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Re: Rigorously defining the interaction of conditional headers

From: Roy T. Fielding <fielding@kiwi.ics.uci.edu>
Date: Thu, 05 Mar 1998 22:39:30 -0800
To: John Franks <john@math.nwu.edu>
Cc: http-wg@cuckoo.hpl.hp.com
Message-Id: <9803052241.aa20112@paris.ics.uci.edu>
X-Mailing-List: <http-wg@cuckoo.hpl.hp.com> archive/latest/5438
>This is certainly clear and simple.  But, why do precondition checks 
>take precedence over validation checks?  While we are on the subject,
>what is the purpose of the precondition checks?

A precondition gives a client the opportunity to require that the
resource (or at least some aspect of the resource) has not changed
before applying a method.  Preconditions are necessary to support
atomic check+set actions which are common in distributed authoring
and version control situations.

Actually, validation checks are also preconditions, but I didn't have a
convenient word for preconditions-that-return-412-when-false vs
The latter is only used for cache validation checks.

The precedence is due to the requirement that 304 be returned only
if the response would otherwise be 200.  Since a 412 is not a 200,
those preconditions win.  Checking them first is a performance issue,
but also simplifies the algorithm description.

Received on Thursday, 5 March 1998 22:46:48 UTC

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