Professional Ruby on Rails™

13.3. Caching

Databases aren't the only places where your Rails application might need a speed boost. Your view code can slow to a crawl as well. This section discusses a way to speed up views, and provides a look at what can slow them down.

Caching is the purest way to speed up your Rails application. By that, I mean that caching has a sort of strict adherence to the Don't Repeat Yourself (DRY) principle — don't redo page generation that you've already done. The basic idea of caching is that your Rails application automatically stores generated HTML for a page or some segment of a page. When the next user requests that part of the application, the cached file is sent to the browser, saving you from regenerating the data. The performance improvement here can be substantial, not just because you are saving the render time, but also because you are saving the bandwidth to and from the database. Where applicable, caching can give you a multiple order-of-magnitude speedup.

Caching is applicable for any page or portion of a page that will be identical for multiple users. Even if the underlying data changes rapidly, you can still get a performance speedup for a cache that is expired frequently. If you're getting 10 hits a second, then even with a cache that lives for 30 seconds, well over 99 percent of your page hits will be served from the cache.

Rails can cache at three different scales: an entire page, an entire controller action, or an arbitrary fragment of a view. Each scale has ...