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Re: rdfms-graph: Food for thought

From: Frank Manola <fmanola@mitre.org>
Date: Tue, 17 Jul 2001 15:43:32 -0400
Message-ID: <3B549564.8000901@mitre.org>
To: Graham Klyne <Graham.Klyne@baltimore.com>
CC: Stephen Petschulat/CanWest/IBM <spetschu@ca.ibm.com>, w3c-rdfcore-wg@w3.org
I think it's important not to let the tail wag the dog too much here. 
My understanding is that the key question involved in "rdfms-graph" is 
one of scope:  we talk about "a graph" or "a model" without being able 
to describe very easily what that consists of, or what kind of thing it 
is (the "model" term is particularly significant when we aren't 
specifically talking about one of the graph-like pictures in the M&S, 
but rather about a collection of triples or some XML serialization). 
 From an abstract point of view, it's obviously some kind of collection 
of RDF statements, but then come the questions, like:

a.  what *kind* of collection is it?
b.  is it a resource (and do we need to explicitly specify how it gets a 
URI?)
c.  what kinds of things go in it?
d.  what is the purpose of defining such collections (e.g., do you scope 
it for the purpose of attributing all of its contents in a common way)?

The issue of whether an isolated subject ought to be permitted in the 
contents of such a collection needs to come out of the answers to these 
(and possibly other) questions (like, what do we intend the meaning of 
such a thing to be?), rather than being simply decided on the basis that 
isolated nodes are legal graphs in graph theory (we may only be talking 
about a certain subset of the graphs definable in graph theory--we could 
still call them "graphs").  Note that even if we don't wind up dealing 
with disconnected nodes (like subjects), we will still wind up dealing 
with "models" or "graphs" that contain disconnected subgraphs in any 
sensible interpretation of "model" or "graph".  For example, many 
collections of RDF statements will consist of disconnected subgraphs, 
each subgroup consisting of the descriptions pertaining to a separate 
subject (Web resource).  (I'm assuming here that you can separately 
scope a collection of RDF statements, even when objects in those 
statements are sometimes URIs of resources (including literals) 
"located" elsewhere.  If such references mean that those referred-to 
resources are also in the graph, then I don't see how we can talk about 
more than one RDF model at all, particularly if literals wind up having 
URIs).

--Frank

Graham Klyne wrote:

> Steve,
> 
> I think I broadly agree with what you say.  My term "awkward" isn't 
> meant to imply problematic, or even difficult.  My purpose of engagement 
> here is based on:
> (a) my perception that representing isolated nodes adds some complexity 
> (though maybe not very much), and
> (b) questioning whether there is any real purpose in adding this small 
> extra complexity to RDF.
> 
> That said, Aaaron's proposal to represent isolated nodes as ( <foo> 
> rdf:type rdfs:Resource ) overcomes those objections (but introduces 
> another because it would make the RDF core dependent on a schema 
> definition, viz rdfs:Resource).
> 
> You also say:
> 
>> "An RDF Subject that does not have any associated Properties 
>> corresponds to
>> a disconnected node in a graph. The value of the about/ID attribute of 
>> this
>> element is the label of the disconnected node."
> 
> 
> With which I'd pick a nit:
> 
> My take on the current M&S is that the concept of "an RDF Subject" is 
> meaningful only in the context of a property -- a "Subject" doesn't 
> exist in isolation.  A resource can be any or all of Subject, Object or 
> Property depending on how it is used.
> 
> (This isn't affected by your rewording in a different message.)
> 
> #g
> -- 
> 
> At 08:44 AM 7/17/01 -0700, Stephen Petschulat/CanWest/IBM wrote:
> 
>> I don't really see this as being about the abstract syntax as much as the
>> graph theoretical model. Right now RDF pays lip service to being a 
>> "graph",
>> but doesn't formalize this in the model. If we do intend to lay down a
>> graph theoretical foundation for RDF then this issue is fundamental. 
>> Graph
>> theory makes use of disconnected nodes in graphs (ie. a graph is defined
>> such that it may contain disconnected nodes) so it would seem we should
>> either explicitly define what it means or have a good reason to disallow
>> out it (an possibly lose out on the body of graph theory that requires a
>> graph be able to have edgeless/arcless nodes). As far as being awkward to
>> define, I don't think this is the case for the graph theoretical model,
>> although I don't know how the logic people would deal with it. The
>> definition can be as simple as:
>> 
>> "An RDF Subject that does not have any associated Properties 
>> corresponds to
>> a disconnected node in a graph. The value of the about/ID attribute of 
>> this
>> element is the label of the disconnected node."


-- 
Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-875
Received on Tuesday, 17 July 2001 15:42:25 EDT

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