics
• XML serialization syntax
• Basic RDF and RDF schema vocabulary
• [[[Datatypes]]]
• MIME type registration: application/rdf+xml

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

$Log: Overview.htm,v $
Revision 1.1  2002/06/24 16:48:33  graham
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Revision 1.4  2002/06/24 16:39:24  graham
Completed initial cut of section 2 text:
- 2.3.1 Semantics from Primer 7.1
- 2.3.2 social meaning adapted from text by DanBri
- 2.3.3-4 from text discussed at face-to-face
Some further renaming of sections

Revision 1.3  2002/06/24 13:27:16  graham
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Revision 1.2  2002/06/24 13:22:24  graham
Transcribe initial issue list to appendix X.
Rearrange outline with new sections for graph syntax
and informal semamntics for RDF vocabulary.

Revision 1.1  2002/06/21 14:57:22  graham
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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