What do linked-data and RDFa have in common?

by Michael Hausenblas

One valid answer to the question in the title would be: I'm both into linked-data and RDFa. Hey, but that's not the answer you are interested in, right? We'll have a look into both and find a better answer by the end of this post. Oh, right, by the way, let me introduce myself shortly. I'm new to xml.com and I try focusing on Semantic Web stuff.

In the beginning, there was the URI. Kingsley recently wrote about it, coming from the plain old untyped @href hyperlink. Then there was RDF, not so well known, and still often confused with one of its serialisations, namely RDF/XML. But there are other ways to deploy RDF as well. In a couple of weeks, presumably, RDFa will be finalised by W3C. RDFa is all about delivering structured metadata in HTML. Much as microformats, RDFa uses attributes to 'hide' - or, more technically: embed - metadata in HTML.

Coming back to URIs: The hyperlinks basically were the success factor of the Web as we know it. Typed or semantic links are expected to be the same for the Semantic Web. TimBL wrote up the so called linked-data principles a bit ago (URI for everything, HTTP URI, RDF properties). An example might help understanding both RDFa and linked-data; compare

this page is under <a href="http://creativecommons.org/licenses/by/2.5/">CC 2.5</a> license
this page is under <a rel="cc:license" href="http://creativecommons.org/licenses/by/2.5/">CC 2.5</a> license

The key is the rel="cc:license" bit. This is actually a piece of valid RDFa (telling that this content is under a certain license) and equally is a typed link. It overloads the simple @href hyperlink and let's an agent (be it a search bot or a syndication site) interpret and follow it properly. I think you get the point, right? To sum up: RDFa is the way doing linked-data. Coming back to the initial question, I guess the main point is that both are manifestations of the real-world Semantic Web emerging these days. While in the last couple of years most of the people involved in Semantic Web stuff maybe thought ontologies and reasoning are the most important issues to deal with, it's a bit like building a marvellous roof and finding out one day that there are no walls, and not even a foundation to put it onto.


Erik Wilde
2008-06-23 18:23:03
actually, without declaring the cc: prefix, your example is not valid RDFa... on some parent element of your examples, there must be a namespace declaration such as xmlns:cc="http://creativecommons.org/ns#", otherwise the cc: does not refer to any declared namespace and the RDFa snippet thus would be invalid.