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caching CGI responses

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 05 Dec 96 12:09:29 PST
Message-Id: <9612052009.AA09857@acetes.pa.dec.com>
To: advax@triumf.ca
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Andrew Daviel writes:
    Why can't (shouldn't) one cache a CGI response ? It seems to me
    more rational to flush cache based on the frequency of hits.

The HTTP/1.1 specification, in fact, does not specifically say
that proxies and clients should not cache the results of a CGI
response.  In fact, section 13.4 (Response Cachability) says

    Unless specifically constrained by a Cache-Control
    directive, a caching system may always store a successful response 
    as a cache entry, may return it without validation if it
    is fresh, and may return it after successful validation. If there is
    neither a cache validator nor an explicit expiration time associated
    with a response, we do not expect it to be cached [...]

In other words, if the server supplies a response with either a
Last-Modified header, or an Expires header (or "Cache-control: max-age")
that gives an expiration time in the future, then the response
*should* be cached.

However, because most existing caches were designed before HTTP/1.1,
and do not expect servers to generate Expires headers (most servers
apparently do not), they often cache responses that have neither
a Last-Modified header or an Expires header.  This is not really
such a great idea, but it "usually" works.  The two well-known cases
that it often does not work in are those where the URL includes a "?"
and those where it includes "cgi-bin" (or a few similar strings).
So it's normal practice for proxies to not cache responses to such
URLs.

Note that section 13.9 says, regarding URLs with "?" in them,
    caches MUST NOT treat responses to such URLs as fresh unless
    the server provides an explicit expiration time.
There is a general consensus (but not unanimity) that it is better
to err on the side of caution in this case.  I.e., since there are
many such URLs for which caching would cause seriously wrong results,
it's better to not cache any of these responses (and thus give up
the ability to cache certain responses that are cachable), rather
than to risk occasionally returning wrong answers.

However, I think everyone agrees with you that it's both possible and
desirable for origin servers to explictly mark all responses
as either non-cachable or cachable, since then the proxies don't
have to play guessing games based on the URL.  E.g., if you are
writing a server that uses CGI or "?" URLs, and you know that
some of these are cachable, if you simply add a Last-Modified
or Expires (in the future) header to the response, then a well
designed proxy will cache the response.  Conversely, if you
mark the response as Expires "in the past", then no well designed
cache should cache it (without at least sending you a conditional
GET to see if the value has changed).

As to why the AltaVista people haven't done this: I don't know.
Some of them work in our building, but I don't have much to
do with their design decisions (and they didn't invite me
for a ride in the blimp!).

It's probably too hard to decide automatically that a response
on a query for "Soccer in Latvia" would be more stable than
a query for "Cool Site of the day", but it should certainly
be possible to set an expiration time reflecting the expected
time between database updates.

-Jeff
Received on Thursday, 5 December 1996 12:23:25 EST

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