Browser Cache Control, Dynamic Content

Question

Problem: I can't seem to get FireFox to cache images sent from a dynamic server

Setup: Static Apache Server with reverse proxy to a dynamic server (mod_perl2) at backend.

Here is the request URL for the server. It is sent to the the dynamic server, where the cookie is used to validate access to the image:

Request Headers

Host:  <OBSCURED>
User-Agent:  Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.15) Gecko/2009102815 Ubuntu/9.04 (jaunty) Firefox/3.0.15
Accept:  image/png,image/*;q=0.8,*/*;q=0.5
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset:  ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive:  300
Connection:  keep-alive
Referer: <OBSCURED>
Cookie:  pz_cred=4KCNr0RM15%2FJCOt%2BEa6%2BL62z%2Fxvbp2xNQHY5pJw5d6Q
Pragma:  no-cache
Cache-Control: no-cache

The dynamic server streams the image back to the server, and provides the following response:

Response Headers

Date:  Tue, 24 Nov 2009 04:28:07 GMT
Server:  Apache/2.2.11 (Ubuntu) mod_apreq2-20051231/2.6.0 mod_perl/2.0.4 Perl/v5.10.0
Cache-Control: public, max-age=31536000
Content-Length:  25496
Content-Type:  image/jpeg
Via: 1.1 127.0.1.1:8081
Keep-Alive:  timeout=15, max=75
Connection:  Keep-Alive

So far, so good (me thinks). However, on reload of the page, the image does not appear cached, and a request is again sent:

Request Headers

Host: <OBSCURED>
User-Agent:  Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.15) Gecko/2009102815 Ubuntu/9.04 (jaunty) Firefox/3.0.15
Accept:  image/png,image/*;q=0.8,*/*;q=0.5
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset:  ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive:  300
Connection:  keep-alive
Referer: <OBSCURED>
Cookie:  pz_cred=4KCNr0RM15%2FJCOt%2BEa6%2BL62z%2Fxvbp2xNQHY5pJw5d6Q
Cache-Control: max-age=0

It doesn't seem that request should happen as the browser should have cached the image. As it is, a 200 response is received, same as the first, and the image appears to be re-fetched (although the browser does appear to be using the cached images).

The problem appears to be hinted at by the Cache-Control: max-age=0 in the reload request header, above.

Does anyone know why this is happening? Perhaps it is the Via header in the response that is causing the problem?

Answer 1

The original request has

Cache-Control: no-cache

which tells all the intermediate HTTP caches (including Firefox's) that you don't want to use a cached response, you want to get the response from the origin web server itself.

The response says:

Cache-Control: public, max-age=31536000

which tells everyone that as far as the origin server is concerned, the response may be cached. The server seems to be configured to enable the PNG image to be cached: HTTP 1.1 (section 14.21) says:

Note: if a response includes a Cache-Control field with the max-age directive (see section 14.9.3), that directive overrides the Expires field.

Your second request says:

Cache-Control: max-age=0

which tells all the intermediate HTTP caches that you won't take any cached response older than 0 seconds.

One thing to watch out for: if you hit the Reload button in Firefox, you are asking to reload from the origin web server. To test the caching of the image, navigate away from the page and back, or open it up in a new tab. Not sure why you saw no-cache the first time and max-age=0 the second though.

BTW, I like the FireBug plug-in for Firefox. You can take a look at the request and response headers with it and all sorts of other good stuff.