My last week of postings have been mostly sauce with no meat, which means it's high time I quit with conjecture and start talking turkey. For those new to the Semantic Web, I'm sure you're saying, "How do I use this stuff?" I know how you feel. When I'm learning a new technology, I'm usually relieved when the writer stops talking around things and finally addresses the topic directly. So for my first "tangle yarn," I'll tackle RDF, the Resource Description Framework, since it's the technological foundation for the whole magilla. There's a lot written about RDF and friends. I'm...

In my last post, we learned that RDF is all about describing resources, and that resources are referenced with URIs. So how does it describe them? Let's start by looking at the source of this very page (View / Page Source). There's two snippets of RDF embedded in the HTML. The first helps tools like Movable Type create what are called "trackbacks", links to this post from other people's posts. Here's the RDF: <!-- <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:trackback="http://madskills.com/public/xml/rss/module/trackback/" xmlns:dc="http://purl.org/dc/elements/1.1/"> <rdf:Description rdf:about="http://bigfractaltangle.com/archive/2003/12/26.jsp" trackback:ping="http://immuexa.com/cgi-bin/mtype/mt-tb.cgi/80" dc:title="RDF intro, part 2" dc:identifier="http://bigfractaltangle.com/archive/2003/12/26.jsp" dc:subject="Tangle Yarns" dc:description="In my last post, we learned..." dc:creator="timothy" dc:date="2003-12-26T21:49:46-08:00" /> </rdf:RDF> --> First,...

Now that you've seen some actual RDF, we can take a step back to put things in context. RDF is essentially a data model -- a way of describing data, or in this case, metadata. There's plenty of data models out there. The ones we're most familiar with are connected to programming languages and their functional flavors. Algol, Pascal, and C have their records and procedures. Prolog has statements and rules. Smalltalk, C++, and Java have objects and methods. SQL has tables and statements. At their core, each of these systems describe data in the same way: atomic data nuggets...

Last time I talked about "triples", which are the elementary nugget in RDF. What's a triple? Have a look: <rdf:Description rdf:about='http://bigfractaltangle.com' dc:title='Big Fractal Tangle' /> This triple is saying, "The resource 'http://bigfractaltangle.com' has the title 'Big Fractal Tangle." It's a single fact, expressed as an RDF statement, or triple. The three parts that make it a triple are: resource: http://bigfractaltangle.com predicate: dc:title object: Big Fractal Tangle Every RDF statement has a resource, a predicate, and an object. To see it another way, we can take the Grammar Rock approach: each sentence has a subject and a predicate. The subject is...

Why is RDF worth our time? My short answer is "because RDF is loose, but not too loose." RDF has enough order to do useful things, but doesn't require us to rewire the world first. If tech were tunes, RDF would be a jazz trio, not a Bach fugue. It lets players who hardly know each other improvise, yet it holds things together beautifully: the quintessential jam. In this way, RDF is much like the current Web. RDF shares many of the benefits that made the first Web a success. As Dave Beckett said, "RDF allows loose collaboration with little...