On Mon, Apr 18, 2011 at 9:18 PM, Yang Zhang <yanghatespam@xxxxxxxxx> wrote: > On Mon, Apr 18, 2011 at 9:08 PM, Amos Jeffries <squid3@xxxxxxxxxxxxx> wrote: >> On Mon, 18 Apr 2011 20:07:33 -0700, Yang Zhang wrote: >>> >>> We're using squid (3.0.STABLE19-1ubuntu0.1) as an application cache >>> and we're trying to cache everything, for a long time, but max-age=0 >>> is throwing off squid. >> >> max-age=0 is called "reload" in HTTP terminology. >> >>> >>> I have this refresh_pattern in squid.conf: >>> >>> $ grep ^refresh_pattern /etc/squid3/squid.conf >>> refresh_pattern ^ftp: 1440 20% 10080 >>> refresh_pattern ^gopher: 1440 0% 1440 >>> refresh_pattern . 525600 100% 525600 override-expire ignore-private >>> ignore-no-cache ignore-no-store >>> >> >> To quote Frankenstein >> "Why? whyyyy?" >> >> >>> I have the following response prepared to be sent from a dummy web server: >>> >>> $ cat response >>> HTTP/1.1 200 OK >>> Server: Apache-Coyote/1.1 >>> Expires: 0 >>> Pragma: no-cache >>> Cache-Control: no-cache, must-revalidate, max-age=0 >>> Content-Type: text/html;charset=UTF-8 >>> Date: Mon, 18 Apr 2011 10:06:26 GMT >>> Connection: close >> >> Those headers say two things: >> * that cached content MUST NOT be supplied to the client. EVER. >> * that any existing cached content also MUST be checked for updates on the >> next request. >> >> Ignoring this requirement *from the server*. Again... why? >> >> >> I can only imagine that you have fallen for the myth that "application >> caches" must always provide HITs. This is false and following it will break >> things for you in potentially nasty ways. >> >> A cachable application will not send max-age=0 or "Expires: 0", both of >> which mean the object was obsolete the second it was produced. >> >> Squid operating as a reverse-proxy will cache the replies and keep updating >> fresh copies as long as the server is available to present new ones. If the >> server dies an 5xx start coming back, the stale versions will start to be >> used. How long for and whether "must-revalidate" produces 4xx/5xx to the >> client depends on particular squid versions. >> I recommend you grab the newest release you can for the best HTTP/1.1 >> caching behaviour. there are still things we are fixing but work is ongoing >> so the newer the release the better things work. I maintain a PPA for Ubuntu >> (https://launchpad.net/~yadi/+archive/ppa) with newer versions. >> >> <snip> >>> >>> This response prevents anything from being cached; all requests for >>> http://localhost:8765 result in TCP_MISS. I believe it's because of >>> the max-age=0 - once I remove that, this request is cached fine. Yet >>> I had set override-expire in the refresh_pattern. >>> (Note that I don't think it's a must-revalidate/ignore-revalidate (new >>> in v3.1) problem - I can leave that in fine.) >> >> However "We're using squid (3.0.STABLE19-1ubuntu0.1)" ... not a v3.1 >> release. >> >> "ignore-revalidate" blocks the *client* from requesting a revalidation >> (max-age=0). But still obeys the server controls. >> >>> >>> Any hints on how to make squid ignore the max-age=0? I've pored over >>> the refresh_pattern documentation a bunch of times but still can't see >>> what's wrong. Thanks in advance for any help. >> >> If you must; >> * "ignore-reload" to ignore the max-age=0 from either client or server >> IIRC. > > Just tried this out - it doesn't work. The comments in the squid.conf > also say that it's for max-age=0 from the client (no mention of > server). > >> * "reload-into-ims" to convert a client max-age=0 ("force a new copy or >> die") into a soft IMS request ("new content or cached copy"). > > The client is not issuing any max-age=0. Tried adding this in anyway > - didn't work. > >> >> 'must-revalidate' will still have affect until you are using a 3.1 release >> with the "ignore-must-revalidate". The result of revalidate should only be >> 304 (TCP_IMS_*) or 200 (TCP_REFRESH_UPDATE), not a "TCP_MISS" since the >> content being validated *is* in the cache. > > My point earlier is that the must-revalidate from the server doesn't > seem be what's preventing the response from getting cached.... Also, FWIW, these -max-age=0 responses are making it *into* the cache (verified by grepping /var/spool/squid3), for whatever reason (not sure why Squid is deciding to keep them if only to later not use them). -- Yang Zhang http://yz.mit.edu/
