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Re: A shot at rdfms-resource-semantics

From: Dan Connolly <connolly@w3.org>
Date: Tue, 08 May 2001 03:31:38 -0500
Message-ID: <3AF7AEEA.4CB8E356@w3.org>
To: Martyn Horner <martyn.horner@profium.com>
CC: w3c-rdfcore-wg@w3.org
Martyn Horner wrote:
> 
> http://www.w3.org/2000/03/rdf-tracking/#rdfms-resource-semantics
[...]
> Please comment before I begin another spiral...

Before I comment on your shot at it, some comments
on the summary of the issue:

The summary includes two specific questions; the
first one is easy to answer from ratified specs:

	For a resource which is for example, a web page,
	is the resource the sequence of bytes representing
	that web page? 

No. cf

      "Resource
         A resource can be anything that has identity.  Familiar
         examples include an electronic document, an image, a service
         (e.g., "today's weather report for Los Angeles"), and a
         collection of other resources.  Not all resources are network
         "retrievable"; e.g., human beings, corporations, and bound
         books in a library can also be considered resources.
         The resource is the conceptual mapping to an entity or set of
         entities, not necessarily the entity which corresponds to that
         mapping at any particular instance in time.  Thus, a resource
         can remain constant even when its content---the entities to
         which it currently corresponds---changes over time, provided
         that the conceptual mapping is not changed in the process."

        -- http://www.ietf.org/rfc/rfc2396.txt

"resource 
     A network data object or service that can be identified by a URI,
as
     defined in section 3.2. Resources may be available in multiple
     representations (e.g. multiple languages, data formats, size, and
     resolutions) or vary in other ways. 
entity 
     The information transferred as the payload of a request or
     response. An entity consists of metainformation in the form of
     entity-header fields and content in the form of an entity-body, as
     described in section 7. "
        -- http://www.ietf.org/rfc/rfc2616.txt
        aka http://www.w3.org/Protocols/rfc2616/rfc2616-sec1.html#sec1.4


To summarize:
	each URI denotes/identifies a resource.
	Each 200 OK HTTP message represents (the state of)
		a resource using an entity.
		(there are other analagous cases: SMTP, FTP, fopen, ...)
	each entity* is a sequence of bytes.

* to be precise: the body of each entity is a sequence of bytes.

In some cases (e.g. the resource
denoted by data:application/octet-stream,abc )
a resource *is* a sequence of bytes. But in the general
case, it's not.

The second specific question in the issue write up is
much more subtle:

	Can two different URI's name the same resource? 

but I think it can actually be reduced to a test case:

/======
@prefix u: <http://www.w3.org/2000/01/rdf-schema#>.

<#Car> u:subClassOf <#Automobile>.
<#Automobile> u:subClassOf <#Car>.
\======

(attached as RDF/xml, cloaked as text/html)

If we decide that subClassOf works like conventional subset
(cf
  http://www.w3.org/2000/03/rdf-tracking/#rdfs-no-cycles-in-subClassOf
) then we can conclude from the above that (the absolute
forms of) #Car and #Automobile denote the same resource.

I suggest the answer is: yes.

(and it's independent of fragment identifier issues;
use uuid:2l3kj23lkj32lkj3 in place of #Car above
and the questions remain the same.)

Now on to your approach...

[...]
> It's clear that the definition of resource in the M&S spec is intended
> to be a `fundamental' definition from which the rest can be built. It's
> clear also that there's a question whether a `resource' as understood in
> RDF represents the bit-sequence (or the actual atoms) of a referenced
> entity or some sort of pledged token of good intent towards an
> significantly-invariant object.

I don't think there's any question about this.
In the general case, a resource is just something;
it's like a point in geometry, or an element in set theory.
The term 'resource' is a primitive notion; it's not decomposed
further.

> I wonder, given the need to emphasise the legal aspects of RDF
> assertion, whether we don't need to define resources reflexively to
> support this.

'reflexively'? I don't understand what you mean by that.

> I was playing with a definition along the lines of `a token for an
> object constrained in a set of dimensions expressed by a set of
> properties and values but otherwise capable of redefinition'.

Please don't try to define 'resource' independent of the
other terms in the specs. You might come up with some words
that sound nice, but I doubt they'll have any useful meaning.

> There's a
> musing somewhere
[... no comments on the rest...]

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/

Received on Tuesday, 8 May 2001 04:31:43 EDT

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