Re: [squid-users] How to make squid ignore max-age?

On Mon, Apr 18, 2011 at 9:08 PM, Amos Jeffries <squid3_at_treenet.co.nz> 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....

>
> Amos
>

-- 
Yang Zhang
http://yz.mit.edu/