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Re: UFDTF Metamodeling Document

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Fri, 30 Nov 2007 05:54:48 -0500 (EST)
Message-Id: <20071130.055448.244410205.pfps@research.bell-labs.com>
To: ekendall@sandsoft.com
Cc: public-owl-wg@w3.org

Unforunately, I think that there often cannot be a distinction between
comments on the metamodels and comments on the related text, as the
related text serves to illustrate and refine the situation expressed in
the metamodels. 

For example, consider "URIREferenceNode, BlankNode, and RDFLiteral form
a complete covering of RDFSResource and are pairwise disjoint."  This
contains one very wrong part and one very right part.  If the right part
(pairwise disjointness) is ignored, then the metamodel is incorrect.
The very wrong part is that RDFSResource is a syntactic superclass of
URIREferenceNode, BlankNode, and RDFLiteral, but this is also reflected
in the metamodel itself.   

The basic problem is that there is a fundamental misconception of the
relationship between RDF syntax and semantics embedded in the RDF
metamodel.  Fixing this misconception appears to me to require a
complete rewrite of the metamodel and accompanying text.

Here are some other problems in the metamodel:

- the metamodel use statements (instead of triples)
- the metamodel does not say anything about the uniquess of statements (triples)
- the metamodel allows statements (triples) to be in multiple graphs,
- the metamodel has a unique reification status for each statement (triple)

As well, the metamodel has the wrong cardinalities for the relationships
between resources (nodes) and comments/labels and between typed literals
and datatypes.  (I'm not prepared to say exactly what is wrong, because
I seem to remember that UML diagrams have the cardinality restrictions
on the other end from that which would be intuitive.)  In actuality a
node can have any number of comments/labels in RDFS and any string can
be used for any number of comments/labels.

peter


From: "Elisa F. Kendall" <ekendall@sandsoft.com>
Subject: Re: UFDTF Metamodeling Document
Date: Thu, 29 Nov 2007 12:56:23 -0800

> Just to follow up on Conrad's response, I would really encourage you to 
> submit comments to the issues list, which would help us improve the 
> specification in the short time remaining.
> 
> I think some of the issues you raised are legitimate, I disagree with 
> others, and some comments may stem from lack of knowledge of MOF, 
> particularly its purpose as a repository/persistence representation. 

Which are which?

> Most are not comments on the metamodels themselves but on the related 
> text, which can be fixed where appropriate. Metamodel changes are 
> harder, due to the stage in the standardization process and number of 
> implementations, but possible, depending on their nature. We should 
> simply work through them, in the OMG forum rather than on this list, and 
> rather than requiring another three years to develop an entirely new set 
> of models.

Well, I am afraid that I am of the opinion that it would be better to
start over.  I agree that it may be better to has this out in some other
forum.  However, I don't have management support for me doing
OMG-related work.

> I would also like to say that the use cases we've been discussing in the 
> user facing documents TF are essential to teasing out metamodel 
> requirements, 

How so?

> which in turn, should have beneficial input to the 
> language development.  We worked synergistically with the Common Logic 
> language developers in creating the metamodel for CL, which had clear 
> impact on the language itself.  The CL metamodel in the ODM 
> specification is the metamodel in the ISO CL specification. Any approach 
> that provides a smorgasbord of diagrams -- UML conceptual view in the 
> structural specification, a different metamodel in a W3C note, and a 
> third international standard metamodel in the ODM, will only serve to 
> confuse and divide the user community. 

Well, maybe, but it may also be that the three set of "diagrams" serve
entirely different purposes and thus deserve to be different.  It may be
that the OMG OWL diagrams are designed to support light-weight
OWLish processing of RDF graphs instead of actually supporting OWL, and
thus my comments on them not being appropriate for OWL are not of
interest to the OMG.

> Elisa


peter
Received on Friday, 30 November 2007 11:11:45 GMT

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