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RE: On the integration of Topic Maps and RDF

From: Martin Lacher <lacher@db.stanford.edu>
Date: Tue, 21 Aug 2001 11:25:22 -0700
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>, <em@w3.org>
Cc: <www-rdf-interest@w3.org>, <gdm@empolis.co.uk>
Message-ID: <NEBBIEAODMJKOIMFPAEBAEFICLAA.lacher@db.stanford.edu>
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
Received on Tuesday, 21 August 2001 14:28:22 GMT

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