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Data Integrity

From: Ted Hardie <hardie@merlot.arc.nasa.gov>
Date: Thu, 15 Feb 1996 13:45:03 -0800 (PST)
Message-Id: <199602152145.NAA18205@merlot.arc.nasa.gov>
To: Larry Masinter <masinter@parc.xerox.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
In the Working group issues list, one of the comments on data
integrity confused me a bit.  The second of the two positions asserts
that it is perfectly reasonable for a cache to do integrity checks, to
avoid the cache serving nonsense to the end user.  So far, I
understand and agree.  It goes on to say, however, that if the cache
serves nonsense to the end user, the end user may re-request the
document, thus correcting the problem in the cache.

I'm not clear from the text whether this is meant to apply to
the condition where integrity checks exist between proxy and server
or whether it is meant to apply to the condition where they do
not.  

If the former, then the integrity check would presumably be applied
the first time the cache stored the item; if it passed somehow or was
corrupted later, re-requesting the item won't clear the problem unless
the proxy goes beyond current practice in re-checking the resource.
Any time-based check, for example, won't fail until the resource has
expired or changed.  

A really aware client might still get the correct resource despite the
cache by reloading with a Pragma: request, but I can't see why the
cache would take that as a signal to update its copy (rather than
simply pipelining the request on to the origin server).  

Do we wish to suggest additions to current practice for the case where
integrity checks are applied between proxy and server?  If so, should
these include:  a method for the client to explicitly request a cache
update, and/or recommendations for additional checks by the proxy when
integrity checks are available? 

					Ted Hardie
					NASA Science Internet
Received on Thursday, 15 February 1996 13:42:18 EST

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