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Re: Returning to OWL and application profiles

From: Mikael Nilsson <mikael@nilsson.name>
Date: Tue, 12 Oct 2010 13:50:19 +0200
To: Antoine Isaac <aisaac@few.vu.nl>
Cc: public-lld <public-lld@w3.org>
Message-ID: <1286884219.4536.113.camel@daneel>
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 
Received on Tuesday, 12 October 2010 11:50:56 GMT

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