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Re: Returning to OWL and application profiles

From: Karen Coyle <kcoyle@kcoyle.net>
Date: Tue, 12 Oct 2010 10:12:48 -0700
Message-ID: <20101012101248.esjmtiou84cwok40@kcoyle.net>
To: Emmanuelle Bermes <manue.fig@gmail.com>
Cc: Mark van Assem <mark@cs.vu.nl>, Mikael Nilsson <mikael@nilsson.name>, Antoine Isaac <aisaac@few.vu.nl>, public-lld <public-lld@w3.org>
I agree with Emmanuelle's points below. Creating subclasses for every  
constraint sounds to me like it will fracture our information world.  
While machines may go merrily along creating inferences from classes  
and sub-classes, I think this greatly increases the complexity for  
humans who must design metadata and who will be creating metadata  

I also really appreciate Michael Panzer's explanation of the  
capabilities of OWL, and wonder if we shouldn't be looking for the  
"sweet spot" between modeling for broad information sharing and  
creating applications that make use of the semantic web in very  
specific ways. To use a library example, library applications may  
limit the number of authors to 3 per record, but that doesn't change  
the meaning of each individual author or his/her relationship to the  
work being defined when that data mingles with other data on the web,  
nor does it mean that someone re-using the data couldn't describe  
additional authors. This is a 1+ for Mikael's remark that the "A" in  
AP is "application" -- and there are some constraints that may be best  
left to applications.


Quoting Emmanuelle Bermes <manue.fig@gmail.com>:

