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Re: Profiles intro

From: Ivan Herman <ivan@w3.org>
Date: Wed, 09 Apr 2008 20:30:22 +0200
Message-ID: <47FD0B3E.2080505@w3.org>
To: Carsten Lutz <clu@tcs.inf.tu-dresden.de>
CC: Bijan Parsia <bparsia@cs.man.ac.uk>, OWL Working Group WG <public-owl-wg@w3.org>

as far as I am concerned, this thread was actually very helpful. 
Followup tomorrow...

Thank you


Carsten Lutz wrote:
> 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?
> greetings,
>         Carsten
> 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     *


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 Wednesday, 9 April 2008 18:31:05 UTC

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