Sjoerd Visscher's weblog

My ideas about new web technology that can change the future of the world wide web.

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10/25/2002; 8:18:16 PM

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Saturday, September 07, 2002

Other uses of semantic schemas

Thinking more about the semantic schemas I wrote about last night, I realized that it has more uses. XLink for example has the same problems as RDF. For XLinks to work, you have to add XLink specific syntax to your document. This is far from ideal, which became painfully clear when the first XHTML 2.0 draft was released without XLink support. A semantic schema for XHTML 2.0 can declare that the href attributes generate XLinks.

Another example. A month ago, when I tried to add support for XHTML 2.0 to the various browsers, I found out that there's no way to tell the browser what the title of the document is. A semantic schema can declare what the title of a document is. Google needs to know what the title of an xml document is too.

Is RDF a failure?

OK, the discussions aren't over yet, but it looks like RDF is no longer an option for RSS. This may worry the RDF fans, because RDF should have been a perfect match for RSS. RDF is designed to be a format to describe website meta-data. And that is exactly what RSS is. So is RDF a failure?

There are two problems that have prevented wide-spread adoption of RDF-style RSS. The first one is that there is no added value in using RDF over plain XML. All applications that use RSS, are RSS specific applications. They don't combine the data with loads of other information, simply because there isn't any except other RSS files. So these applications don't need a general semantic format, because they have the RSS semantics built-in. The second problem is the added complexity of the syntax. It's a bit ugly and the purpose of the extra syntax is far from self-explaining, which quickly lead to mistakes in implementations. Also the first mentioned problem makes it just not worth the trouble.

Note that these 2 problems are hardly RSS specific. They will also arise in other attempts to deploy RDF in a community. RDF seems doomed. But I think RDF still has the future, or at least the model of RDF has the future. The problems are in the serialization of RDF. Serialization is all about making concessions to conciseness, file size, readability, streamability etc. It depends on the situation which of these properties are more important than others. So if you design a general format, it is always going to be inferior to a specificly designed format. With RDF this is even worse because RDF can make absolutely no concessions towards semantics.

Given all this it is unlikely that RDF as serialized in XML is going to be widely adopted. People want their XML the way they want it, and don't want to be bothered with the semantics. That's also because they already know the semantics of their own XML. It's those that want to put it all together that want a semantic format, not those who design a single format. But the information is already there, in all those XML files everywhere. Once there's a way to specify how to convert an XML format to the RDF model, you have both the freedom of the XML format, and the exciting features of the RDF model that will give us the Semantic Web. And once there's such a “semantics schema” for RSS, I'm sure the RSS developers are very willing to add that one extra attribute or element to point to the schema, if that's the only thing it takes to take part in the Semantic Web.