- From: Jim Hendler <hendler@cs.umd.edu>
- Date: Fri, 9 May 2003 18:56:16 -0400
- To: webont <www-webont-wg@w3.org>
The following is my proposed response to the message from Martin Merry, which is found at: http://lists.w3.org/Archives/Public/public-webont-comments/2003May/0046.html Thanks for your comment. We believe it raises two important questions, one is whether the design of OWL DL as is would require a long CR period, the other is whether dropping #hasValue and #oneOf should be done by the WG. Let me answer these in reverse order. 1 - a Long CR: From a Process point of view (and ignoring your technical point for a moment, I promise to return to it), I would point out that there is no requirement that all features of a recommendation must be handled by all implementations. The process document states that the requirement to move from CR to PR is: each feature of the technical report has been implemented. Preferably, the Working Group should be able to demonstrate two interoperable implementations of each feature. For every part of the OWL DL specification we currently have multiple implementations that can implement it, and in fact for every subset of the language features that doesn't contain BOTH "inverseOF" and "OneOf" we have multiple working and interoperable implementations either complete at this time, or coming soon. So from a process point of view, we believe the current OWL DL implementation meets the requirements for advancement. 2 - Your second issue is a more important one, you suggest that for the current OWL DL, which includes OneOf there is a lack of practical implementation experience -- this is not quite correct. We have numerous implementations that support oneOf quite well. What is correct is that for the whole of OWL DL, which includes BOTH owl:inverseOf and owl:oneOf there is a problem -- if both are used together, in the same ontology, there is a possibility that the solution to some queries will take a very long time (that is, that there is no effective algorithm). This is true -- although the language containing both of those features is decidable (it is, and it's not just Ian's assertion, the proofs are quite well known and studied), the algorithmic complexity is potentially quite high. However, there are other solutions than removing owl:oneOf, for example removing owl:inverseOf would work as well. In fact, the problem is that there are two useful features in OWL DL (inverse properties and designators such as oneOf and hasValue) that are widely used in practice. The WG believes that removing either one of these would cause there to be many valuable use cases that could not be represented in OWL DL. However, there is no problem with documents that use either Inverses XOR designators, it is only when both are used together that the problem occurs. A "common sense" analogy is found in medicine where there are many drugs that are each beneficial, but when used together they can cause bad side effects -- inverse and oneOf are analogous to two such drugs. In your comment you write: We find that the draft documents make it clear that OWL Full systems will not have full reasoning support and that therefore users will not be too surprised when there is a resulting migration cost from one OWL Full system to another. so we believe that if the documents made clearer that using BOTH oneOf and inverseOf (and their various forms) could lead to an unexpected rise in complexity, we would set the expectation correctly. In that way the current OWL DL subset would be easier to understand, and the design rationale behind it better understood. Thus, given these two below, we propose that the WebOntology working group will make the issue above clearer and will write text to appear in the Reference, S&AS and Test documents that explain the above. However we would also propose not to remove these very useful features from OWL DL or to have a long CR period mandated by this specific issue. If this outline of a solution is acceptable to you, we will produce proposed text properly setting the expectations with respect to inverse and oneof, and request your approval before closing this comment. We look forward to your response Jim Hendler WebOnt Co-Chair, on behalf of the Working Group At 15:18 +0100 5/9/03, Merry, Martin wrote: We wish to comment on the usefulness of OWL DL as a sensible subset of OWL Full. We're concerned that OWL users should have their expectations met when they use OWL compliant systems. We find that the draft documents make it clear that OWL Full systems will not have full reasoning support and that therefore users will not be too surprised when there is a resulting migration cost from one OWL Full system to another. We are concerned, however, that OWL DL is presented as a sensible stopping point before OWL Full, where there are greater guarantees. The theoretical results for the decidability of OWL DL are interesting but not particularly helpful. OWL Lite is justified by practical results in DL systems (primarily from Ian Horrocks). There is no such practical experience for the OWL DL subset. We would like to see such practical experience before OWL exits candidate recommendation. In particular, we would like to see adequate practical implementation experience of the OWL DL constructs owl:oneOf and owl:hasValue. We believe that this should include the goal that OWL DL reasoners can make a reasonable attempt at classic NP complete problems (such as the 3-SAT problem and the subgraph isomorphism problem) which can be straightforwardly encoded within OWL DL. For example, any such problem that can be solved in seconds by a specialised reasoner should be soluble by a general OWL DL reasoner in minutes rather than years. An alternative, would be to redefine OWL DL downwards, excluding owl:oneOf and owl:hasValue, which would then be subject to the health warnings of OWL Full - i.e. use of these constructs means that your ontology is likely to be outside the limits of practical reasoning. Such a redefinition of OWL DL, could sensibly accompany a redefinition of OWL Lite to exclude complete class definitions. Martin Merry HP Semantic Web Programme Manager Martin Merry HP Semantic Web Programme Manager -- Professor James Hendler hendler@cs.umd.edu Director, Semantic Web and Agent Technologies 301-405-2696 Maryland Information and Network Dynamics Lab. 301-405-6707 (Fax) Univ of Maryland, College Park, MD 20742 240-731-3822 (Cell) http://www.cs.umd.edu/users/hendler
Received on Friday, 9 May 2003 18:56:20 UTC