In HTTP, since it's stateless, you don't "tell" the client anything without it first asking. The usual way to bust the cache is to add a junk parameter to the end of a GET request.
I see. I assume you would change the html code to say <img src="image.png?cache=no"> or something like that to force the browser to redownload it? What if the html page itself were cached for a year? Is there an Apache setting that can give a global "no caching" command, or something like that?
Yep, exactly -- not only can the images be cached, but the HTML too!
The ideal way to do it is have the "loader file" (index.html) only cached with last-modified date, so as soon as it changes the client is aware. The client requests the file each time, and is returned the full file or a simple Not Modified response.
Within the file, you have references to permanently-cached, versioned resources (<img src="/images/foo.png?build=123" />). If the cache expiration is far enough away, the browser won't even issue the request to check for a new version.
Some browsers don't cache query params so you might use rewriting rules to change foo.png to foo.123.png. This rewriting is done automatically for you with Google Page Speed module for Apache.
It sounds like a much deeper subject than I first appreciated. I'll definitely read up on this. Using URL rewriting with caching is interesting, I've not seen that before.
I work with one lady that complains about a slow loading JQuery slideshow, and smarter caching may very well be the solution (at least, after the first load).
I've experimented and managed to shave 2 seconds off a client's website upon reloads - that's significant! Still playing with it, but I've already learned a lot.
