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.
* "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").
'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.
Amos
