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Re: ISSUE 131 (OWL R Unification): Different semantics on syntactic fragment

From: Ivan Herman <ivan@w3.org>
Date: Tue, 12 Aug 2008 16:54:03 +0200
Message-ID: <48A1A40B.2020009@w3.org>
To: Ian Horrocks <ian.horrocks@comlab.ox.ac.uk>
CC: Michael Schneider <schneid@fzi.de>, public-owl-wg@w3.org
Ian Horrocks wrote:
> Ivan,
> I'm not sure what it means for an implementation to be "perfectly o.k." 
> or "proper". According to what we discussed, implementations based on 
> the OWL RL rules is that they are sound for OWL Full semantics and also 
> complete for query answering (the key entailment in the kinds of 
> application that motivate OWL R) for ontologies inside a given syntactic 
> fragment. This was always clearly stated (see Section 4.4 of the 
> existing spec).

As I said: my mistake not to have realized the full depth of that 
statement. Thanks to Michael for clarifying it (at least for me).

>          The only thing that I have heard against this are some 
> strange arguments about OWL RL reasoners not being "competitive" with 
> OWL DL/Full reasoners/semantics. 

This was not my argument, or at least not the intention of it:-).

Referring to another mail of yours in the thread[1] I think we actually 
agree on what you called there 'advantage' and 'disadvantage'. Yes, 
keeping the rules separate has the 'advantage' that implementers of 
those rules would be able to declare themselves sound and complete w.r.t 
that semantics. However, we seem disagree on the relative 
weight/importance of those advantages and disadvantages...

One way forward could be to formulate this question in precise manner 
and ask the community, eg, potential RDF implementers and users both 
within and outside the Working Group, to see how they would feel about 
that 'advantage'/'disadvantage', whether that would be a major issue (I 
am thinking about Oracle, HP, OpenLink, Franz Inc, and others that I 
might very well forget in the implementation space, and there are a 
number in the user space, too). We are actually talking about marketing 
type of issues here and, let us face it, we are certainly not experts of 
that... Such a feedback may help us in moving forward.



