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Re: Metadata, XML, and RDF

From: Sankar Virdhagriswaran <sv@crystaliz.com>
Date: Mon, 15 Nov 1999 19:01:00 -0500
Message-Id: <199911160004.TAA02421@hunchuen.crystaliz.com>
To: Michael Uschold <michael.f.uschold@boeing.com>, www-rdf-interest@w3.org
As far as I know there is no white paper. There is a W3C report that
compares RDF to UML. 

I am sure others would have different views on this, but the way I have
thought about this is to think of RDF as a conceptual modeling language and
XML-Schema as a logical schema language. RDF comes from the knowledge
representation community and hence worries a lot about semantic
representation and reasoning through them. The most practical application of
this sort of stuff has been in conceptual modeling languages such as UML
(which is why reading the comparison report is interesting) and in expert
systems (or limited versions of these).

XML-Schema is verbose and voluminous, but is driven by the need to support
exchange of document + ORDBMS data and with the need to support different
namespaces for schema. There are a lot of practical applications for this
sort of stuff.

RDF on the other hand worries about a new way modeling resources from which
'automated' agents could reason. One way to think about RDF is Prolog
clauses. The analog of the Prolog unification (i.e., inferencing) is what
automated agents that process RDF will have to write. The choice of syntax
in RDF makes it difficult for folks who are used to the style of mark-up in
XML and HTML. 

As such, representing your information in RDF allows for a lot of power and
flexibility. As can be seen from recent traffic on this group, one can start
with querying and go all the way up to inferencing. Furthermore, RDF Schema
makes expression of object oriented models fairly straight forward.

So, the map as I see it as follows:

At the logical schema level we went from Relational tables in the 70s, to
ORDBMS structures in the 90s. This stream merged with structured document
representation (a la SGML) to produce XML-Schema, XLink, XPath, etc.

At the conceptual schema level we went from entity-relationship
representations in the 80s to predicate calculus and object based (e.g.,
UML) models in the 90s (actually predicate calculus has been around since
the 80s, but had not been applied in semantic mediation). 

Notice that conceptual models were useful only in the context of working
with logical schemas. However, that was the closed world of DBMSs. On the
Web, one can have completely different 'services' develop even with
separation between these. Unfortunately, one is affected by mind space and
resources to address both. 

But, hey, the world is big. Maybe the best that can come out is some set of
people will work on services that leverage the logical schema level while
others will offer services based on conceptual schema level. Who knows?

Sankar

PS: I wish the W3C would say some thing like the above or some thing that is
a theory like the above to help people. This lack of theory is what folks
are complaining about.
Received on Monday, 15 November 1999 19:03:03 GMT

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