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From Brian Akins <>
Subject Re: [PATCH] mod_disk cached fixed
Date Thu, 05 Aug 2004 16:49:36 GMT wrote:

> I thought you were referring to the scheme you proposed in an
> earlier email where you WERE planning on doing just that...

Once again, I probably was just uncelar or, frankly, said what I didn't 
mean :)

> Take your logic just a tiny step farther.
> I you know EVERYTHING about the data... then you CERTAINLY can/would.

In the situations in which we are speaking of currently, the "origin" 
and the "cache" may indeed be the same box, or one controlled by the 
same person.  In these situations, you should be able to "influence" the 
Vary'ing scheme.

> When does a response REALLY (actually) Vary and
> why should you have to store tons and tons of
> responses all the time?
Like I said, if you know alot about the content, then you know when to 
REALLY vary and when to "cheat."

> If you MD5 and/or hard-CRC/cheksum the actual BODY
> DATA of a response and it does not differ one iota
> from another (earlier) non-expired response to a
> request for the same URI... then those 2 'responses'
True.  But you have to make the decision of which version to server 
based purely on the clients request headers.

> Even if you get 25,000 different 'User-Agents' asking
> for the same URI... there will most probably only be
> a small sub-set of actual RESPONSES coming from the
> COS.

Correct.   This how using SetEnvIf to set an environment variable is a 
better solution than varying on the "string value" of the User-Agent header.

> Topic for discussion?
> Kindling for a flame ware?
> Not sure... but you raise an interesting question.
Yes :)

> Only the most recent versions of SQUID make any attempt
> to support 'Vary:' at all. 

We know a few things about squid.  And what you say is true.  It 
supports it "well enough" for what we use it for.  We serve 2 billion + 
hits a day using Squid.

Note: this does not imply that CNN or Time Warner in any way endorse 
squid or any mentioned product.

> It should remain THEIR choice, no matter what.
Yes.  But in the case of caching you own content, as long as you leave 
the Vary headers in tact, it shouldn't matter how you actually handle 
the Vary.

Brian Akins
Senior Systems Engineer
CNN Internet Technologies

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