Re: [squid-users] Squid Reverse Proxy (accel) always contacting the server

On 1/04/2012 3:53 a.m., Daniele Segato wrote:
> On 03/31/2012 05:01 PM, Daniele Segato wrote:
>> On 03/31/2012 10:13 AM, Amos Jeffries wrote:
>>> max-age
>>>
>>> The max-age response directive indicates that the response is to
>>> be considered stale after its age is greater than the specified
>>> number of seconds.
>>>
>>> "
>>>
>>> The logic goes like this:
>>>
>>> Object modified ... 22:14:20
>>> Valid +3600
>>> ==> fresh until 23:14:50
>>> Current time: 23:27:57
>>>
>>> 23:14:50 < 23:27:15 ==> currently stale. must revalidate.
>>>
>>> Expires header can be used to set an absolute time for invaldation.
>>> max-age is relative to age.
>
> Ok I think I now understood you...
>
>
>> you are saying that max-age is added to "last modified date"
>> but that doesn't make much sense to me.
>>
>> If the server (parent cache) is returning the content at 23:27:00 saying
>> max-age 3600 I would expect that 3600 start from "now".
>
>
>> anyway, I thought about this before and I also tried to modify the
>> content, then immediately giving two request to squid.
>
> apparently this was caused by a mistake I did with the server (see below)
>
>
>> this time, suppose:
>>
>> Object modified ... 00:00:00
>> Valid +3600
>> ==> fresh until 01:01:00
>> Current time: 00:05:00
>>
>> 01:01:00 > 00:05:00 ==> currently fresh. shouldn't bother the server.
>>
>> instead what's actually happening is that squid is doing a request to my
>> server, only header, but it's still doing it.
>>
>> My server, to compute the "Last-Modified" date has to do all the job of
>> collecting the data, looping to each data element and extract, for each,
>> the last modified date, then compute the "last one".. it build a "model"
>> that is then rendered: it's pretty short anyway since it's gzipped text.
>>
>> So the big work of my server is to collect the data, and my server have
>> to do it both if you do a GET both if you do an HEAD request.
>>
>> I would like squid to revalidate with my server every, say 1 minute,
>> even 10 seconds is ok.. but it shouldn't revalidate every single request
>> it is receiving.
>>
>> I hope I made my point.
>
>
> this question is still in place :)
>

revalidation is more of a threshold which gets set on each object. Under
the threshold no valdation takes place, above it every request gets
validated. BUT ... a 304 response revalutating the object can change the
threshold by sending new timestamp and caching headers.

>
>
>> I wanted to give you an example but now squid is always giving me a
>> TCP_MISS
>
> this was my mistake, the Last-Modified date format was wrong from
> server :)
>
> please ignore the debug and everything behind this point in my
> previous email...
>
> Now it's giving cache hits in ram!
>
>
> I think I can summarize my question in this two questions:
> 1) can I make squid3 update the cache with my server every, say, 1
> minute (at most) but use it's cache otherwise without bothering the
> server (not even for headers)? how?
>
> Avoiding to call the server for 1 hour, I think, it's a bit too much:
> the content can change in the meanwhile and I don't want the user to
> wait 1 hour for it.
>
> On the other part I don't want every single request after that hour is
> pass to see squid contacting my server to check if the last modified
> date is changed.
>

You have the two options of max-age or Expires. The thing to remember is
to increment the value / threshold forward to the next poitn where you
want revalidation to take place.

with a max-age N value which you generate dynamically by: calculate
current age of object when responding, add 60.

with Expires: you simply emit a timestamp of now() + 60 seconds on each
response.

Other useful things to know;
   Generating an ETag label for each unique output helps caches detect
unique versions without timestamp calculations. The easy ways to do this
are to make ETag a MD5 hash of the body object. Or a hash of the
Last-Modified timestamp string if the body is too expensive to locate
MD5 for. Or some other property of the resource which is guaranteed to
change any time the body changes and not otherwise.

   Cache-Control:stale-while-revalidate tells caches to revalidate, but
not to block the client response waiting for that validation to finish.
Clients will get the old object until a new one or 304 is received back.

>
>
> 2) which is the best way to debug why squid3 is deciding to keep a
> cache entry, contact the server or not? looking at the huge debug log
> is not very simple maybe some log option to filter it with the cache
> decisions informations only would help

debug_options 22,3
... or maybe 22,5 if there is not enough at level 3.

Amos
Received on Sun Apr 01 2012 - 01:21:46 MDT