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Re: Proposed response to Martin Merry, HP

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Sun, 11 May 2003 18:45:05 +0100
Message-ID: <16062.35873.318435.816245@merlin.horrocks.net>
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 GMT

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