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Re: resource and representation

From: Paul Prescod <paul@prescod.net>
Date: Thu, 04 Jul 2002 11:09:40 -0700
Message-ID: <3D248F64.73B78727@prescod.net>
To: Patrick Stickler <patrick.stickler@nokia.com>, WWW TAG <www-tag@w3.org>

Patrick Stickler wrote:
> So when a Semantic Web agent dereferences the URI denoting an RDF
> Schema containing statements, what it gets back may not be that
> actual schema, yet it may in fact be encoded in RDF/XML so the
> agent has no way to know if the schema it recieves is the actual
> and complete schema it asked for?

What it got back was a representation of the schema. Presumably the
provider feels that that representation is sufficiently representative
of the schema. You are trusting the provider in any case, so I don't see
a problem.

> And *here* is the crux of the issue. REST/HTTP is for human consumption,
> and the fact that GET can return something other than the resource
> (or as Jonathan puts it, a representation of 'full fidelity') makes
> it unsuitable for the Semantic Web.

You have known since the beginning of the discussion that GET could
return something other than the resource or "a representation of full
fidelity." Now you are shifting the goal posts.

> Fair enough, but in practice, folks *do* use the same URI to denote
> both the non-digital resource and some representation of the
> resource.

They cannot. By definition their URI denotes the resource (whether
digital or not).

> That is *exactly* what will be *recommended* by having namespace
> names resolve to namespace documents! No?!

Not at all. The URI does not denote the namespace document. There
happens to be a well-established algorithm for certain kinds of URIs to
fetch representations associated with them. That does not change what
the URI denotes. If you are a programmer then you are probably aware
that you can put a pointer in N hashtables and thus have N different
things associated with that pointer. That doesn't change what the
pointer points to.

> RDF is fully able to reason about both resources and representations
> of resources. Presuming the resources and all its representations
> have different URIs that are used consistently.

It would be trivial to extend RDF (or perhaps URIs) to make both
resources and representations *first class*. And in fact, this is
necessary because it is ALREADY THE CASE that some URIs have both an
XHTML, and an XML and a Docbook representation. Therefore if you want to
reason about them, you need a way of describing which representation you
are reasoning about. Once you've solved this problem the solution for
resources will just fall out of it.

How would you use RDF to say that the "fifth element of the XML
representation of 'foo' is an element of type 'bar'" and the "'seventh
element of the XHTML representation of 'foo' is an element of type 'A'".
If, given the appropriate primitives (element numbering, element types,
etc.) RDF cannot say this because it cannot address representations or
distinguish between representations and resources then RDF is not yet
sufficient to work with the deployed Web architecture. Once you can
distinguish between representations, it will be trivial to add a little
feature for distinguishing between representations and resources.
Come discuss XML and REST web services at:
  Open Source Conference: July 22-26, 2002, conferences.oreillynet.com
  Extreme Markup: Aug 4-9, 2002,  www.extrememarkup.com/extreme/
Received on Thursday, 4 July 2002 14:10:15 UTC

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