apache - random numbers after a ? on an image is this for far expires headers? -

i pooking around random site on internet , noticed on images have numbers prefixing it: icons-16.png?1292032550

i've heard of people optimising websites far expires headers. if changes content doesn't change often, cache won't refreshed. ergo new image won't re-downloaded someones cache. because filename has change.

yes, intent force refresh of browser cache. however, i not recommend approach:

• many proxies (and possibly browsers) not cache anything query string, regardless of cache-control headers. you're shooting in foot if include superfluous query string – you'll needlessly consume own bandwidth sending images should cached, aren't.
• depending on how configure server, user agents periodically make request cached resources, if-modified-since and/or if-none-match header. if client's cache date, server responds 304 not modified , stops; otherwise responds normal 200 ok , sends new content. not have change resource's file name in order client caches updated when resource changes. trying clever query string serves defeat caching mechanisms.

• that said, if optimize caching setting expires date year out (and if last modified date of resource long ago), user agents may check updates infrequently. if unacceptable you, have 2 options: either reduce amount of time before resource expires (so browser issue get request , can respond 304 or 200 appropriate), or use "url fingerprinting," random token included in path, instead of in query string. example:

  /img/a03f/image.png 

  instead of

  /img/image.png?a03f 

  this way, resources still cached proxies. you'll want in using mod_rewrite allow include token in path. of course, need able change references url whenever change resource.

for further reading, highly recommend google's page speed best practices, section on optimizing caching.