W3C home > Mailing lists > Public > http-caching-historical@w3.org > February 1996

Re: Ordered 'opqque' validators

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Tue, 06 Feb 96 16:44:52 PST
Message-Id: <9602070044.AA18338@acetes.pa.dec.com>
To: "Roy T. Fielding" <fielding@avron.ICS.UCI.EDU>
Cc: HTTP Caching Subgroup <http-caching@pa.dec.com>
    Yep, though what I'd really like is the words to put in the spec so
    that we can transfer our group's accumulated understanding about
    all these niggling details into something an implementor can work from.
I've been trying to collect some of these little pearls of wisdom
as we seem to reach consensus on them.  For now, I'm sticking
them into my "cachemodel.txt" draft, but ultimately the important
ones will have to migrate into the HTTP/1.1 spec.

You wrote in an earlier message:
    A sensible design would have the server send the "new version" it
    has in its cache and then the client can compare the Date on its
    stored version to the Date on the "new version" -- if the "new
    version" is not newer according to the Date comparison, then the
    client can invalidate both and send a "no-cache" request as a

How about if I paraphrase this as:

    If a client does a [conditional?] request for a resource that it
    already has in its cache, and the response it receives
    contains a Date: value that appears to be older than the
    one it already has in its cache, then the client SHOULD repeat the
    request unconditionally, and include
	Cache-control: [ no-cache | reload ]
    to force any intermediate caches to obtain a new copy
    from the origin server.  This prevents certain paradoxes
    arising from the use of multiple caches.

We can argue at some other time about the specific Cache-control:
syntax, OK?  And this might be a case where "Cache-control: revalidate"
is useful, since it's quite possible that the client's current
copy is actually valid (since it has the newer Date:).

Received on Wednesday, 7 February 1996 00:56:23 UTC

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