> Regards,
> Ian
> On 12 Aug 2008, at 09:35, Ivan Herman wrote:
>> Hi Ian,
>> In a thread you started a while ago (after a long discussion on 
>> another thread):
>> http://www.w3.org/mid/AEDBB4D5-D0D4-4B0D-8A2B-B308C51E8518@comlab.ox.ac.uk 
>> we seemed to converge toward a solution based on first finalizing the 
>> conformance issues. However, _my_ assumption back then was that if an 
>> implementation implements the OWL R Rule set (and _only_ that), then 
>> this would be a perfectly o.k. OWL R implementation for all those RDF 
>> graphs that fall under the OWL R syntactic restrictions. The issue we 
>> had, and for which the thread offered a good way forward, was what 
>> would happen to graphs that fall outside the syntactic restriction of 
>> OWL R.
>> However... I was clearly wrong. Michael's example shows that the Rule 
>> set would not provide a proper implementation of OWL R. A conformant 
>> OWL R implementation must do more than just implementing the rules or, 
>> put it another way, an implementation using the rules only could not 
>> claim to be OWL R conformant. This is a different and pretty serious 
>> issue which, I must admit, would make me vote against unification at 
>> this point.
>> In another mail Michael says that we should close this issue without 
>> action. Maybe so. At the minimum we should acknowledge that there is 
>> no consensus on this issue at the moment...
>> Sigh...
>> Ivan
>> Ian Horrocks wrote:
>>> Michael,
>>> It has never been claimed that OWL R DL and OWL R Full are completely 
>>> equivalent on the syntactic fragment. What is claimed (see Section 
>>> 4.4 of the existing Profiles doc) is that for ontologies within the 
>>> syntactically defined fragment the OWL RL rules will be complete 
>>> w.r.t. certain kinds of entailment -- in particular w.r.t. those 
>>> entailments that correspond to query answering.
>>> Your example is a good illustration of why it would be *a very bad 
>>> idea* to define a 3rd semantics for OWL based on the OWL RL rules. 
>>> According to this semantics, it would NOT be the case that 
>>> owl:intersectionOf (C D) is a subClassOf D. Any reasoner finding this 
>>> entailment would be unsound and non-conformant w.r.t. this semantics. 
>>> This would, IMHO, be highly counter-intuitive. Moreover, I expect 
>>> that many rule based implementations *will* find this entailment, or 
>>> at least answer "yes" to a query asking if it holds, because they 
>>> will use the standard rule-based technique for answering such a 
>>> subsumption query, i.e., assert a fresh individual to be an instance 
>>> of owl:intersectionOf (C D) and check if it is entailed that this 
>>> individual is an instance of D. This entailment *will* follow from 
>>> the rules.
>>> Regarding the guarantees that are made, guaranteeing soundness and 
>>> completeness w.r.t. OWL RL Full semantics for ontologies in the 
>>> syntactic fragment and certain kinds of entailment seems to be the 
>>> best that we can do -- as you yourself have clearly illustrated. I 
>>> don't pretend to fully understand your statements about reasoners 
>>> based on subsets of the OWL RL rules, but they seem to be based on a 
>>> misunderstanding about what is being stated w.r.t. soundness and 
>>> completeness, and on some strange assumptions about how conformance 
>>> will be defined.
>>> Regards,
>>> Ian
>>> On 10 Aug 2008, at 15:45, Michael Schneider wrote:
>>>> Dear WG!
>>>> I was able to find an example, which demonstrates that the semantics 
>>>> of OWL R DL and OWL R Full differ on the syntactic fragment of a 
>>>> unified OWL R language (see below). The example shows that 
>>>> entailments exist w.r.t. the semantics of OWL R DL (actually, the 
>>>> OWL 2 DL semantics), for which there is no equivalent sequence of 
>>>> rule applications in OWL R Full. This means that the semantics of 
>>>> OWL R ontologies, which match the syntactic fragment of OWL R, 
>>>> cannot be defined in parallel by both the DL semantics *and* the 
>>>> ruleset.
>>>> An alternative approach would be to define the semantics of OWL R on 
>>>> the syntactic fragment by the OWL 2 DL semantics only. However, I 
>>>> would expect that this would lead to significant acceptance problems 
>>>> for implementers, who want to base the implementation of their OWL R 
>>>> reasoner on the OWL R ruleset (and for customers/users of such 
>>>> reasoners, either).
>>>> An implementer, who plans to create a reasoner based on the ruleset, 
>>>> certainly wants to be ensured that his reasoner implements *all* of 
>>>> OWL R, not only a subset, i.e. is complete w.r.t. the OWL R 
>>>> semantics. It would at least be very confusing and hard to accept 
>>>> that it is not sufficient to just implement the "OWL R rules", in 
>>>> order to create a complete OWL R reasoner. But with my counter 
>>>> example, it now turns out that completeness will not be achievable 
>>>> for such a rulebased reasoner.
