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Re: On the integration of Topic Maps and RDF

From: Leo Obrst <lobrst@mitre.org>
Date: Tue, 21 Aug 2001 20:56:25 -0400
Message-ID: <3B830339.E79D71E6@mitre.org>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
CC: lacher@db.stanford.edu, em@w3.org, www-rdf-interest@w3.org, gdm@empolis.co.uk
I have to agree with Peter on this. I really can't understand the
alternative. Unless you really do formalize a third representation
language which attempts to "preserve" the semantics of the mappings
between your other two languages (say, with a notion of the formal
properties represented and preserved on each side through those
"mappings" or morphisms), then you are spinning in air. Semantic
interoperability or semantic "mapping" requires a commensurate
language/model. How else can it work? Modeling the semantics of a model
in the syntax of another model just can't work. You need to preserve the
semantics of the original model when you translate it into the syntax of
the other model (or approximate it to a greater or lesser degree of
possible formalization). 

Leo

"Peter F. Patel-Schneider" wrote:
> 
> I think that we will have to ``agree to disagree'' on whether mappings
> between Topic Maps and RDF should preserve meaning.
> 
> I strongly, no, passionately, believe that such mappings have to be
> model-mappins that preserve meaning, at least if one is to hold the view
> that RDF is a representation formalism.  If RDF is a representation
> formalism, then positive ground binary relations have to be represented as
> RDF triples.  Otherwise, RDF is just some syntactic encoding, and the
> entire meaning is conveyed in some outside-of-RDF (and, probably, outside of
> the web) side agreement.
> 
> Any approach that requires an outside-of-RDF approach to ascribe meaning to
> the resulting RDF has, in my opinion, lost everything.  Yes, an approach
> that stays within RDF has the potential of losing some things, but at least
> the portion that can be naturally represented in RDF is completely
> captured.
> 
> Peter F. Patel-Schneider
> Bell Labs Research
> 
> From: "Martin Lacher" <lacher@db.stanford.edu>
> Subject: RE: On the integration of Topic Maps and RDF
> Date: Tue, 21 Aug 2001 11:25:22 -0700
> 
> > Hi Peter,
> >
> > Thank you very much for your comments. Your question concerns a very
> > important point in the mapping.
> >
> > > One important aspect of such mappings, for me, is whether information
> > > expressed naturally in the source formalism (Topic Maps) and then
> > > translated into the target formalism (RDF) can be naturally
> > > integrated with
> > > information expressed naturally in the target formalism.  If this is not
> > > the case, then I claim that there is something wrong with the translation.
> >
> > You are right, there are several ways to perform such a mapping. Graham
> > Moore has nicely summarized the mapping approaches in his paper on RDF/Topic
> > Maps. He called the approach you propose "mapping the model" and the
> > approach we took "modeling the model".
> > When mapping the model, the semantics of the primitives of one model have to
> > be mapped to the semantics of the primitives in the other model. It is sort
> > of an all-in-one approach, which attacks the problem in one piece (i.e.
> > mapping all semantics of Topic Maps in one piece to all semantics that RDF
> > offers). The downside of this approach is that one is likely to incur loss
> > in the mapping, since the primitive constructs won't be close enough.
> > When modeling the model, one model is expressed in the syntax of another.
> > The advantage of this approach is, that loss is less likely to be incurred,
> > since only the syntax primitives have to be mapped.
> > As an example, mapping the model one has to map what an association conveys
> > in Topic Maps to what a property conveys in RDF. That is virtually
> > impossible, since associations in Topic Maps are much richer. When modeling
> > the model in this case on only has to map what the graph model primitives in
> > the Topic Map model convey to what the graph model primitives in RDF can
> > convey.
> > The unifying view on both of the approaches is that modelling the model is
> > just again a mapping the model on another layer (see paper of Sergey Melnik
> > on layered interoperability model). That means that our graph translation is
> > again a mapping of the graph semantics of Topic Maps (which is expressed in
> > XTM syntax) to the graph semantics of RDF ( any syntax). However, we believe
> > that by performing the mapping between the primitives, we incur less loss.
> >
> > > Suppose some facts about natural resources come from topic maps, and are
> > > represented in this translation to RDF, and other facts about natural
> > > resources come from a natural RDF representation.  How can one query the
> > > RDF to find the union of the facts?  Even if it is possible to
> > > write a such
> > > a query is it at all possible to write such a query without knowing that
> > > some of the natural resource facts come from topic maps?
> >
> > No, the query processor will have to be aware of what source it is querying.
> > But that is not a bad thing, since the specifics of the source can be
> > expressed in a rule language. On top of that rule base one couled possibly
> > imagine some universal query laguage. However, the question is, whether that
> > is desirable at all - since one would lose all the specifics of the
> > respective formats. For example, one could possibly not query for a unique
> > property in DAML data anymore. Or one could not query for an association
> > scope of a Topic Map association anymore. That probably depends on the
> > application context.
> >
> > So the definite advantages of our approach are:
> > - only (n-1) mappings between formats required instead of n(n-1)/2 for an
> > all-capable query engine
> > - mapping between the higher-level constructs can be expressed declaratively
> > - existing RDF infrastructure can be re-used for Topic Maps
> >
> > Cheers,
> >
> > Martin

-- 
_____________________________________________
Dr. Leo Obrst		The MITRE Corporation
mailto:lobrst@mitre.org Intelligent Information Management/Exploitation
Voice: 703-883-6770	7515 Colshire Drive, M/S W640
Fax: 703-883-1379       McLean, VA 22102-7508, USA
Received on Tuesday, 21 August 2001 20:57:45 GMT

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