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Re: why no-cache is not reload

From: Roy T. Fielding <fielding@avron.ICS.UCI.EDU>
Date: Sun, 18 Feb 1996 00:33:08 -0800
To: Shel Kaphan <sjk@amazon.com>
Cc: hardie@nasa.gov, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <9602180033.aa02508@paris.ics.uci.edu>
> Roy T. Fielding writes:
 >> > 	On a related note, does cache-control: max-age 0 also force
 >> > a reload (by asking for a copy of the resource that is no older than
 >> > 0 seconds old?).
 >> No, it forces a "refresh" (i.e., a conditional GET on the next inbound
 >> server w/max-age=0, which results in a conditional GET on the origin).
 >> It is possible that the refresh will result in a reload, but only if
 >> the server decides that the resource has changed.
> How is a request containing
> 	cache-control: max-age=0
> but NOT
> 	if-modified-since:  <last modified date of resource>
> conditional?   Without  if-modified-since, the origin server would
> always have to return a full response, wouldn't it? 

Yes, but I said the request *would* include an IMS by saying "conditional GET"
(the reason for not saying IMS is because there are now other proposed ways
to make a request conditional).

What "cache-control: max-age=0" does is force the cache to do a refresh,
and the refresh action is a conditional GET on the next inbound server.
By including max-age in the request (in addition to IMS), the
conditional GET can only be answered by a cache with age <= max-age
or by the origin server.  Thus, max-age=0 will effectively propagate
a condional GET all the way to the origin.

 ...Roy T. Fielding
    Department of Information & Computer Science    (fielding@ics.uci.edu)
    University of California, Irvine, CA 92717-3425    fax:+1(714)824-4056
Received on Sunday, 18 February 1996 00:37:24 UTC

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