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Re: Profiles intro

From: Carsten Lutz <clu@tcs.inf.tu-dresden.de>
Date: Wed, 09 Apr 2008 19:49:23 +0200 (CEST)
To: Ivan Herman <ivan@w3.org>
Cc: Bijan Parsia <bparsia@cs.man.ac.uk>, OWL Working Group WG <public-owl-wg@w3.org>
Message-id: <Pine.LNX.4.64.0804091944510.8529@frege.inf.tu-dresden.de>

A wrap-up after my longish mail. My view on the use of fragments is:

- you want to design an ontology and efficiently reason about it
   -> EL++
- you want to query large-scale data w.r.t. a very lightweight ontology
   -> DL-Lite
- you want to enrich RDF with some OWL expressivity retaining efficiency
   of reasoning
   -> OWL-R

I expect that not everybody agrees. True?


PS: Zhe: can you provide a one-liner describing the most typical scenario
     for OWL-R? We are clearly not restricted to a one-liner in our
     documents, but it would help to understand your view.

On Wed, 9 Apr 2008, Carsten Lutz wrote:

> I totally agree with Bijan to put a note to the intro for now. Still,
> here is a bit of discussion of fragments (maybe for future use).
> On Wed, 9 Apr 2008, Ivan Herman wrote:
>> [[[
>> - "The EL++ profile is for efficient reasoning about large-scale ontologies 
>> formulated on a high level of abstraction."
>> ]]]
>> To be honest, I do not understand that. I am not playing dumm; I _really_ 
>> do not understand what you mean!
> Can you elaborate? "Efficient reasoning about large-scale ontologies"
> sounds reasonably comprehensible. Is it the high level of abstraction
> that bothers you?  Here is a more detailed explanation of what I mean,
> phrased in a possibly more controversial style that I would not use in
> the documents:
> EL++ is designed for use in the construction of real (and large scale)
> ontologies such as SNOMED and NCI. IMHO, this already distinguishes it
> from DL-Lite and OWL-R as, in my very personal opinion, the latter two
> are not good as ontology languages:
> - DL-Lite is very weak: it does not allow at all to relate two classes
>  in terms of a property (neither SomeValuesFrom nor AllValuesFrom
>  available).
> - OWL-R allows different constructs on the left-hand side and right-hand
>  side of SubClassOf. For example, ObjectSomeValuesFrom is allowed only
>  on the left. This means that I cannot use *definitions* in my ontology,
>  i.e., statements that give both necessary and sufficient conditions for
>  class membership. As an example, take
>    "A Father is a Human which has a Child"
>  To say this, I need to say that i) Father is a subclass of the
>  intersection of Human and (someValuesFrom hasChild Human) and
>  (ii) the intersection of Human and (someValuesFrom hasChild Human)
>  is a subclass of Father. Such symmetric definitions are standard,
>  but cannot be made if the left- and right-hand sides of SubClassOf
>  admit different constructs.
> This does not at all mean that DL-Lite and OWL-R are useless. To me,
> they are constraint languages rather then ontology languages, and they
> are useful for data access and from an RDF perspective. But if you
> want to *design an OWL ontology*, you may not be happy with DL-Lite and OWL-R 
> (and indeed, I know of no ontologies at all formulated in
> these languages).
> Back to the "high level of abstraction": the benefit of EL++ over full
> OWL DL as an ontology language is that is admits efficient (in the
> sense of polytime) reasoning. You have to pay for this advantage by
> giving up expressive power. But we should not say to a user "use it if
> you want to give up expressive power". That's simply not the right way
> to put it. The typical use pattern of EL++ is to give a much less
> fine-grained modelling than with a more expressive language, see
> e.g. SNOMED and NCI. In other words: representing things in a formal
> language *always* means abstraction. Even in full OWL 2 and even in
> first-order logic, there are a lot of standard things that are not
> expressible. When using EL++ instead of OWL DL, this simply means that
> you have to abstract even further than in OWL DL. This is what the
> switch to a more lightweight ontology language means to the user.
>> [[[
>> - "The DL-Lite profile is for using conventional database systems to 
>> efficiently query large amounts of data in the presence of a very 
>> lightweight ontology."
>> - "The OWL-R profile is for efficient rule-based reasoning about 
>> lightweight ontologies and potentially large amounts of data."
>> ]]]
>> The interesting thing is that, from a user point of view and based on those 
>> two statements, there is no clear reason why choosing one over the other!
> There is, and I tried to capture it in the slogans: If you want to use
> a conventional and off-the-shelf database system without modifying it
> in any way whatsoever, you can only use DL-Lite, but (provably!) not
> OWL-R. If you want to do forward chaining, OWL-R is for you (though
> you could also use DL Lite, and would then get a different kind of
> expressive power).
> But let me also say this: it is naive to think that there is always a
> clear reason to use an ontology language and not another. In the end,
> users will anyway have to look at the provided expressivity to decide
> whether they can live with it.
>> Both profiles are for lightweight ontologies and large amount of data.... 
>> _That_ statement I do understand and like. But then... why having _both_ 
>> DL-Lite and OWL-R?
> Because they are orthogonal in expressive power, and target a different
> use (conventional DBs vs rules, see above).
>> Is there a way to differentiate between the terms 'lightweight' ontologies? 
>> Can we say that OWL-R is 'lighter' than DL-Lite?
> They are orthogonal in expressive power, but to me DL-Lite "feels"
> lighter than OWL-R. I intended to reflect this in my slogans: "very
> lightweight" for DL-Lite as opposed to "lightweight" for OWL-R.
>> To be more positive:-), I guess _a_ differentiating factor is the 
>> technology that can be used to implement those. Rule based means that I can 
>> implement OWL-R via either some simple (Horn?) rule engine or a simple 
>> procedural environment easily. I am not sure how one can characterize the 
>> DL-Lite implementation engine.
> Query rewriting + off-the-shelf database system.
> greetings,
> 		Carsten
> --
> *      Carsten Lutz, Institut f"ur Theoretische Informatik, TU Dresden 
> *
> *     Office phone:++49 351 46339171   mailto:lutz@tcs.inf.tu-dresden.de 
> *

*      Carsten Lutz, Institut f"ur Theoretische Informatik, TU Dresden       *
*     Office phone:++49 351 46339171   mailto:lutz@tcs.inf.tu-dresden.de     *
Received on Wednesday, 9 April 2008 17:50:09 UTC

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