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Resource Description Framework (RDF) is a model for describing collections of formalized statements about a Web resource. You can also think of RDF as a metadata system for the Web. Learn about this standard and about its relationship with the Semantic Web, Web Ontology Language (OWL), and more.
For almost as long as the World Wide Web Consortium
(W3C) has worked toward next-generation markup technology in the form of
XML, it has also worked toward next-generation technology for formal
description of Web resources.
Resource Description Framework (RDF)
[W3C Recommendation] is a model for describing
collections of statements about a Web resource. These statements are
conceptualized as triples, each of which has a subject,
which is a Uniform Resource Identifier (URI); a predicate,
which is also a URI; and an object, which is a URI or literal
data value. To get the flavor of such statements, think of an HTML meta tag that gives the description of a Web page.
If recast as RDF, the subject would be the URI of the Web page itself,
the predicate would be a standard URI signifying a general description,
and the object would be the actual text of the description. By making
heavy use of URIs, RDF seeks to minimize ambiguity about the identity of
components of these statements and thus make them formal enough for
machine processing. Whether RDF succeeds in this goal is up for debate,
but RDF is well-known for its active community and broad range of
tools.
RDF is the backbone of the W3C's Semantic Web activity; the Semantic Web is a vision of a Web not just of presentation of content, but also of annotation of content to help frame its meaning. For example, in a Semantic Web, one can make a distinction between the concept of "python," a kind of snake, and "python," a computer programming language, while describing Web resources. RDF is standardized in a large stack of specifications, including the following:
- Resource Description Framework (RDF): Concepts and Abstract Syntax [W3C Recommendation] sets forth the goals, core concepts, basic data model, and abstract syntax of RDF.
- RDF/XML Syntax Specification [W3C Recommendation] defines a general-purpose XML representation of RDF. Many observers have complained about the poor markup design of the RDF/XML syntax.
- RDF Vocabulary Description Language 1.0: RDF Schema [W3C Recommendation] defines an RDF vocabulary that you can use to define other RDF vocabularies.
- RDF Semantics [W3C Recommendation] is not for the faint at heart and goes into formal mathematical theory underlying the RDF data model.
Web Ontology Language (OWL) [W3C Recommendation] is an application of RDF, often encoded in RDF/XML, that adds a rich vocabulary that you can use to formally classify and reason about RDF resources. SPARQL Query Language for RDF [in development] is a special syntax for querying RDF data, and Gleaning Resource Descriptions from Dialects of Languages (GRDDL) [in development] is a system for extracting RDF data from XML documents. GRDDL has recently come into some prominence with the popularity of microformats on the Web. Another RDF-related specification that fits into the microformats puzzle is RDFa Syntax [in development], which is a special set of attributes for embedding RDF data within XML formats such as XHTML.
- In The
future of the Web is Semantic, Naveen Balani provides an overview of
Semantic Web technology (developerWorks, October 2005). You can find
another such overview in Uche Ogbuji's The Languages of the Semantic Web, which explores the
motivations and basics of RDF and Semantic Web technologies primarily
geared toward Web developers (Dr. Dobb's Portal, March 2002).
- Learn about SPARQL in Search
RDF data with SPARQL by Philip McCarthy (developerWorks, May
2005).
- Learn about RDF and RDF Schema in the context of
rich Web applications in The
ultimate mashup -- Web services and the semantic Web, Part 3: Understand
RDF and RDFs by Nicholas Chase (developerWorks, September 2006). In
The ultimate mashup -- Web services and the semantic
Web, Part 4: Create an ontology, Nicholas Chase and Michel Mitri
introduce OWL (developerWorks, October 2006).
- The W3C's RDF Primer is a pretty decent starting point. You can also
check out W3C primers for RDFa and GRDDL.
- Read about other XML standards:
Index of XML
standards.
- Participate in any of several XML-centered forums:
XML
zone discussion forums.
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- Find out how you can become an IBM-Certified
Developer in XML and related technologies at IBM XML
certification.
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technical articles and tips, tutorials, standards, and IBM Redbooks at
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technical library.
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