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RE: Terminology (was Re: article on URIs, is this material that can be used by the)

From: Rhys Lewis <rhys@volantis.com>
Date: Fri, 6 Jul 2007 03:11:48 -0700 (PDT)
To: "Pat Hayes" <phayes@ihmc.us>, <noah_mendelsohn@us.ibm.com>
Cc: "Dan Brickley" <danbri@danbri.org>, "Henry S. Thompson" <ht@inf.ed.ac.uk>, "Tim Berners-Lee" <timbl@w3.org>, <www-tag@w3.org>
Message-ID: <004d01c7bfb6$54b88380$78a6f40a@volantisuk>

 
Hello Pat, 

I'd like to ask my first dumb questions, if I may. They concern your
recent response to Noah's comments.

Noah wrote:
"Here's what I think may be the essence of the confusion:  there are 
certain systems in which it is by definition possible to attempt to 
access anything that can be referenced."

You responded:
"Indeed this may well be part of the confusion. OK, there are a few such
systems (a VERY few), but the Web is not one of them; or at least, not if
its understood as described in the Web Architecture document. That
document is at pains to explain, quite early, that resources identified by
URIs can be physical things not connected in any way to the Internet, such
as books and people. From that point on, all talk about attempting to
access things that can be referenced is obviously crazy. You can't use
HTTP or any other xxTP to access people and books. (You can maybe access
some kind of description of them, which could be called a representation
of them, although not in the same sense of "representation" used by you
and the architecture document; and not by getting a TP poke to them and
causing them to emit a representation in response. But you didn't say
'representation': you said, access anyTHING that can be referenced.)"

My question concerns whether I've interpreted your response correctly. I
was somewhat surprised by your apparent assertion that the Web is not a
system which Noah characterised as one 'in which by definition it is
possible to ATTEMPT (my emphasis) to access anything that can be
referenced'.

Let me first try and explain my surprise.

It seems to me that the Web has always had the property that you can
attempt to access that which a URI identifies (I'm assuming HTTP
throughout this), but there is no guarantee that the access operation will
be successful. Indeed, there are no guarantees about what might be
returned in response to a successful access operation. This is a
consequence of the localisation, of the definitive information about that
which a URI identifies, to the resource itself. It is, I believe, in large
part the source of the Web's scalability. 

Clearly, it would be strange to try and access something via the Web which
was known to be 'physical' and hence not able to emit anything in response
to a transport protocol 'poke'. However, in general, the Web doesn't
provide any guaranteed way to know that in advance. It's not possible to
know whether that which a URI identifies will respond to such a 'poke', by
returning some material, without actually trying it.

Now, I agree with you that it could well be described as crazy to make
subsequent attempts to access something via the Web that you already know
is 'physical'. But using the core facilities provided by the Web, it's not
possible to know that for sure without attempting that first access.

Some systems may, of course, provide mechanisms for making assertions
about URIs that can help avoid the need to attempt an access in order to
find out about what is being referenced. But since URIs are universal,
other more general systems, encountering such URIs, may attempt access
because they are unaware of that additional information and hence don't
know any better. Only by attempting an access can they find out anything
more about the resource. You are probably aware that the behaviour
associated with attempts to access URIs that identify physical things
occupies a large part of the httpRange-14 finding on which the TAG is
currently working.

Ok, so now I'd better try and phrase the questions. First, does it sound,
from what I've written here, as though I understood your response
correctly, or have I simply missed the point? Second, if I have understood
your response, does this throw any light on why it's important that
attempts to access physical things identified by URIs need to be supported
by the Web in order for there to be a general mechanism by which it is
possible to discover that the thing is indeed physical?

Very best wishes

Rhys Lewis
Received on Friday, 6 July 2007 10:12:03 GMT

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