Question

Caching is something that I kind of ignored for a long time, as projects that I worked on were on local intranets with very little activity. I’m working on a much larger Rails 3 personal project now, and I’m trying to work out what and when I should cache things.

1. How do people generally determine this?
2. If I know a site is going to be relatively low-activity, should I just cache every single page?
3. If I have a page that calls several partials, is it better to do fragment caching in those partials, or page caching on those partials?

The Ruby on Rails guides did a fine job of explaining how caching in Rails 3 works, but I’m having trouble understanding the decision-making process associated with it.

Answer 1

Don’t ever cache for the sake of it, cache because there’s a need (with the exception of something like the homepage, which you know is going to be super popular.) Launch the site, and either parse your logs or use something like NewRelic to see what’s slow. From there, you can work out what’s worth caching.

Generally though, if something takes 500ms to complete, you should cache, and if it’s over 1 second, you’re probably doing too much in the request, and you should farm whatever you’re doing to a background process…for example, fetching a Twitter feed, or manipulating images.

EDIT: See apneadiving’s answer too, he links to some great screencasts (albeit based on Rails 2, but the theory is the same.)