Comparing Topic Maps and RDF

<< 2007-01-06 18:52 >>

The Topic Maps and RDF technology stacks are quite complex, and it's not easy to see how the various pieces compare. So I thought I'd write a little bit about this. It was Trond's comment that triggered this, but I've seen no end of people struggle with this. I've written about this before, but I think it's possible to do a lot better. So, here goes.

The big picture

The big picture

The figure above illustrates the basic relationships between the various pieces of the puzzle. How this works is best expressed with some examples:

• Let's say you want to express that "Lars Marius is employed by Ontopia". For this you would use either Topic Maps (ie: TMDM) or RDF (ie: the basic triple model). That's because these are the basic data models in which all information is expressed, and the statement is what I'd call "instance information", or "application data".
• If you wanted to say that "employment must relate a person to an organization", on the other hand, you would have to use TMCL (on the ISO side) or RDFS (on the W3C side). (You're probably wondering about OWL now, but we'll return to that.) This is because we are now talking about constraints, and no longer about instance information.
• Now, if you wanted to find "all people employed by Ontopia" you would have to use TMQL (ISO) or SPARQL (W3C). These are the standard query languages in the two technology families.
• Finally, if you wanted to take the statement about Ontopia and me and export it from your application for transfer somewhere else (or just plain storage) you'd have to use XTM (with Topic Maps) or RDF/XML (with RDF). These are the standardized XML-based interchange formats on the two sides. (You could also use LTM or n3, but these are text-based, non-standard, and optimized for human readability.)

So where does OWL come into this? Well, in one sense OWL is just the big brother of RDFS, which lets you say lots of things that RDFS isn't powerful enough to let you do. However, OWL is not really a constraint language, but a language optimized for describing ontologies so that you can do inferencing on top of the instance data based on that information. I've blogged about this difference before, so go there if you want to know more.

The questions

With this out of the way it becomes easier to answer the questions Trond asked:

I am curious as to why the XTM community "always" (papers, mailing lists, etc.) compare XTM to RDF/XML and not to e.g. OWL Lite.

The reason should be quite plain now: these are the two standard XML-based interchange syntax in the two technology stacks, so RDF/XML really is the natural equivalent of XTM on the RDF side.

RDF/XML lacks the ability to define new classes and relations / ontologies, which is the purpose of RDFS & OWL (as you would know). Is it really fair to compare XTM to a technology which was not designed to do this, when alternatives / more expressive competing technologies do exist? I do understand Ontopia's (and other companies') motivation behind this, but is the XTM community really better off by ignoring OWL as a competing language/tool?

The simple answer to this is that TMCL is the natural technology to compare OWL with. Of course, the problem is that TMCL is not yet really here yet. However, I have reason to believe that the next draft will take us to the point where TMCL really is here, even if not 100% finalized yet.

If I was new to XTM and in need of a technology in which to define my ontologies, I'd most def. not compare XTM to RDF/XML, but to OWL (which, according to Anne Cregan, may also be used in order to express Topic Maps).

Well, you can't define your ontologies any more in XTM than you can in RDF/XML (or in Topic Maps than in RDF, which I think Trond really meant). To be able to define an ontology beyond just creating the various concepts and wiring them together you really need TMCL.

As for being able to express Topic Maps in RDF, what Anne Cregan really did was to create an RDF vocabulary for expressing Topic Maps. That is, she made a set of RDF properties and RDF types so that Topic Maps information could be represented in RDF as triples. I did the same thing way back in 2002, but I knew even then, that this was not the right way. The thing that Anne did, which I did not, was to describe her vocabulary with OWL, which let OWL work out some of the computed properties of TMDM for you.

I also recently got an email from Patrick Durusau where he wrote:

I am in a conference call presentation where topic maps were described as being "less expressive" that RDFS/OWL.

Patrick disagreed with this, but I'm afraid I do not, if you interpret "expressive" in the sense it's used in logic, where it refers to the amount of logical inferences that some language or representation allows you to draw on the basis of other data. RDFS/OWL really do allow more inferences to be drawn than the basic Topic Maps standard (TMDM) does. I think it's likely that they will also allow more inferences than TMCL, but that TMCL will reduce the gap significantly.

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