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Re: resource and representation

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Thu, 04 Jul 2002 11:04:50 +0300
To: "ext Roy T. Fielding" <fielding@apache.org>
CC: ext Paul Prescod <paul@prescod.net>, WWW TAG <www-tag@w3.org>
Message-ID: <B949DC52.17DB1%patrick.stickler@nokia.com>

On 2002-07-04 4:07, "ext Roy T. Fielding" <fielding@apache.org> wrote:

>> Web applications do not have access to the actual resource, no, but
>> a representation of the resource is analogous to getting the actual
>> resource. For that to happen, the resource has to be digitally
>> encoded in some fashion.
> 
> We have had this discussion far too many times.  Web applications may
> or may not have access to the actual resource, depending on the properties
> of the scheme and the naming authority under that scheme.  In all cases,
> however, a URI is an identifier (AND NOTHING MORE THAN AN IDENTIFIER) for
> a resource, which in turn is always a semantic mapping to zero or more
> representations.  Give me a more restrictive definition and I will show
> you a deployed implementation that contradicts your definition.
> 
> A URI (hopefully) only identifies one resource.  That goal has no
> relation, whatsoever, to any argument of any sort over the thing
> received when a resource is accessed with GET over HTTP (or file
> or wais or gopher or telnet or ...).  If you do not accept that fact
> then you don't recognize the existence of the Web, and therefore any
> further discussion is pointless.  There is absolutely no/zero/nil
> opportunity for ambiguity once the representation(s) of a resource
> are not considered the resource itself, which is, of course, why we
> defined them that way in the first place.

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?

And the Web is then supposed to be the foundation for the Semantic
Web, employing such ambiguous and unreliable behavior?

> A resource is not the thing that is transferred across the wire or
> picked up off the disk or seen from afar while walking your dog.  Each
> of those is only a representation.  The same is true of physical objects
> encountered in life and never identified with URI and never made
> accessible on the net.  Yes, it does present a bit of a quandary, but
> it is one that we have all learned to live with.  Our eyes are not
> powerful enough to see identity through the representations, but our
> minds are powerful enough to associate identity to that which we see.

Yes, *HUMAN* minds are powerful enough to deal with the ambiguity.

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.

If I ask for a digital resource, such as a specific RDF/XML instance,
I expect to get it, or know that I have not gotten it.

Thus, it appears that HTTP needs some rework if it is to meet the
higher precision needs of the Semantic Web.

> Do I think of a different identifier every time I see my dog,
> or do I simply think of my dog as one identity and experience
> many representations of that identity over time (and on into memory
> and imagination)?
> 
> One of my favorite quotes from TimBL is:
> 
>   "I don't want the Web to constrain what people do:
>    the Web is not there to constrain society.
>    It's there to model society in its completeness, in its entirety."
>   -- Tim Berners-Lee (GNN Interview, 1994)
> 
> Resources are not transferred, just as identity within the real world
> is not transferred when it is accessed.  That doesn't stop us from
> reasoning about rocks, plants, chairs, or my dog.  In fact, the
> separation of identity from representations of an identity is
> necessary to reason about them at all.  That does not mean that the
> resource and the representation are both identified by the same URI;
> they are not the same resource.  It means that you can reason about
> resources and reason about representations of resources, even if you
> don't know "the most appropriate URI" that does identify the
> representation as a *separate* resource.

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

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

> In short, the only reason this gives anyone in RDF land heartburn is
> simply because their definition of resource doesn't match that of the
> Web, or that of reality.

No, it is because RDF and the Semantic Web requires a much higher
degree of precision in identity than the Web, because there are
not intelligent humans to realize "oh, I got a picture of Paris
rather than the actual city, but of course, that makes sense..."

> I absolutely refuse to consider RDF as a useful technology or the
> Semantic Web as the future of human communication if its reasoning
> power is incapable of describing one of the fundamental facts of life,
> particularly since the only reason it is incapable of doing so is
> because a few people suffer from the unfortunate belief that it is
> easier to change the Web than it is to change their preconception
> of what it means to be a resource in RDF.

That is not the issue. The issue is about the ambiguity that
arises when one uses, on the Web, a URI to denote one resource
with full knowledge that another resource will always be returned
by a GET, yet without any indication from the HTTP server that
what is returned is not the resource denoted by the URI provided
to the GET request.

Yes, folks can argue till their blue in the face about using
Content-Location, etc. but let's face it, folks don't do that,
and the namespace document issue illustrates that fact. Where
is the language in the TAG proposal that says that, yes, a
namespace document can be returned by a GET but it is required
that a different URI be specified for the Content-Location to
indicate that it is a different resource from the namespace
itself...? Eh?

> Change that preconception
> and RDF becomes capable of reasoning about both resources and
> the representations of resources through one level of indirection,
> just like the rest of us mere mortals.

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.

Patrick

--
               
Patrick Stickler              Phone: +358 50 483 9453
Senior Research Scientist     Fax:   +358 7180 35409
Nokia Research Center         Email: patrick.stickler@nokia.com
Received on Thursday, 4 July 2002 04:04:47 GMT

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