Module ActionController::Caching::Actions

Action caching is similar to page caching by the fact that the entire output of the response is cached, but unlike page caching, every request still goes through Action Pack. The key benefit of this is that filters run before the cache is served, which allows for authentication and other restrictions on whether someone is allowed to execute such action. Example:

  class ListsController < ApplicationController
    before_filter :authenticate, :except => :public

    caches_page   :public
    caches_action :index, :show
  end

In this example, the public action doesn’t require authentication so it’s possible to use the faster page caching. On the other hand index and show require authentication. They can still be cached, but we need action caching for them.

Action caching uses fragment caching internally and an around filter to do the job. The fragment cache is named according to the host and path of the request. A page that is accessed at david.example.com/lists/show/1 will result in a fragment named david.example.com/lists/show/1. This allows the cacher to differentiate between david.example.com/lists/ and jamis.example.com/lists/ — which is a helpful way of assisting the subdomain-as-account-key pattern.

Different representations of the same resource, e.g. david.example.com/lists and david.example.com/lists.xml are treated like separate requests and so are cached separately. Keep in mind when expiring an action cache that :action => 'lists' is not the same as :action => 'list', :format => :xml.

You can modify the default action cache path by passing a :cache_path option. This will be passed directly to ActionCachePath.path_for. This is handy for actions with multiple possible routes that should be cached differently. If a block is given, it is called with the current controller instance.

And you can also use :if (or :unless) to pass a proc that specifies when the action should be cached.

Finally, if you are using memcached, you can also pass :expires_in.

The following example depicts some of the points made above:

  class ListsController < ApplicationController
    before_filter :authenticate, :except => :public

    caches_page :public

    caches_action :index, :if => proc do
      !request.format.json?  # cache if is not a JSON request
    end

    caches_action :show, :cache_path => { :project => 1 },
      :expires_in => 1.hour

    caches_action :feed, :cache_path => proc do
      if params[:user_id]
        user_list_url(params[:user_id, params[:id])
      else
        list_url(params[:id])
      end
    end
  end

If you pass :layout => false, it will only cache your action content. That’s useful when your layout has dynamic information.

Warning: If the format of the request is determined by the Accept HTTP header the Content-Type of the cached response could be wrong because no information about the MIME type is stored in the cache key. So, if you first ask for MIME type M in the Accept header, a cache entry is created, and then perform a second request to the same resource asking for a different MIME type, you’d get the content cached for M.

The :format parameter is taken into account though. The safest way to cache by MIME type is to pass the format in the route.