>>>> Even worse, having such a concrete counter example makes it easy to 
>>>> create a proper *superset* of the current ruleset, which will still 
>>>> be sound w.r.t. the OWL R semantics. This means that the implementer 
>>>> cannot even claim that his reasoner is at least the "best possible" 
>>>> triple-rule based implementation of OWL R.
>>>> The only thing which the implementer can claim is that his reasoner 
>>>> is a "valid" OWL R reasoner, i.e. is sound w.r.t. the OWL R 
>>>> semantics. But this will also be the case for any reasoner, which is 
>>>> based on an arbitrary *subset* of the OWL R rules, and even for 
>>>> trivial reasoners, which do not produce any inferences at all. So, 
>>>> OWL R soundness alone isn't a very interesting property of an OWL R 
>>>> reasoner in practice.
>>>>> From this p.o.v., I would expect that, after a unification, 
>>>>> implementers will not regard the OWL R ruleset to be relevant as a 
>>>>> base for the implementation of OWL R reasoners. IMO, the OWL R 
>>>>> rules will then be no more than just an arbitrary example for how 
>>>>> one MAY create a valid OWL R reasoner. Without completeness 
>>>>> guarantees, this ruleset won't be better than any other ruleset 
>>>>> which provides formal OWL R soundness. Compared with the ruleset's 
>>>>> current status in OWL R Full, where it specifies the semantics of 
>>>>> the language, this would be a significant demotion.
>>>>    * * *
>>>> The example below is not simply a bug in the current ruleset, which 
>>>> could easily be fixed. Instead, it shows two general problems:
>>>>  (1) The ruleset would need to be able to create fresh bNodes on the 
>>>> right hand side of derivations. To my understanding, it was a design 
>>>> goal to avoid this. At least, none of the current rules creates such 
>>>> new bNodes on the consequent side.
>>>>  (2) Since the semantics of the current OWL R DL language is the DL 
>>>> semantics, it allows for very flexible reasoning with complex class 
>>>> expressions, as long as an ontology conforms to OWL R DL's syntactic 
>>>> restrictions. The current triple ruleset isn't able to compete with 
>>>> this amount of flexibility. And don't believe that this can be 
>>>> overcome.
>>>> Here is the example. We start from the RDF graph
>>>>   G_L := {
>>>>     ex:C rdf:type owl:Class .
>>>>     ex:D rdf:type owl:Class .
>>>>     ex:C rdfs:subClassOf ex:D .
>>>>   }
>>>> This is a valid OWL R DL ontology in RDF graph form, since only named
>>>> classes are used on the LHS and RHS of the subclass axiom (see sec. 
>>>> 4.2.3
>>>> of [1]).
>>>> The second RDF graph is
>>>>   G_R := {
>>>>     ex:C rdf:type owl:Class .
>>>>     ex:D rdf:type owl:Class .
>>>>     _:X owl:intersectionOf ( ex:C ex:D ) .
>>>>     _:X rdfs:subClassOf ex:D .
>>>>   }
>>>> Again, this is a valid OWL R DL ontology, because this graph 
>>>> RDF-maps to the
>>>> following expression in Functional Syntax (omitting declarations):
>>>>   SubClassOf( IntersectionOf(ex:C ex:D) ex:D )
>>>> According to sec. 4.2.3 of [1], intersections of named classes may 
>>>> occur on
>>>> the LHS of subclass axioms.
>>>> The semantic meaning of these two graphs is given in OWL R DL by 
>>>> applying
>>>> the OWL 2 DL semantics. According to the OWL 2 DL semantics, the 
>>>> entailment
>>>>   G_L |= G_R
>>>> is true, because from
>>>>   C subset D
>>>> follows
>>>>   (C and D) subset D
>>>> However, there is no sequence of applications of OWL R Full rules, 
>>>> which
>>>> starts in G_L, and which produces G_R (or a super graph of G_R) as a 
>>>> result.
>>>> Actually, there is no single triple rule in OWL R Full that would 
>>>> allow to
>>>> produce the bNode '_:X', which occurs in G_R but not in G_L.
>>>> Regards,
>>>> Michael
>>>> [1] <http://www.w3.org/2007/OWL/wiki/Profiles#Class_Expressions_3>
>>>> -- 
>>>> Dipl.-Inform. Michael Schneider
>>>> FZI Forschungszentrum Informatik Karlsruhe
>>>> Abtl. Information Process Engineering (IPE)
>>>> Tel  : +49-721-9654-726
>>>> Fax  : +49-721-9654-727
>>>> Email: Michael.Schneider@fzi.de
>>>> Web  : http://www.fzi.de/ipe/eng/mitarbeiter.php?id=555
>>>> FZI Forschungszentrum Informatik an der Universität Karlsruhe
>>>> Haid-und-Neu-Str. 10-14, D-76131 Karlsruhe
>>>> Tel.: +49-721-9654-0, Fax: +49-721-9654-959
>>>> Stiftung des bürgerlichen Rechts
>>>> Az: 14-0563.1 Regierungspräsidium Karlsruhe
>>>> Vorstand: Rüdiger Dillmann, Michael Flor, Jivka Ovtcharova, Rudi Studer
>>>> Vorsitzender des Kuratoriums: Ministerialdirigent Günther Leßnerkraus
>> -- 
>> Ivan Herman, W3C Semantic Web Activity Lead
>> Home: http://www.w3.org/People/Ivan/
>> PGP Key: http://www.ivan-herman.net/pgpkey.html
>> FOAF: http://www.ivan-herman.net/foaf.rdf


Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
PGP Key: http://www.ivan-herman.net/pgpkey.html
FOAF: http://www.ivan-herman.net/foaf.rdf

Received on Tuesday, 12 August 2008 14:54:35 UTC

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