Apache mod_expires question

Question

Hi Folks,

I'm really confused by all that caching stuff. I'm trying to setup mod_expires to reduce the number of HTTP Requests from my website to the server.

I did well so far, I installed mod_expires and wrote a little .conf file from the instructions on http://httpd.apache.org/docs/2.0/mod/mod_expires.html.

Now, for instance, all my .png, .gif, .jpeg files have a Cache-Control header. My expected result was, that the browser won't do any GET Request within the time period (given from the Cache-Control value). But it does, every single file fires a request and receives HTTP 304 not Modified.

That is the wrong behavior isn't it ? It should load that files from the internal cache. One thing I don't understand is, that the browser sends a Response header: Cache-Control: max-age=0. Should it be like that?

Here is an complete example Request + Response headers for a single .png file:

Request

Host             dev-mgg.localdomain
User-Agent     Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.2.8) Gecko/20100722     Firefox/3.6.8
Accept         image/png,image/*;q=0.8,*/*;q=0.5
Accept-Language  de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding  gzip,deflate
Accept-Charset   ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive     115
Connection     keep-alive
Referer       http://dev-mgg.localdomain/css/global/icons.css?18224
Cookie         IR_SQLPwdStore=; IR_SQLUser=sysadm
If-Modified-Since   Thu, 24 Jul 2008 06:24:11 GMT
If-None-Match   "4010127-3c4-452bf1aefd8c0"
Cache-Control   max-age=0

Response

Date         Mon, 02 Aug 2010 14:00:28 GMT
Server     Apache/2.2.9 (Debian) PHP/5.2.6-1+lenny8 with Suhosin-Patch mod_perl/2.0.4     Perl/v5.10.0
Connection   Keep-Alive
Keep-Alive   timeout=15, max=59
Etag         "4010127-3c4-452bf1aefd8c0"
Expires   Mon, 02 Aug 2010 14:04:28 GMT
Cache-Control   max-age=240

Answer 1

It looks like the Expires header being sent back by the server is only 4 minutes in the future. The algorithm by which web browsers decide whether or not to actually make a request (based on Expires values) uses the "closeness" of the current time to the expiration date - so unless the expiration date is still a long time away (weeks, months, years), you can be pretty sure that the browser will make a request for the file.