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resource and representation

From: Roy T. Fielding <fielding@apache.org>
Date: Wed, 3 Jul 2002 18:07:11 -0700
Cc: ext Paul Prescod <paul@prescod.net>, WWW TAG <www-tag@w3.org>
To: Patrick Stickler <patrick.stickler@nokia.com>
Message-Id: <5EE217AE-8EEA-11D6-833B-000393753936@apache.org>

> 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.

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.
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.

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.

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.  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.


Cheers,

Roy T. Fielding, Chief Scientist, Day Software
                  (roy.fielding@day.com) <http://www.day.com/>

                  Chairman, The Apache Software Foundation
                  (fielding@apache.org)  <http://www.apache.org/>
Received on Wednesday, 3 July 2002 21:06:56 GMT

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