RDF: Using XML to Describe Data (Ralph Swick)
May 24, 1999
The W3C's Resource Description Framework (RDF)
plays an important role in enabling a wide variety
of new metadata applications including sitemaps, stream channel definitions,
search engine data collection (web crawling), digital library collections, and distributed authoring.
XML is used as the underlying interchange syntax.
Resource Description Framework (RDF) Model and Syntax Specification
became a W3C Recommendation in February 1999.
According to the description of the WWW8 Metadata workshop by
Carl Lagoze, Eric Miller, and Stuart Weibel:
"Metadata is 'structured information about resources' that enables users and
organizations to not only describe the resources (documents, images,
databases) that they publish on the Internet, but also to effectively manage
and organize these electronic resources and the services that provide them.
The Web is on the verge of a 'metadata revolution' due to the convergence of
the development of XML and RDF and the maturation and standardization of
the Dublin Core."
The basic RDF data model consists of three object types:
- resource - anything that can be specified by a URI, such as a web page, and entire web site, a specific newsgroup message
- properties - characteristics or attributes of a resource, along with some notion of meaning, valid values, etc.
- statements - resource + named property + value of property, expressed as a tuple {subject predicate object}
RDF Schemas are what it means to be a publisher of a particular datatype; this is vocabulary
rather than syntax.
The RDF Schema and XML Schema
Working Groups are now merging their efforts.
A simple example
(from the W3C Metadata Activity page)
of RDF being used to describe metadata about the Consortium's Prospectus follows.
"The basic concept is that metadata about this item on the Web is
described through a collection of properties called an RDF Description." The DC namespace prefix stands for Dublin Core.
<RDF xmlns:DC="http://purl.org/DC">
<Description about="http://www.w3.org/folio.html">
<DC:Title>The W3C Folio 1999</DC:Title>
<DC:Creator>W3C Communications Team</DC:Creator>
<DC:Date>1999-03-10</DC:Date>
<DC:Subject>Web development, World Wide Web
Consortium, Interoperability of the Web</DC:Subject>
</Description>
</RDF>
This trivial example belies the fact that RDF is complicated to write, so Ralph suggested that
it will often be generated from databases. He mentioned that metadata for images could be
expressed using either XML directly or RDF. Using RDF is one step beyond using a DTD to add
semantics to content.
The Netscape Mozilla project is using RDF
"to aggregate and display information about all kinds of Internet resources, including email,
UseNet news, site maps, bookmarks, and browser history."
Readers new to RDF should take a look at the brief RDF FAQ.
A major RDF reference is
Dave Beckett's Resource Description Framework (RDF) Resources.
See Ralph Swick's talk, Using XML to Describe Data, for details.
Also see his April 1998 at WWW7 for more details, although some parts may be out of date.
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