What Is RDF
by Tim Bray
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Pages: 1, 2, 3
The Devil is in the Details
The four general rules given above define the central ideas of RDF. It turns out that it takes quite a lot of abstract terminology and XML syntax to define them precisely enough that people can write computer programs to process them. In particular, turning Statements into Resources is quite tricky. It also turns out that in a (very) few cases, you do need to order your properties, and this requires quite a bit of syntax.
This article doesn't explain all these details; there are a variety of excellent resources to be found at http://www.w3.org/RDF that are designed to do just that.
Vocabularies
RDF, as we've seen, provides a model for metadata, and a syntax so that independent parties can exchange it and use it. What it doesn't provide though is any Properties of its own. RDF doesn't define Author or Title or Director or Business-Category. That would be a job for GOD, if there were one. Since there isn't, it's a job for everyone.
It seems unlikely that one Property standing by itself is apt to be very useful. It is expected that these will come in packages; for example, a set of basic bibliographic Properties like Author, Title, Date, and so on. Then a more elaborate set from OCLC and a competing one from the Library of Congress. These packages are called Vocabularies; it's easy to imagine Property vocabularies describing books, videos, pizza joints, fine wines, mutual funds, and many other species of Web wildlife.
What RDF Might Mean
The Web is too big for anyone person to stay on top of. In fact, it contains information about a huge number of subjects, and for most of those subjects (such as fine wines, home improvement, and cancer therapy), the Web has too much information for any one person to stay on top of and much of anything else .
This means that opinions, pointers, indexes, and anything that helps people discover things are going to be commodities of very high value. Nobody thinks that everyone will use the same vocabulary (nor should they), but with RDF we can have a marketplace in vocabularies. Anyone can invent them, advertise them, and sell them. The good (or best-marketed) ones will survive and prosper. Probably most niches of information will come to be dominated by a small number of vocabularies, the way that library catalogs are today.
And even among people who are sharing the use of metadata vocabularies, there's no need to share the same software. RDF makes it possible to use multiple pieces of software to process the same metadata, and to use a single piece of software to process (at least in part) many different metadata vocabularies.
With any luck, this should make the Web more like a library, or a video store, or a phone book, than it is today.
Getting started with RDF
Since RDF became a W3C Recommendation in February 1999, a number of tools have been created by developers working with RDF. For an in-depth treatment of these, consult the W3C RDF home page. A number of other listings are available, including XML.com, XMLhack and Dave Beckett's RDF Resource Guide.
Developer Community
The main email list for RDF developer discussion is W3C's RDF Interest Group. A number of other RDF-related discussion lists exist, including the Mozilla-RDF forum (the Mozilla and Netscape 6 browsers make heavy use of RDF). More recently, the RDF-Logic list has been announced, providing a forum for the discussion of formal, logic-based approaches to knowledge representation for the Web. DARPA's DAML (DARPA Agent Markup Language) initiative uses the RDF-Logic list for discussions and announcements.
The RDF developer community is rather diverse, which is reflected in the nature of online discussions on the RDF lists. While one strand of RDF development is concerned with highly formal topics (RDF-Logic, DAML and so on), others are busy deploying simpler, more pragmatic applications for Web-based content and metadata syndication. All these themes meet (sometimes productively, sometimes confusingly) on the RDF Interest Group list, but they also typically each have a dedicated email list. For example, the RSS-DEV group has produced the RDF Site Summary (RSS) 1.0 Specification, which provides an RDF-based channel format, designed for interoperability with high level vocabularies such as Dublin Core as well as a variety of more application-specific RDF vocabularies.
Notes on Update (Dan Brickley)
This update to the 1998 article serves only to synchronize it with recent RDF terminology. Since this document was first published, the W3C has published the Model and Syntax specification as a Recommendation.
I have updated the markup example to use current RDF 1.0 syntax. There have also been some terminology changes: 'PropertyType' became 'Property', 'Property' became 'Statement'. I have also added a brief mention of subject/predicate/object terminology, and lowercased a few mentions 'Value' (since rdf:object replaced rdf:value for talking about the object of a statement).
