[squid-users] cache dynamically generated images

Hi All

I'm having no luck doing something which I expected to be very simple, =
yet has me completely stumped. I'll give you an overview of what I am =
trying to do and then my problems. Any help is appreciated.

In short, I want to put squid in front of apache which is serving images =
dynamically (ie generating them from a script) and cache them for the =
period specified by the script as it will vary per image. I am going to =
have different lifespans for the images and I am testing with a max-age =
of 10 minutes but in production it could be as little as 2 seconds. At =
this point I'm hoping you're thinking "A basic http accelerator, ok".

So one of my questions might not really squid specific but if anyone =
know you will. Squid is not caching the images and I suspect it's due to =
the headers being generated. I get TCP_REFRESH_MISS/200 every time, even =
with a 10 minute age. The browser is sending a "If-Modified-Since" =
header as well as a "Cache-Controlmax-age=3D0" and I suspect this is =
part of the problem. I have set an Expires, Etag, "Cache-Control: =
max-age=3D600, s-max-age=3D600, must-revalidate", "Content-Length and =
Last-Modified headers in the response to no avail. Can you advise on =
how to set the headers properly to get squid to cache them? Here is an =
example request/response. My test script is pretty dumb so I can't keep =
track of a last-modified date across sessions, but if I do not send a =
last modified header in the response the browser does not use an =
if-modified-since header in the subsequent request, but there is still =
no caching.

Request headers:

Host: 192.168.13.31
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; =
rv:1.9.2.13) Gecko/20101203 Firefox/3.6.13
Accept: text/html,application/xhtml+xml,application/xml;q=3D0.9,*/*;q=3D0.=
8
Accept-Language: en-us,en;q=3D0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=3D0.7,*;q=3D0.7
Keep-Alive: 115
Connection: keep-alive
If-Modified-Since: Fri, 18 Feb 2011 15:42:29 GMT
If-None-Match: 6382.jpg
Cache-Control: max-age=3D0

Response headers:

Date: Fri, 18 Feb 2011 15:42:33 GMT
Server: Apache/2.2.14 (Win32) SVN/1.6.15 mod_python/3.3.2-dev-20080819 =
Python/2.6.4 DAV/2 mod_perl/2.0.4-dev Perl/v5.10.1
Expires: Fri, 18 Feb 2011 15:52:35 GMT
Etag: 6382.jpg
Cache-Control: max-age=3D600, s-max-age=3D600, must-revalidate
Content-Length: 15965
Last-Modified: Fri, 18 Feb 2011 15:42:35 GMT
Content-Type: image/jpg
X-Cache: MISS from PowerSpecG158
X-Cache-Lookup: HIT from PowerSpecG158:80
Via: 1.1 PowerSpecG158:80 (squid/2.7.STABLE8)
Connection: keep-alive

If you think this approach is bad and you have a better way, I am all =
ears. I have tried a mod_rewrite rule to check for the existence of a =
static file and serve it if it exists, otherwise call the script which =
generates both the static file and returns the content, but the files =
would need to be purged when they are stale and at rapid refresh rates I =
found I could easily get timing issues with the check for the static =
file succeeding, but then apache not being able to find the find when it =
goes to serve it.

Lastly, I was unable to setup squid on an alternate port - say 8081, and =
use an existing apache on port 80, both on the same box. This is for =
testing so I can run squid in parallel with the existing service without =
changing the port it is on. Squid seems to want to use the same port =
for the origin server as itself and I can't figure out how to say =
"listen in 8081 but send requests to port 80 of the origin server". Any =
thoughts on this? I am using another server right now to get around =
this, but it would be more convenient to use the same box.

tia,
charles
Received on Fri Feb 18 2011 - 15:53:34 MST