- From: Ian Horrocks <horrocks@cs.man.ac.uk>
- Date: Sun, 11 May 2003 18:45:05 +0100
- To: Jim Hendler <hendler@cs.umd.edu>
- Cc: webont <www-webont-wg@w3.org>
Jim, I think that this is a good response. A couple of small suggestions: 1. You could point to the extensive discussion we had regarding the features to be in/out of OWL DL and Lite (Dan seems to like this kind of thing as it shows that the WG have considered the issue); 2. You might want to say "removing owl:inverseOf would work *almost* as well" (or some such), given that removing nominals gets you to a logic for which implemented DL reasoners exist, whereas removing inverse only gets you to a logic for which we know in principal how to build such reasoners. Ian On May 9, Jim Hendler writes: > > 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 Sunday, 11 May 2003 13:45:15 UTC