Jul 10 2009 2:35AM GMT

The Semantic Web: RDF and SPARQL, part 2

Roger King Profile: Roger King

This posting is a continuation of the previous posting. We are discussing RDF, the “triples” language that is serving as a cornerstone of the Semantic Web effort. In the previous posting, we looked at a simple RDF program, which creates a relationship between a web-based resource and the term “funstuff”; the relationship is called “topic”, thus telling us that the resource located at the given URL is something fun.

RDF and URI’s.

One interesting fact is that, although we only used URI’s for two parts of the RDF triple embedded in this RDF program, we could have used URI’s for all three pieces of the triple. Thus, the program from the previous blog posting (immediately below) might be changed to look like the second program below, which now has two triples in it:

<rdf:RDF

xmls:rdf=”http://www.w3.org/1999/02/22-rdf-syntax-ns#”>
xmls:zx=”http://www.someurl.org/zx/”>

<rdf:Description

rdf:about=”http://www.awebsite.org/index.html”>
<zx:topic>funstuff</zx:topic>

</rdf:Description>

</rdf:RDF>

————-

<rdf:RDF

xmls:rdf=”http://www.w3.org/1999/02/22-rdf-syntax-ns#”>
xmls:zx=”http://www.someurl.org/zx/”>

<rdf:Description

rdf:about=”http://www.awebsite.org/index.html”>
<zx:topic>funstuff</zx:topic>

<zx:created-by>http://www.anotherurl.org/buzz</zx:created-by>

</rdf:Description>

</rdf:RDF>

RDF and decentralized information.

As a reminder, the triple expressed in the first program can be stated as:

www.awebsite.org/index.html <topic> funstuff

So, what did we add in the second program? There is a new triple that has been added. It can be roughly stated as:

www.awebsite.org/index.html <created-by> http://www.anotherurl.org/buzz

In other words, our vocabulary defined at http://www.someurl.org/zx apparently has another standardized term called “created-by”. The added triple in our second program says that the resource found at www.awebsite.org/index.html was created by someone who is identified by the url http://www.anotherurl.org/buzz.

We see that the value in the first triple, which concerns the “topic” of our resource, consists of a character string, but the value in the second triple, which concerns the “created-by” of our resource, is actually a URL.

This is big. It shows us that all three parts of a triple in RDF can be URI’s, and they can be distributed around the Internet. This means that the information embedded in the triple is highly decentralized.

The bottom line

This illustrates the power of RDF. It can be used to express information which is not controlled in any centralized fashion. RDF is thus the glue that can be used to bring diverse pieces information together. And it can use standardized, shared terminologies to precisely dictate the semantics of the triples in RDF programs. In our example, the resource is defined by one URI, the kind of relationship is defined by another URI, and the value of that relationship is defined by yet another URI.

We will continue this in the next posting.