Caching, Weak Etags, and Last-Modified Issues

Question

I'm having trouble with a few images on a site that I'm developing. Some images are being cached correctly and their headers seem correct (strong etags, correct last-modified). But other images have weak etags (preceded with W/) and a last-modified header that is always equal to the current time. Does anyone know how I can resolve this issue? I've looked around quite a bit and I can't seem to find anything that works. The image that isn't being cached is a background image and it causes a background flash on every page reload, which is annoying.

Headers:

Accept-Ranges:bytes
Connection:Keep-Alive
Content-Length:6879
Content-Type:image/jpeg
Date:Wed, 15 Dec 2010 07:41:44 GMT
Etag:W/"4e863ea-1adf-49772cd85b340"
Keep-Alive:timeout=15, max=98
Last-Modified:Wed, 15 Dec 2010 07:41:44 GMT
Server:Apache/2.2.11 (Ubuntu) PHP/5.2.6-3ubuntu4.6 with Suhosin-Patch

Thank you in advance for any help. I'm not an expert in this sort of stuff, so I apologize if I'm missing something obvious.

Answer 1

If you want the ensure that your images are cached you can not rely on validation headers (last-modified, etag). For ensuring caching you need to use Expires or Cache-control. Validation is used only after cache expiration mechanism says that content is stale. If you do not use Expires or Cache-control headers, content freshness is not defined and depends only on the specific implementation of the browser so in your case it can behave unexpectedly.

Answer 2

I'm not sure what changed, but it looks like the problem is gone.

Headers:

Accept-Ranges:bytes
Connection:Keep-Alive
Content-Length:6879
Content-Type:image/jpeg
Date:Wed, 15 Dec 2010 16:46:14 GMT
Etag:"4e863ea-1adf-49772cd85b340"
Keep-Alive:timeout=15, max=99
Last-Modified:Wed, 15 Dec 2010 13:20:37 GMT
Server:Apache/2.2.11 (Ubuntu) PHP/5.2.6-3ubuntu4.6 with Suhosin-Patch