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Re: Returning to OWL and application profiles

From: Karen Coyle <kcoyle@kcoyle.net>
Date: Tue, 12 Oct 2010 15:02:17 -0700
Message-ID: <20101012150217.e6bczz0r9cggckw0@kcoyle.net>
To: Antoine Isaac <aisaac@few.vu.nl>
Cc: public-lld <public-lld@w3.org>
Quoting Antoine Isaac <aisaac@few.vu.nl>:

> Interesting example. But in fact if we went for OWL modelling in this
> AP sequence, there would be nothing that prevents the re-use of, say,
> dc:creator everywhere:
> - 1. domain vocabulary coins dc:creator  in its ontology
> - 2. ontology for AP1 defines a new class of books with a cardinality
> restriction (3) for its usage of dc:creator
> - 3. other applications can freely re-use the dc:creator statements of
> application 1. If they don't re-use the class defined in the ontology
> for AP1, then there is nothing against having dc:creator applied more
> than 3 times for their resources.

Yes, I agree with this. I do still worry that having every combination  
of constraints be a new class will become un-usable. So a usage  
restriction of 3 plus a value vocabulary v. usage restriction of 3 and  
no stated vocabulary; a restriction of 2 plus that same value  
vocabulary; a restriction of 5 and vocabulary or no vocabulary -- it  
gets out of hand pretty easily. (Note: in one system I recall having  
to limit author names to 99 because there were records that exceeded  
that!). It's not a matter of whether it CAN be done, but whether doing  
it this way becomes a hindrance to metadata creation.


> Note that in relation to your last sentence I've used "ontology" for
> both the domain vocabulary and the one for AP1. The SW stack assigns no
> intrinsic commitment wrt. a given application level for ontologies:
> ontologies are meant to define constraints or data production rules,
> irrespective of whether these will be used for an entire domain or a
> specific application.
> Antoine
>> 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 22:02:51 UTC

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