DJ’s Weblog

RDF is a framework for describing resources. We know that. We also know (from various sources, such as the RDF Model and Syntax Specification, or Pierre-Antoine Champin’s RDF Tutorial (recommended!)) that an RDF document consists of a number of descriptions, and these descriptions look like this:

<rdf:RDF ... >

  <rdf:Description ... >
    ...
  </rdf:Description>

  <rdf:Description ... >
    ...
  </rdf:Description>

  ...

</rdf:RDF>

How come then, that instances of two of the more well-known RDF applications, RSS and FOAF, don’t seem to reflect this format? Following the root rdf:RDF node and the declarations of the namespaces, we have, respectively:

<channel rdf:about="http://www.pipetree.com/qmacro">
  <title>DJ's Weblog</title>
  ...
</channel>

and

<foaf:Person rdf:ID="qmacro">
  <foaf:mbox rdf:resource="mailto:dj.adams@pobox.com"/>
  ...
</foaf:Person>

What, no rdf:Description? Let’s have a look at what’s happening here. In the RSS example, we have channel - or in its fully qualified form http://purl.org/rss/1.0/channel - a class, of which http://www.pipetree.com/qmacro is declared as an instance with the rdf:about attribute.

The RDF subject-predicate-object triple looks like this:

http://www.pipetree.com/qmacro rdf:type http://purl.org/rss/1.0/channel

or in other words “the URI (which is about to be described) is a channel”.

Because RDF is about is declaring and describing resources, it becomes clear that this sort of statement (technically the rdf:type triple, above) is very common. And what we saw in the RSS snippet above was the special RDF/XML construction that may be used to express such statements. If we didn’t have this special construction, we’d have to write:

<rdf:Description rdf:about="http://www.pipetree.com/qmacro">
  <rdf:type rdf:resource="http://purl.org/rss/1.0/channel" />
  <title>DJ's Weblog</title>
  ...
</rdf:Description>

which is a tad long winded. Similarly, the long winded equivalent for the FOAF example would look like this:

<rdf:Description rdf:ID="qmacro">
  <rdf:type rdf:resource="http://xmlns.com/foaf/0.1/Person" />
  <foaf:mbox rdf:resource="mailto:dj.adams@pobox.com"/>
  ...
</rdf:Description>

So there you have it. The rdf:Description isn’t there because a special construction is being used in both examples. Many thanks to Jon Hanna for turning the light bulb on in the first place.