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Re: statements about resources vs. representations

From: <noah_mendelsohn@us.ibm.com>
Date: Fri, 21 Nov 2008 20:06:50 -0500
To: Pat Hayes <phayes@ihmc.us>
Cc: Alan Ruttenberg <alanruttenberg@gmail.com>, Harry Halpin <hhalpin@ibiblio.org>, Jonathan Rees <jar@creativecommons.org>, "public-awwsw@w3.org" <public-awwsw@w3.org>
Message-ID: <OFF1829AF8.93B862AF-ON85257509.00054DD6-85257509.00061F74@lotus.com>

Reflecting on the note I sent below, I realize that there's a subtlety I 
didn't deal with.   There are at least two abstractions we might want to 
identify:

1) A representation in the sense of a particular set of bits sent at a 
particular time from an HTTP resource to a client or proxy, or conversely, 
such a representation sent upstream with PUT or POST.  By this definition, 
any representation involved in a subsequent interaction would necssarily 
be a different one.

2) A representation defined to be the information sent with a GET, PUT, 
POST, etc.  By this definition, it's at least possible that the same URI 
would be used to identify the representation that resulted from, say, two 
or more GETs.  I believe the natural way to go down this path is to allow 
the same URI to be used to identify representations with the same content, 
even if provided as representations from very different resources.  For 
example, the representation that is Content-type: text/plain with entity 
body "hello world" could have the same URI even if returned on GETs to 
many different resources.

When I claimed that I wanted to indicate that a given representation was 
poorly formed, I could have meant in the sense of (1) or (2).  If I want 
to make a statement that a particular representation was received at 4PM, 
I probably mean in sense (2).  Or maybe for that case I need a URI for the 
HTTP response, which contains a representation but potentially contains 
additional information as well. 

Anyway, I realized that there is potentially such an ambiguity, and I 
think we should be very clear which of the possible abstractions we refer 
to when we decide to identify a "representation" with a URI.

Noah

--------------------------------------
Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
1-617-693-4036
--------------------------------------








Noah Mendelsohn
11/21/2008 06:22 PM

        To:     Pat Hayes <phayes@ihmc.us>
        cc:     Alan Ruttenberg <alanruttenberg@gmail.com>, Harry Halpin 
<hhalpin@ibiblio.org>, Jonathan Rees <jar@creativecommons.org>, 
"public-awwsw@w3.org" <public-awwsw@w3.org>
        Subject:        Re: statements about resources vs. representations


Pat Hayes writes:

> I think what Harry should have said is that they are too 
> ephemeral for someone to want to give them an enduring name or 
> identifier. But there are other ways to refer to things than 
> baptizing them with a URI for all time.

On this I don't think I agree.  We're talking about the Web here, and 
what's more, I think a representation is an information resource.  I mean, 

not only can the thing be represented as a computer message, the whole 
point of it is to be sent in a computer message!  The key architectural 
imperative for the Web is "Identify with URIs."  I see no reason why, in 
cases where you do want some means of identifying a particular 
representation, a URI wouldn't be the way to do it.  When I make that 
choice, I get a variety of advantages:  I can make Semantic Web statements 

about the representation (it was buggy, it took a long time to arrive, it 
was cached at proxy p1, etc.) in the natural way without resorting to 
indirection;  I think I could even choose to run a server that would 
respond to GETs with representations of, well, the representation.  I 
think the usual rules of the Web apply well here:  when you need to 
identify something, do it with URIs.

Noah

[1] http://www.w3.org/TR/webarch/#pr-use-uris

--------------------------------------
Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
1-617-693-4036
--------------------------------------
Received on Saturday, 22 November 2008 01:07:35 GMT

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