Re: [squid-users] Re: refresh_pattern question

On Wed, 2009-12-23 at 18:10 -0800, RW wrote:
> On Wed, 23 Dec 2009 16:34:37 -0800
> Manjusha Maddala <mmaddala25_at_nextag.com> wrote:
>
> > >From "Squid - the definitive guide", a simplified description of the
> > refresh_pattern algorithm is:
> >
> > - The response is stale if the response age is greater than the
> > refresh_pattern max value.
> > - The response is fresh if the LM-factor is less than the
> > refresh_pattern percent value.
> > - The response is fresh if the response age is less than the
> > refresh_pattern min value.
> > - Otherwise, the response is stale.
> >
> > The webserver I have neither sends a Last-modified header nor an
> > Expires header.
>
> That's odd, if you don't have Last-Modified, or an equivalent header,
> you shouldn't see TCP_REFRESH_MISS. Does it have a cache-control header
> instead of Expires?

The only headers the web server sends with the HTTP response to Squid
are "Date", "Server", "Set-cookie", "P3P", "Content-type", "Vary",
"Content-Encoding", "Content-length", "Connection".

>
> Are you sure you aren't hitting a previous refresh_pattern line?
>

Exactly, that's something I don't understand. As per the Squid
configuration several years ago, the refresh_pattern looked like:

refresh_pattern . 30day_equivalent_minutes 100% 30day_equivalent_minutes
override-expire

An year ago, the max age was changed to 15 days from 30 and Squid was
restarted, but still I see the 30day thing coming into effect even
today. Once the squid.conf is edited, what are the next steps to make
sure Squid restarts with the newer configuration?

>
> > > >From squid.conf,
> > > > refresh_pattern . 21600 100% 21600 override-expire
> > > >
> > > > That is, a cached page is fresh if its age in cache < 15 days
> > > > (21600=15*24*60).
> > >
> > >
> > > not quite, an object without an explicit expiry time, but that
> > > can be validated, could be stale in less than 15 days.
> > >
> >
> > So, won't all pages with response age < 15 be considered fresh?
>
> If an object lacks explicit expiry information and can be validated,
> it's governed by the percentage and max-age only, and is stale if either
> is exceeded e.g. a object that was last-modified 5 days ago and
> validated 3 days ago will have exceeded your 100% limit.

I didn't quite get that. Lets say, a request hits Squid with the URL:
http://someurl. If thats the first ever request for http://someurl,
Squid fetches the page from the web server and caches it. The server
does not send any Cache-control/Expires/Last-Modified-Date headers with
the response. Squid sends the response to the client and caches the
content on the disk. 10 days later, another request comes for the same
url. Note that the page was not validated during the last 10days as
there were no requests.

Going by the refresh_pattern algorithm with this directive:
refresh_pattern . 21600 100% 21600 override-expire

- The response is stale if the response age is greater than the
refresh_pattern max value.

        = response age=10, which is less than refresh_pattern max (15); hence
goto next step

- The response is fresh if the LM-factor is less than the
refresh_pattern percent value.

        = there is no "Last-Modified-Date"; hence skipping this step.

- The response is fresh if the response age is less than the
refresh_pattern min value.

        = 10 < 15, so the cached page is still fresh; end of algorithm

- Otherwise, the response is stale.

Isn't that how its supposed to work?
Received on Thu Dec 24 2009 - 18:18:59 MST