ETag vs Header Expires

Question

I've looked around but haven't been able to figure out if I should use both an ETag and an Expires Header or one or the other.

What I'm trying to do is make sure that my flash files (and other images and what not only get updated when there is a change to those files.

I don't want to do anything special like changing the filename or putting some weird chars on the end of the url to make it not get cached.

Also, is there anything I need to do programatically on my end in my PHP scripts to support this or is it all Apache?

Answer 1

Etag and Last-modified headers are validators.

They help the browser and/or the cache (reverse proxy) to understand if a file/page, has changed, even if it preserves the same name.

Expires and Cache-control are giving refresh information.

This means that they inform, the browser and the reverse in-between proxies, up to what time or for how long, they may keep the page/file at their cache.

So the question usually is which one validator to use, etag or last-modified, and which refresh infomation header to use, expires or cache-control.