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Re: Generic-Resources-53: URIs for representations

From: <noah_mendelsohn@us.ibm.com>
Date: Fri, 6 Oct 2006 23:05:27 -0400
To: Patrick Stickler <patrick.stickler@nokia.com>
Cc: "Dan Connolly" <connolly@w3.org>, "ext Booth, David (HP Software - Boston)" <dbooth@hp.com>, raman@google.com, "Williams, Stuart (HP Labs, Bristol)" <skw@hp.com>, www-tag@w3.org
Message-ID: <OFFFEF5C8D.72070E36-ON852571FF.006C3476-85257200.0010FD04@lotus.com>

Patrick Stickler writes:

> (b) if is known (by extra-web means) that a given URI denotes a 
> representation, then the agent is licensed to expect that every time
> it dereferences that URI it will get the same exact byte sequence

Yes, if you really mean "that" representation, but I think we're glossing 
over an ambiguity.

Consider one of my favorite examples, which is a clock resource.  The 
clock updates in real time, and representations of it change accordingly. 
For this example, assume that the clock resource at 
http://example.org/clock supports content negotiation.  It returns the 
representation of the clock as your choice of text/plain, in which case 
you get back the date and time as text, or image/jpeg, in which case you 
get the image of a round analog clock showing the current time.

Your analysis seems to apply to the case where we want a URI for the 
particular resource returned at, say 3PM, and I think your analysis is 
coherent for that case.  I don't think it's the main case of interest. 
What I think we're mostly considering is more along the lines of two 
additional resources which might be named:


These would not in fact return the same representation on successive 
accesses, but would invariably return representations using the particular 
media types.  I also think this is an appropriate use of URIs for 
representations.  So, I don't think that in all useful cases "URI denotes 
representation" implies "denoted representation octet stream is 
invariant".  I think the two URIs above denote representations of the 
original clock resource if the authority at example.org says they do.

Also, I think I'm right that the term "representation" is appropriately 
applied not just to the octet sequence returned, but to some of the 
associated control data such as Content-type that's typically carried in 
some of the returned HTTP headers.


Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
Received on Saturday, 7 October 2006 03:05:39 UTC

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