W3C home > Mailing lists > Public > ietf-http-wg-old@w3.org > September to December 1995

Re: Location Proposals

From: John Franks <john@math.nwu.edu>
Date: Wed, 6 Sep 1995 14:01:23 -0500 (CDT)
Message-Id: <199509061901.OAA10340@hopf.math.nwu.edu>
To: Paul Leach <paulle@microsoft.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
According to Paul Leach:
>
> Tagging a document with a very ancient "Expired:", when handled 
> consistent with the above considerations, will almost completely 
> automatically cause the behavior that has been asked for: you always 
> get the latest copy, saving the cost of getting it from the server when 
> the cached copy is still valid, even though there is no promise that it 
> still is.  The only slightly "funny" thing is that the agent has to 
> ignore the Expires when making replacement decisions, instead only 
> considering the actual reference pattern and other info, such as 
> Last-Modified; in the absence of knowledge of this usage style, one 
> might be tempted to toss resources out of the cache that had expired a 
> long time ago.
>

Am I the only one who finds it extremely counter-intuitive to "ignore
the Expires when making replacement decisions" for a cache?  I
strongly agree that "one might be tempted to toss resources out of the
cache that had expired a long time ago" and I would suggest that 
adopting a semantics for the Expires header where the date has nothing
to do with whether or not a document can be removed from the cache
is inviting immense confusion.


> To add some other header to try and create this behavior will only 
> result in there being two ways to do the same thing, one completely 
> natural, and the other (IMHO) forced. The result will be confusion.
> 

I don't understand why there would be two ways to do the same thing.
Caches would discard documents after they have expired.  That seems
pretty natural to me.  

There is already a proposed Control-Cache: header.  Adding an
additional possible value like "Control-Cache: use-get-if-modified" or
some equivalent is extremely clear.  It says this information is for
caches and it says what it wants the cache to do.  In contrast,
Expires: 1900 or Expires: <yesterday> says this document is no longer
valid.  Remember that browsers get the Expires header also, and some
browsers will display headers to users.  How would an end user
interpret this header?

Also current practice must be for caches to discard expired documents.
An old style cache interacting with a new style server under this
proposal will simply never cache documents which the server wants
cached and which should be cached.


John Franks
Received on Wednesday, 6 September 1995 12:05:55 EDT

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