> On Tue, Oct 12, 2010 at 4:08 PM, Mark van Assem <mark@cs.vu.nl> wrote:
>> Hi Mikael,
>> I just defended my PhD thesis [1] last week and it contains a section 7.5
>> devoted to APs specified in OWL (also referring to your AP constraint
>> language). I suggest you have a read :-)
>> What I propose is to create subclasses of a particular class such as
>> lib:Book, e.g. my:Book and constrain property values on my:Book so that no
>> inconsistency arises with his:Book (which may have entirely different
>> constraints).
> This approach seems very good from the modelling point of view, but
> I'd like to ask wether it is realistic in a pragmatic Linked Data
> world.
> If I want to add specific constraints on, for instance, dct:creator,
> and I create my:creator, I will reach interoperability with others
> using dct:creator only through inferencing. This doesn't seem very
> straightforward to me. We are lacking actual use of Linked Data today,
> and I feel that adding more complexity in the data model is likely to
> create more barriers.
> As librarians sitting on a big mass of data, our immediate need is to
> be make our data understandable in the global Linked Data world, and
> I'm not sure that encouraging us to systematically create our own
> classes and properties rather that reusing existing ones, for the sake
> of expressing patterns, is the right way to go. I see a risk to
> encourage the creation of a lot of redundant, slightly different but
> almost similar vocabularies (we already have 3 flavours of FRBR out
> there...)
> What I like in the application profile approach is the idea that I can
> reuse *existing* classes & properties, and at the same time, express
> the pattern that my community should reuse for describing similar
> resources.
> It's actually 2 separate needs :
> - a need for visibility and interoperability, that can be fulfilled by
> existing vocabularies with their OWL semantics that are consistent
> globally
> - a need for describing domain-specific patterns.
> We call this "collection-specific value ranges" (with
>> collections referring to cultural heritage collections), but cardinality
>> constraints is basically the same story.
>> The only thing that then has to be added is a way to classify every book as
>> either my:Book or something else (the "something elses" representing wrongly
>> specified books, i.e. violations of the AP constraints). This is probably
>> abusing OWL a bit, but it can probably be done.
>> An alternative is to just accept OWL as a "syntax" for the AP constraints,
>> and implement your own checker on top of that. This removes the need to
>> develop your own language (and parser) which will contain almost the same
>> syntactical elements anyway.
> I like the idea that the AP should be something that could be
> implemented following different syntaxes, maybe including OWL, but not
> excluding other approaches that wouldn't make it mandatory to declare
> local properties and classes systematically when additional semantics
> or constraints are needed.
> Emmanuelle
>> (As far as I can tell this is also what DC-APs are intended to do. I had
>> discussions with Tom Baker about this, and he was part of the committee that
>> accepted my thesis last week so apparently I didn't write rubbish :-)
>> Best,
>> Mark
>> [1]http://www.cs.vu.nl/~mark/papers/thesis-mfjvanassem.pdf
>> On 12/10/2010 13:50, Mikael Nilsson wrote:
>>> Hi Antoine,
>>> tis 2010-10-12 klockan 13:11 +0200 skrev Antoine Isaac:
>>>> Hi Mikael,
>>>> Thanks for starting this interesting thread :-)
>>> Thanks for an interesting reply :)
>>>> Well, I have what I think is a quite traditional SW background, and I'd
>>>> be tempted to turn the argument the other way round ;-)
>>>> If the instances of one class in an AP have one title and the instances
>>>> of that class in another AP have two titles, then it is perhaps that these
>>>> two APs are thinking of two different classes, really. Possibly they could
>>>> be two subclasses of a common superclass, but two different   
>>>> classes, still.
>>>> I think this is quite in line with Jeff's message suggesting you could do
>>>> things like this
>>>> baz:Widget rdfs:subClassOf foo:Widget.
>>>> to make the commitment of your various APs a bit clearer.
>>> Yes, I do believe that conceptually, most APs can be described using the
>>> trick of the creation of subclasses, even though the subclass may well
>>> consist of the same individuals (!)
>>>> I'm also a bit puzzled by your "APs define domain-specific structural
>>>> constraints while OWL adds semantics to existing classes." Again, I am
>>>> pretty new to the APs as practiced in the DC realm, but why wouldn't you
>>>> consider classes (and APs built on top of them) from a (SW) semantic
>>>> perspective?
>>>> I understand that OWL and RDF will fail for arrangement/presentation of
>>>> data (order of XML elements, e.g.), which is not really about   
>>>> semantics. But
>>>> to me--and to many in the SW community--cardinality belongs to   
>>>> semantics of
>>>> classes and properties.
>>> Let me give a simple example. Let's assume I am designing a simple REST
>>> service for retrieving metadata records from a library repository (never
>>> mind the 303s etc).
>>> I decide to return Turtle representations of the form
>>> myrepo:book123 rdf:type lib:Book,
>>>                dct:title "Moby Dick",
>>>                dct:creator myrepo:author345 .
>>> I can define this pattern using OWL restrictions on the lib:Book class.
>>> Later on, I want to define an extended API for use by library partners.
>>> This API returns records of the form
>>> myrepo:book123 rdf:type lib:Book,
>>>                dct:title "Moby Dick",
>>>                dct:creator myrepo:author345,
>>>                lib:numCopies "5".
>>> This record describes the exact same individual but using a different
>>> application profile.
>>> To solve this with the subclass method I would need to define a subclass
>>> lib:ExtendedBook that captures the additional property, but has the same
>>> extension.
>>> My point is that application profiles describe metadata *patterns*,
>>> independently of the semantics. None of the above properties are likely
>>> required (cardinality>  0) for the Book class in any case. And there may
>>> well be multiple such patterns for the same class or set of things.
>>> In the DC world, the use of application profiles is often framed in this
>>> way - an application profile does not describe a class, it describes the
>>> metadata records emitted or accepted by a system (hence the term
>>> "application").
>>>>> But that's exactly it - the semantics is "alternative" and on its face,
>>>>> the semantics of the published OWL file is something else entirely.
>>>>> As a concrete example, if I serve an OWL file from my web server, using
>>>>> application/rdf+xml as suggested by the OWL specs [1], the
>>>>> interpretation will be as RDF triples using the RDF and OWL built-in
>>>>> semantics, thus resulting in the generation of new triples, potentially
>>>>> contradiction with other ontologies, and not in validation as expected.
>>>> Well, contradiction (aka inconsistency) is what is used for data
>>>> validation in the RDF/OWL world. So if we can detect contraditions I
>>>> wouldn't be unhappy, at least from the perspective of us able to
>>>> validate some data :-)
>>> My point is that the two ontologies needed to describe the two uses of
>>> lib:Book above would be contradictory. One would the be left wondering,
>>> "what is the semantics of the lib:Book class, is it as defined by
>>> ontology A or B?" and that's not good. The truth is that the semantics
>>> of lib:Book is less constrained (all the above properties optional), and
>>> the constraints appear only to describe certain *records*, not the
>>> lib:Book class itself.
>>>> It's really a situation where the cons for an OWL approach (and being
>>>> awkward to manipulate is not the least default OWL has, sure) could be
>>>> balanced by some strong pros, and we should not discard them too
>>>> quickly!
>>> I agree that OWL has advantages, but we need to develop an understanding
>>> of what kinds of validation we can achieve.
>>> Maybe we arrive at the conclusion that we need to accept a different set
>>> of use cases for OWL-based ontologies. I'm not very happy with that
>>> thought, however.
>>> I therefore lean towards a solution based on true syntactical constraint
>>> language on RDF graphs. Alistair Miles did some experiments in that
>>> direction, but they seem to have disappeared from the net.
>>> It would be extremely interesting to try to figure out whether there are
>>> significant use cases for purely syntactic constraints in the LD domain.
>>> /Mikael

Karen Coyle
kcoyle@kcoyle.net http://kcoyle.net
ph: 1-510-540-7596
m: 1-510-435-8234
skype: kcoylenet
Received on Tuesday, 12 October 2010 17:13:26 UTC

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