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Re: FW: Http.ow and jar-diagram-7.pdf l (was RE: JAR conflict for July 7 AWWSW telecon)

From: Jonathan Rees <jar@creativecommons.org>
Date: Tue, 14 Jul 2009 21:29:11 -0400
Message-ID: <760bcb2a0907141829n76cacecdo93c0f065acc6df54@mail.gmail.com>
To: "Williams, Stuart (HP Labs, Bristol)" <skw@hp.com>
Cc: AWWSW TF <public-awwsw@w3.org>
On Mon, Jul 6, 2009 at 11:47 AM, Williams, Stuart (HP Labs,
Bristol)<skw@hp.com> wrote:
>> [JAR] I would rather remove the date/time so that we can more often say
>> the same representation corresponds to multiple resources, a la the
>> genont theory. I have no idea how carefully last-modified is
>> maintained by servers and proxies; perhaps what you suggest will work.
>
> I don't know either - I expect that for things based on filing systems last modified come straight from there. For things that are the output of processes... Last modified either relates to some underlying storage (if the information is available from there) or is 'now' - modulo possibly some server side caching that mitigates constantly recomputing the same responses - eg. I use a CMS for my canoe club web site [1] which caches the composite pages that it builds.
>
>> Let's try to figure this out over the coming months...
>
> Ok...

I thought of the following case: Consider 302 and 307 redirects from a
URI to A URI B (Location: B). 2616 talks of the resource residing
elsewhere, but I would choose to interpret the redirect as saying that
representations corresponding to B are, for the time being, also
representations corresponding to A. I have also chosen to interpret
"Last-modified:" to mean that the representation has been a
representation of the requested resource (the one named in the GET
request leading to the 200 response containing the Last-modified:
field) since the given time.

So how do go from a statement that Z has been a representation of
resource B since time T, to a statement that Z has been a
representation of A since time T?  To me this doesn't follow, but I
can only come up with a counterexample as a thought experiment:
Suppose we have a current-version resource A advancing to new versions
from time to time. Suppose that this is implemented not by varying 200
responses, but by modulating 302 or 307 redirects, so that as A
advances from V1 to V2, its representations change from those of some
resource B1 to being those of some other resource B2. Now suppose that
a representation of B2 is made available (as a draft, say) in advance
of this switchover for A. The repr. of B2 has some Last-modified: that
precedes the point in time at which it becomes a representation of A.

Kind of twisted? So tell me how where I've gone wrong, and what the
better story is? Are representations you get via redirection not
representations that correspond to the originally requested resource,
as well as to the redirect target?

Jonathan
Received on Wednesday, 15 July 2009 01:29:49 GMT

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