Even a fairly light-weight page will probably load some CSS and JS, and a few images – it’s pretty easy to find each page load triggering at least a dozen subsequent fetches. Now this doesn’t mean the files will be re-fetched – a sensible webserver setup will respond with
304 (Not Modified) and no content, so the browser will use its cache, but those are requests that don’t need to be made, and only so many that your server will handle at once.
Where static files are concerned, caching is simple: all you need are three easy things:
Set expiry headers
There are two important HTTP headers here:
Cache-Control: max-age. They have slightly different semantics, but the distinction is minor – Apache and nginx both provide a single configuration directive to set both. They tell caches how long they can keep files for – you’ll want to set this to a year from access. Why a year? Historically this used to be the maximum allowed value – that restriction has been lifted, but in many cases, there’s little point in setting a larger value, and I don’t consider it worth it.
For Apache, this means first ensuring that
mod_expires is enabled:
then, in the configuration block serving your static files:
For nginx it’s much simpler (nearly all distros package nginx with the
This is a slightly misleadingly-named class: what it does is rename your static files to include a hash of their content. If the content changes, the filename changes, so clients will get changes immediately rather than waiting for cached content to expire.
The “Cached” part of the name is because it uses the Django cache framework to store the hashes – Django 1.7 brings
ManifestStaticFilesStorage which instead writes them to a JSON file it loads at startup – a much-awaited improvement!
There are two types of caches: private (e.g. a user’s browser) and public (e.g. an institutional shared proxy, or a local CDN).
Cache-Control: public with your static files tells shared caches that it’s ok to retain them.
For Apache, this means ensuring that
mod_headers is enabled:
then adding the following to the configuration block for your static files:
append rather than
set so as not to overwrite the
Cache-Control: max-age setting described above;
add would be equally valid)
Caching configuration is really useful, and simple to set up. If you’ve not done it yet for your apps, follow this guide and speed things up for your users.