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re: Is a graph an information resource?

From: Chimezie Ogbuji <ogbujic@ccf.org>
Date: Mon, 12 Oct 2009 12:13:56 -0400
To: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
Message-ID: <C6F8CE04.CFB1%ogbujic@ccf.org>
See response below.

Also, note my response to Steve regarding the use of the term networked RDF
knowledge:

http://lists.w3.org/Archives/Public/public-rdf-dawg/2009OctDec/0124.html


> Chimezie, all,
> I'm trying to understand the implications of the following sentence in the
current HTTP Protocol WD:

>"In this way, an HTTP request can route operations towards a named graph in an
RDF dataset via its URI. However, in using URIs in this way, we are not directly
identifying the RDF graphs but rather the networked RDF knowledge they
represent."

> So, the way I interpret this is that what you call "networked RDF knowledge"
is an information resource, whereas the graph itself is not. Is this a correct
interpretation?

Yes.  The difference is very subtle but it is a part of SPARQL.  See the end
of 8.2.2 (Specifying Named Graphs) - I remember this from the previous DAWG.

"
The FROM NAMED syntax suggests that the IRI identifies the corresponding
graph, but the relationship between an IRI and a graph in an RDF dataset is
indirect. The IRI identifies a resource, and the resource is represented by
a graph (or, more precisely: by a document that serializes a graph). For
further details see [WEBARCH].
"
My understanding is that this applies not just to the use of IRIs in the
dataset clause portion of a query but to all IRIs of graphs in a dataset.

At the time this distinction was made, I didn't quite understand it, but in
reading web architecture literature more carefully, it makes sense to me.

> I can see arguments for making this distinction, but I'm not sure it is
useful. In light of httpRange-14, such an interpretation would seem to dictate
that when dereferencing a named graph via its URI, a 303 would need to be
returned to redirect the client to an information resource with the data itself.

If upon dereferencing a named graph via its URI you get a 303, all this says
is that:

"the response to the request can be found under a different URI and SHOULD
be retrieved using a GET method on that resource. [..] The new URI is not a
substitute reference for the originally requested resource.
"

If httpRange-14 *requires* the behavior above, that would seem very
unfortunate IMHO.  I think that (intuitively) for resources that are
represented by RDF triples it *is* the case that their ".. essential
characteristics can be conveyed in a message."

Where a serialization of RDF triples is that message.

However, I can see how considering httpRange-14 as a normative reference can
force this (rather unpleasant) requirement. I worry about going down that
route.  This is yet another point that seems to dabble in philosophy at the
expense of pragmatism.  Thanks for bringing it up, though - I hadn't
considered this particular implication.

I'm willing to consider just differentiating 'networked RDF knowledge' from
resources (in general) rather than information resources (specifically), but
I hope you can see how it is counter-intuitive given the fact that RDF's
ability to represent 'things' with reasonable fidelity (via model theory,
for instance) suggests not only that the things it represents are different
from typical web content  but that they are well represented (and thus
information resources).  Otherwise, what is the value proposition of the
semantic web?

> Actually, I've always looked upon an RDF graph as an information resource.

Do you mean an RDF graph in the abstract sense or its serialization (the
difference matters)?

>One may not bother to make it dereferencable, but that doesn't disqualify it
from being an information resource, since a 4xx return code just leaves it
undefined what it is. To me, an RDF graph is just a bunch of triples, i.e.
information. What would be the shortcomings of such a naive view?

Well, you would have to reconcile it with the subtle separation the current
SPARQL specification seems (to me) to make between an RDF graph, its URI,
and what it represents.

-- Chimezie


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Received on Monday, 12 October 2009 16:15:08 GMT

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