Saturday, August 04, 2007

OWL Inexpertise

One of my concerns about Talking to Talis yesterday (interesting pun between a verb and a noun there) was in making criticisms of some of the people working on OWL, when I'm really not enough of an expert to make such a call.

I expressed concern over the "logicians" who have designed OWL as being out of touch with the practical concerns of the developers who have to use it. While I still believe there is basis for such an accusation, it is glossing over the very real need for a solid mathematical foundation for OWL, and is also disrespectful to several people in the field whom I respect.

Knowing and understanding exactly what a language is capable of, is vital in its development. Otherwise, it is very easy to introduce features that conflict, or don't make sense in certain applications. Conflicting or vague definitions may work in human language, but is not appropriate when developing systems with the precision that computers require. I have to work hard to get to the necessary understanding of description logic systems, which is why I respect people like Ian Horrocks (or Pat Hayes, or Bijan Parsia, the list goes on...) for whom it all seems to come naturally. Without their work, we wouldn't know exactly what all the consequences of OWL are, meaning that OWL would be useless for reasoning, or describing much of anything at all.

However, coming from a perspective of "correctness" and "tractability", there is a strong desire in this community to keep everything within the domain of OWL-DL (the computationally tractable variant of OWL). Any constructs which fall outside of OWL-DL (and into OWL Full) are often dismissed. Anyone building systems to perform reasoning on OWL seems to be limiting their domain to OWL-DL or less. There appears to be an implicit argument that since calculations for OWL Full cannot be guaranteed to complete, then there is no point in doing them. Use of many constructs is therefore discouraged, on the basis that it is OWL Full syntax.

While this makes sense from a model-theoretic point of view, pragmatically it doesn't work. Turing machines are not tractable (for instance, one can create an infinite loop), and yet no one has suggested that Turing complete languages are not important! Besides, Gödel taught us that tractability is not all that it's cracked up to be.

A practical example of an OWL Full construct is in trying to map a set of RDBMS tables into OWL. It is very common for such tables to be "keyed" on a single field, often a numeric identifier, but sometimes text (like a student number, or SSN). Even if these fields are not the primary key of the table, a good mapping into a language like OWL will need to capture this property of the field.

The appropriate mapping of a key field on a record is to mark that field as a property of type owl:InverseFunctionalPredicate. However, it is not legal to use this property on a number or a string (an RDF literal) in anything less than OWL Full.

There are workarounds to stay within OWL-DL. However, this is one of many common use cases where workarounds are required to stay within the confines of OWL-DL. While theoretically possible that using owl:InverseFunctionalPredicate on a "literal" would cause intractability, most use cases will not lead to this. It would seem safe in many systems to permit this - with an understanding of the dangers involved. Instead, the unwillingness of the experts to let people work with OWL Full, has caused onerous restrictions on many developers. This in turn leads to them simply not bothering with OWL, or to go looking for alternatives.

I can appreciate the need to prevent people from shooting themselves in the foot. On the other hand, preventing someone from taking aim and firing at their feet often leads to other difficulties, encouraging them to just remove the safety altogether.

It's an argument with two sides. There may well be many logicians out there who agree that a practical approach is required for developers, in order to make OWL more accessible to them. However, my own observations have not seen any concessions made on this point.
There. It reads much better here than the bald assertion I made for Talis. :-)

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