Caching

A fundamental trade-off in dynamic Web sites is, well, they’re dynamic. Each time a user requests a page, the Web server makes all sorts of calculations, from database queries to template rendering to business logic to creating the page that your site’s visitors see. This is a lot more expensive, from a processing-overhead perspective, than your standard read-a-file-off-the-filesystem server arrangement.

For most Web applications, this overhead isn’t a big deal. Most Web applications aren’t washingtonpost.com or slashdot.org; they’re simply small- to medium-sized sites with so-so traffic.

But for medium- to high-traffic sites, it’s essential to cut as much overhead as possible.

That’s where caching comes in. To cache something is to save the result of an expensive calculation so that you don’t have to perform the calculation next time. Here’s some pseudocode explaining how this would work for a dynamically generated Web page:

given a URL, try finding that page in the cache if the page is in the

cache:

return the cached page

else:

generate the page

save the generated page in the cache (for next time)

return the generated page

Django comes with a robust cache system that lets you save dynamic pages so they don’t have to be calculated for each request. For convenience, Django offers different levels of cache granularity: You can cache the output of specific views, you can cache only the pieces that are difficult to produce, or you can cache your entire site.

Django also works well with downstream caches, such as Squid and browser-based caches. These are the types of caches that you don’t directly control, but to which you can provide hints (via HTTP headers) about which parts of your site should be cached, and how.

Back to Tutorial