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 the Action Pack. The key benefit of this is that filters are run before the cache is served, which allows for authentication and other restrictions on whether someone is allowed to see the cache. Example:

class ListsController < ApplicationController
  before_filter :authenticate, :except => :public
  caches_page   :public
  caches_action :index, :show, :feed
end

In this example, the public action doesn't require authentication, so it's possible to use the faster page caching method. But both the show and feed action are to be shielded behind the authenticate filter, so we need to implement those as action caches.

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

Different representations of the same resource, e.g. http://david.somewhere.com/listsand http://david.somewhere.com/lists.xmlare 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 set modify the default action cache path by passing a :cache_path option. This will be passed directly to ActionController::Caching::Actions::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.

class ListsController < ApplicationController
  before_filter :authenticate, :except => :public
  caches_page   :public
  caches_action :index, :if => Proc.new { |c| !c.request.format.json? } # cache if is not a JSON request
  caches_action :show, :cache_path => { :project => 1 }, :expires_in => 1.hour
  caches_action :feed, :cache_path => Proc.new { |controller|
    controller.params[:user_id] ?
      controller.send(:user_list_url, controller.params[:user_id], controller.params[:id]) :
      controller.send(:list_url, controller.params[:id]) }
end

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

Namespace
Methods
E
Instance Protected methods
expire_action(options = {})
# File actionpack/lib/action_controller/caching/actions.rb, line 71
def expire_action(options = {})
  return unless cache_configured?
  if options[:action].is_a?(Array)
    options[:action].dup.each do |action|
      expire_fragment(ActionCachePath.path_for(self, options.merge({ :action => action }), false))
    end
  else
    expire_fragment(ActionCachePath.path_for(self, options, false))
  end
end