On Wed, 25 May 2011 10:16:46 -0700, melissa schellenberg wrote:
We're performing an upgrade on the CMS that is sitting behind Squid,
and we want Squid to serve up ALL pages from its cache during the
hour
or two of the upgrade, so that no requests are made to the CMS during
that time. ÂIs there a "hero mode" setting that we can toggle in
Squid? ÂOr should we be pre-loading all cached pages with long expiry
times beforehand?
I've been reading some rather old threads "offline mode" but that
seems to be applying only to forward proxying. ÂThanks in advance for
any help!
There is a magic option. The very badly named "offline_mode" causes
squid to grab things as greedily as possible for caching. Turn it ON for
a while leading up to the outage, some days usually.
Also, run a check of the Expires: headers being used by site content.
That is an absolute limit on cache storage. Bumping up the short ones to
after the outage is over will reduce unavailable objects for the
duration.
Check max_stale (if available in your Squid) is set much longer than
the outage time. Several multiples of the outage period would probably
be best, this has to cope with data stored at the start of the
offline_mode turn-on as well as stuff requested just before outage.
Remove "must-revalidate" and "proxy-revalidate" cache controls wherever
possible for a while leading up to the outage. This trades problems with
unavailable objects for problems with stale objects, so care is needed.
In general if you can easily remove a must-revalidate safely you may
benefit long-term by leaving it that way :)
Also, maybe have a "sorry downtime" page to redirect posts to:
acl POSTs method POST PUT
deny_info http://example.com/sorry.html POSTs
http_access deny POSTs
You likely will miss some things. But those will help a lot.
Alternatively, if this is super critical you could start the outage by
taking a static mirror of the site then pointing Squid to use that
temporarily. Sending this static copy with a fixed Expires: set to the
end of the upgrade outage will make all new requests transition to the
new site version at a easily determined time.
Amos
