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Idempotent sequences of methods

From: Miles Sabin <msabin@cromwellmedia.co.uk>
Date: Fri, 21 Jan 2000 16:47:15 -0000
Message-ID: <AA4C152BA2F9D211B9DD0008C79F760A67525B@odin.cromwellmedia.co.uk>
To: http-wg@hplb.hpl.hp.com
The discussion in the 'On piplining' thread reminded me of
something I found puzzling in 9.1.

I have a strong suspicion that although 9.1.2 suggests that
an idempotent sequence of methods might contain non-safe
idempotent methods (PUT, DELETE), this can't actually be the

Here's the argument,

* A sequence which contains a PUT/DELETE <uri1> after a 
  GET/HEAD/TRACE/OPTIONS <uri2> is idempotent only if
  uri1 is independent of uri2.

* There is nothing in the definition of an HTTP URI which
  allows us to infer that two URIs are independent.

Suppose, for the sake of argument, that a URI path maps
in a natural way onto part of the servers filesystem tree.
In this case, it's clear that we can't infer that,


is independent of,


on a purely textual basis because if we were to DELETE the 
latter we would have also deleted the former.

Again, with the filesystem mapping assumption, we can't even
assume that,


is independent of,


because they might both reference the same underlying file
in the servers filesystem (eg. via a symlink, or via an
HTTP server path mapping).

So, we can't use textual difference of URIs to infer 
independence. Given that we haven't anything else to go on at 
the user-agent end I conclude that an idempotent sequence of
methods can't contain any non-safe methods.

QED ;-)




Miles Sabin                       Cromwell Media
Internet Systems Architect        5/6 Glenthorne Mews
+44 (0)20 8817 4030               London, W6 0LJ, England
msabin@cromwellmedia.com          http://www.cromwellmedia.com/
Received on Friday, 21 January 2000 16:48:46 UTC

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