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LANG: Versioning Use Case

From: Jeff Heflin <heflin@cse.lehigh.edu>
Date: Fri, 20 Sep 2002 17:27:02 -0400
Message-ID: <3D8B92A6.AB15B099@cse.lehigh.edu>
To: WebOnt <www-webont-wg@w3.org>

Hello everyone,

At yesterday's telecon I took an action to provide a use case for
ontology versioning, and in particular backward-compatibility. Here it
is:

Let's assume that we have an an ontology V0 that has 50 classes. On each
of 10 different days, we add a new class to this ontology. The proper
way to do this in OWL (and in RDF, see RDFSS 4.1.2) is to create a new
namespace for each version. Thus, we now have 11 versions, V0 through
V10, of our ontology.

I posit that the following two things are desirable: 

D1: that we only want to refer to a single namespace in any documents
that use this ontology. Why? Because, it will be difficult to remember
which concepts were in which version, and because the alternative
doesn't scale up well (our documents would keep needing more and more
namespaces as the Web grows older).

D2: that we would like to be able to integrate resources that committed
to different versions of this ontology. Since the change is only
additive, we should be able to merge the graphs of various documents
that commit to these ontologies and know that a "#Foo" in one version is
also a "#Foo" in another version. We need to do this because the owners
of documents that commit to ontologies may be different from the owners
of the ontologies themselves, and cannot be made to instantly upgrade
their data. If we don't establish correspondences, then we reduce
interoperability.

Assuming you agree with these desirable properties, let's examine
possible solutions to the problem.

Attempt #1:
Don't have versions at all. Instead simply have each new version import
the previous version and describe its one new class. However, in this
alternative, any document that wishes to talk about classes in different
versions of the ontology must specify the different namespaces for each
class it mentions. This goes against the first desirable property, D1,
above. You may ask why should this be any different than when you import
multiple distinct ontologies. The answer is that in versions, the
objects are conceptually closely related, and users benefit from keeping
them organized as such.

Attempt #2:
Create the new version as a duplicate of the old and add the new class.
Now, this version has local identifiers for every concept from the old
ontology, meaning documents can use a single namespace to refer to
concepts from the ontology (satisfying D1). In order to satisfy D2, we
have to create links between the concepts in the new version and their
companions in the prior version. In RDFS, this would be done using
subClassOf and subPropertyOf (see RDFSS 4.1.3). In OWL, we have the more
accurate sameClassAs and samePropertyAs. Therefore, the ontology must
include 50+ sameClassAs statements to specify the correspondence with
identifiers in the previous version. This is certainly doable, but a
little unwieldy. It would be very easy to forget one, and becomes
particularly burdensome if the ontologies are very large.

Attempt #3:
Provide a simple syntactic sugar for the last part of attempt #2. Call
this backCompatWith. If an ontology is backCompatWith another ontology,
then the 50+ sameClassAs statements are implicitly created. Nothing more
is implied by the semantics, although various advanced application may
experiment with using this statement in interesting ways. This is
essentially what I proposed in [1].

Jeff

[1] http://lists.w3.org/Archives/Public/www-webont-wg/2002Sep/0090.html
Received on Friday, 20 September 2002 17:27:05 GMT

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