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Re: HTTP Caching Model?

From: Roy T. Fielding <fielding@avron.ICS.UCI.EDU>
Date: Thu, 15 Dec 1994 00:46:05 -0800
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <9412150046.aa28029@paris.ics.uci.edu>
I wrote:
>>  2) Patch the current model by adding a "Variants:" header like
>>        Variants: text/html;qs=1;bs=9653;ua="Mozzilla",
>>                  text/html;qs=0.9;bs=8753,
>>                  text/plain;qs=0.2;bs=7989,
>>                  text/plain;qs=0.3;bs=8989;lang="en/gb",
>>                  application/postscript;qs=0.7;bs=90267
>>     which would ONLY be sent when the given URI allows variants;

and Dan replied:

> This is kinda humorous... the answer has been in the spec all along:
> ...
> URI-header      =       "URI" ":" 1#( URI [";" vary] )
> vary            =       "vary" "=" <"> 1#vary-param <">
> vary-param      =       "type" / "language" / "version" / "encoding"
>                 /       extension-vary
> extension-vary  =       token

No, that isn't enough information to allow a proxy to decide
what is available.  For instance, let's say we have the same document
in four natural languages: English (en), French (fr), Spanish (es),
and Maori (mi).  Under the current scheme, the proxy would make its
first request for Jacque (Accept-Language: fr) and get:

    Content-Language: fr
    URI: <http://www.roy.com/Maori/museum>;vary="language"

    Bonjour -- uma rapate, uma rapate, uma uma uma

so the proxy could be smart enough to keep track that this is the
French version.  Okay, so Charles requests the same document
w/(Accept-Language: en).  Knowing that it only has a French version,
the proxy can pass the request on and get:

    Content-Language: en
    URI: <http://www.roy.com/Maori/museum>;vary="language"

    G'day mate, welcome to my museum of New Zealand native culture.

No problem so far -- we now have both the French and English versions
cached.  Unfortunately (in more ways than one ;-), our next request comes
from Hone Heke w/(Accept-Language: mi;q=1, en;q=0.2).  He wants to see the
version in Maori:

    Content-Language: mi
    URI: <http://www.roy.com/Maori/museum>;vary="language"

    Teenaa koutou katoa, Aotearoa haere mai!  Ke te pehea koe?
and he's going to be very upset when he gets the English version
instead.  But, as far as the proxy can tell, that is the only acceptable
version available.

The same problem is generic to all the negotiated dimensions.  There are
only three solutions that preserve transparent behavior:

1) Have the proxy revert to the source if it receives a request which
   includes a variant category that has not already been cached;

2) Tell the proxy about all the possible variants so that it can make
   the decision itself;

3) Use URCs so that the decision can be made within the client and not
   as part of the request.

I think we can live with (1) for HTTP/1.0, but we should change to (3)
for the next version.

......Roy Fielding   ICS Grad Student, University of California, Irvine  USA
Received on Thursday, 15 December 1994 00:48:23 UTC

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