Since the Steve Sounder’s High Performance Web Sites everyone knows to set far future Expires header on their static assets. What he didn’t say was how to deal with the file changes. You can’t instruct browser to download the file so you actually have to change the file’s URL.

Ruby on Rails handles this by automatically adding file’s timestamp as a query-string when using build-in helper methods — simple and not requiring server-side URLs rewriting. This is good enough, but only for files accessed from HTML directly and doesn’t work for files accessed from CSS: background images (unless you have dynamically generated styles).

I was thinking for some time about those background images, because they can be heavy and if you’re not using CSS sprites they make lots of requests. As I don’t like dynamically generated style sheets, I didn’t want any additional processing. And so it came to me: why not incorporate some changing value into the domain name? You could put a revision number or a timestamp in domain name of your statics server, just like this: static-1566.example.com. Both Rails and Django provide easy ways to change static assets domain.

This requires you to set-up wildcard subdomain, but is’s easier than processing CSS files. It works because url function in a style sheet is resolved using the style sheet’s own address.

PS: This is only a brain-dump — I didn’t test it.