Re: [squid-users] Re: Stale-if-error , does it work?

Looking at the response headers, it's possible that the URL isn't
being cached at all by squid because there's no Last-Modified or Cache-
Control: max-age header. IIRC (Amos, correct me if I'm wrong here),
Squid will assume that content without a Last-Modified header is
dynamic content and not cache it, unless there's a max-age header
giving it an explicit cache age.

And if squid isn't caching the URL, then there's nothing to serve up
if the backend server is unavailable.

You can test this by looking for "X-Cache: HIT" (or MISS) the second
time you request the URL before you shut the back-end server down.

If the URL isn't being cached, the easiest fix IMO would be to add
"max-age=XXX" to your Cache-control: header, as in "Cache-Control: max-
age=300, stale-if-error=300". See if that solves the problem.

HTH,

-Chris

On Mar 27, 2009, at 8:53 AM, Dingwell, Robert A. wrote:

> I¹m using 2.7STABLE6 which is why I¹m confused as to why it¹s not
> working as
> I thought it would. Are there any other headers that need to be
> set as
> well in order for the header to be used? I guess the other question
> is am I
> wrong in my assumption to how this is suppose to work? My assumption
> is that
> if squid receives a stale-if-error cache directive it will cache that
> response and then if there is an error contacting the origin server
> the the
> cached response will be served up instead. Is that correct?
>
> Below are the headers that are being emitted from the server I am
> testing
> against.
>
>
> HTTP/1.1 200 OK
> Date: Thu, 26 Mar 2009 13:59:36 GMT
> Server: Apache-Coyote/1.1
> Content-Type: text/html;charset=ISO-8859-1
> Content-Language: en-US
> Content-Length: 384
> Cache-Control: stale-if-error=300
> Keep-Alive: timeout=5, max=100
> Connection: Keep-Alive
>
> I currently expect squid to cache the content from this request.
>
> Then I shut the origin down and the apache acting as a reverse proxy
> ships
> back a service unavailable response.
>
> HTTP/1.1 503 Service Temporarily Unavailable
> Date: Thu, 26 Mar 2009 14:00:26 GMT
> Content-Length: 323
> Connection: close
> Content-Type: text/html; charset=iso-8859-1
>
> This is were I would expect squid to give me back the first request.
> Is this
> a correct assumption?
>
> Thanks for the help.
>
> Rob
>
>
>
>
>
>
>
> On 3/26/09 10:52 PM, "Amos Jeffries" <squid3_at_treenet.co.nz> wrote:
>
>> Dingwell, Robert A. wrote:
>>>
>>>
>>> On 3/26/09 11:01 AM, "Dingwell, Robert A. (BOBD)" <bobd_at_mitre.org>
>>> wrote:
>>>
>>>> I¹ve been trying to get a caching architecture setup so that if
>>>> a server I
>>>> am caching content for goes down or is more likely just
>>>> unavailable to do
>>>> network issues I can have the cache serve up the stale content.
>>>> In the
>>>> production environment that this will be used I will not have the
>>>> ability to
>>>> have all of the services involved set cache control headers on
>>>> their data.
>>>> We
>>>> are operating on a wan with people all over the globe so it
>>>> would be really
>>>> difficult to get everyone to do that. What I can do though is to
>>>> make sure
>>>> that everything goes through apache acting as a reverse proxy
>>>> which then
>>>> sets
>>>> the Cache-Control header, which I have done and verified that it
>>>> does indeed
>>>> do what I want it to.
>>>>
>>>> So now I fire up squid and apache and attempt to access a server
>>>> I have
>>>> control over and I get back the data I expect to get back. Look
>>>> in the
>>>> squid
>>>> log file see the header in the log ³Cache-Control: stale-if-
>>>> error=300² so I
>>>> know it¹s being sent. Now I shutdown the server I just accessed
>>>> and attempt
>>>> to access the same url. I expect to get back the same data that I
>>>> just got
>>>> back but instead I get back the error page saying the service is
>>>> unavailable.
>>>> Shouldn¹t squid be serving up the stale content? Or am I missing
>>>> a step.
>>>> The
>>>> other odd thing is once I start the server back up it takes a
>>>> couple of
>>>> minutes to get back the page from server as squid seams to be
>>>> caching the
>>>> error page.
>>>>
>>>> I know that stale-if-error is only going to work for 500-504
>>>> errors but
>>>> looking through the squid logs again I see that I am getting an
>>>> error code
>>>> back when I try to access the page when it¹s down ³HTTP/1.1 503
>>>> Service
>>>> Temporarily Unavailable².
>>>>
>>>> One last thing this article , http://www.mnot.net/blog/2007/12/12/stale
>>>> ,
>>>> makes it sound like squid has a configuration parameter that is
>>>> used to
>>>> enable
>>>> stale-if-error even when the server does not send the control
>>>> header. Does
>>>> anyone know if this exists or what the magic incantation is to
>>>> get it
>>>> working?
>>>>
>>>> Anybody have any ideas as to what I may be doing wrong, or an
>>>> alternative
>>>> approach to accomplishing what I am trying to get working? I
>>>> already tried
>>>> using offline mode which did not work and I think isn¹t really
>>>> what I would
>>>> want to do anyway as I need squid to attempt to access the server
>>>> and serve
>>>> up
>>>> the stale data only when the server is down.
>>>>
>>>> Any help is appreciated ,
>>>>
>>>> Thanks
>>>> Rob
>>>
>>
>> Depends.
>> Squid-2.7 supports it, the others do not yet though there is a
>> patch for
>> Squid-3.1 in bugzilla awaiting someone to test it properly.
>>
>> Amos
>> --
>> Please be using
>> Current Stable Squid 2.7.STABLE6 or 3.0.STABLE13
>> Current Beta Squid 3.1.0.6
>>
>
Received on Sun Mar 29 2009 - 22:51:08 MDT