RDFa, Drupal and a Practical Semantic Web

By Dee-Ann LeBlanc and Brice Dunwoodie (@deele) Mar 25, 2009

6 comments

In the march toward creating the semantic web, web content management systems such as Drupal (news, site) and many proprietary vendors struggle with the goal of emitting structured information that other sites and tools can usefully consume. There's a balance to be struck between human and machine utility, not to mention simplicity of instrumentation.

With RDFa (see W3C proposal), software and web developers have the specification they need to know how to structure data in order to lend meaning both to machines and to humans, all in a single file. And from what we've seen recently, the Drupal community is making the best of it.

Introducing RDFa

RDFa is a set of XHTML attributes meant in particular to augment visual data with machine-readable hints. In layman's terms, RDFa was created to help machines understand what humans intuitively get while browsing around the web. The hints in this case will strike those familiar with microformats and the rel nofollow open standard as rather familiar.

Actually, RDFa goes beyond this, providing somewhat of a circular benefit. While this standard helps machines understand what humans see, it also has applications for providing metadata to augment content. Machines then display the augmented content and humans suddenly understand even better the context of what they're seeing.

RDFa, RDF, XML and XHTML

Okay, too many acronyms you say? It can easily happen — especially while skipping around W3C specifications. Let's untangle just a bit.

RDFa is based upon the principles of RDF. RDF stands for Resource Description Framework. It's an endorsed W3C Recommendation implemented as an XML syntax, and is meant to be a language for representing information about stuff found on the web (things that have URIs).

If you need to assign importance to this RDF thing, consider this: RDF is at the heart of what we call the semantic web.

The Heart of RDF

The most important RDF concept to understand is that of the RDF Triple. A triple has, as one might guess, three parts: the Subject, the Predicate and the Object (you will also see Subject, Property and Value). An expression in RDF (also refered to as an RDF Graph) then is a collection of these triples.

Resource Description Framework (RDF) Triple — Subject, Predicate (or Property) and Object (or Value)

The meaning (or semantic value) of an RDF expression is that some relationship — defined by the RDF Predicate/Property — exists between the RDF Subject and the RDF Object/Value. In the end, that is as simple as the semantic web gets.

RDF in Action

The idea behind RDF is to give us geeks a simple way to make statements about things on the web and have machines understand us. Let's look at an example.

Let's say I want to express that the website found at http://www.google.com was created by Larry Page. In this case we have the following 3 things that comprise our assertion:

The subject: http://www.google.com
The predicate or property: creator
The object or value: Larry Page

Now to put this into RDF syntax, we do the following:

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:dc="http://purl.org/dc/elements/1.1/">
  <rdf:Description rdf:about="http://www.google.com">
        <dc:creator>Larry Page</dc:creator>
  </rdf:Description>
</rdf:RDF>

RDFa is Much Simpler Than RDF

What we have above is Greek to most of us (unless you're Greek and then it's something else). Fortunately, RDFa significantly simplifies the implementation of RDF.