Page caching is an approach to caching where the entire action output of is stored as a HTML file that the web server can serve without going through Action Pack. This is the fastest way to cache your content as opposed to going dynamically through the process of generating the content. Unfortunately, this incredible speed-up is only available to stateless pages where all visitors are treated the same. Content management systems — including weblogs and wikis — have many pages that are a great fit for this approach, but account-based systems where people log in and manipulate their own data are often less likely candidates.

Specifying which actions to cache is done through the caches_page class method:

  class WeblogController < ActionController::Base
    caches_page :show, :new
  end

This will generate cache files such as weblog/show/5.html and weblog/new.html, which match the URLs used to trigger the dynamic generation. This is how the web server is able pick up a cache file when it exists and otherwise let the request pass on to Action Pack to generate it.

Expiration of the cache is handled by deleting the cached file, which results in a lazy regeneration approach where the cache is not restored before another hit is made against it. The API for doing so mimics the options from url_for and friends:

  class WeblogController < ActionController::Base
    def update
      List.update(params[:list][:id], params[:list])
      expire_page :action => "show", :id => params[:list][:id]
      redirect_to :action => "show", :id => params[:list][:id]
    end
  end

Additionally, you can expire caches using Sweepers that act on changes in the model to determine when a cache is supposed to be expired.