Contributors:
  Tom Fakes (tom@craz8.com) - Initial implementation, plugin implementation, x-sendfile work
  Scott Laird				- Ideas for the timed expiry and programmable fragment_key features

=== Action Cache Upgrade

This is a drop in replacement for the Rails Action Cache.  When this plugin is
installed, the new behavior will take effect without any further configuration.

All documentation for the Rails Action Cache is still relevant.  Sweepers still work, all the
fragment stores are supported.

See my blog at http://blog.craz8.com to find some interesting uses of the extended behavior
provided by this plugin

=== Major Change!

This version uses a different cache key generation mechanism.  Instead of setting 
ActionController::Caching::Actions::ActionCacheFilter.fragment_key, the cache code calls out to
the action_fragment_key method on the current controller.  A default version of this method is
supplied that emulates the Rails built in Action Cache.  If you haven't set the fragment_key
in your code, then nothing changes.  If you have set the fragment_key, then you will need
to move that code to the application controller for your code to continue working. 

=== Features

1. Store cache entries as YAML streams so the Response headers from the original
   response can be returned with cache hits

2. Add a 'last-modified' header to the response to get the client to use a 
   get-if-modified request

3. If the client has the response we have cached, don't send it again, send a 
   '304 Not Modified' response to reduce data on the wire

4. Fix a bug in the original Rails code where responses other than '200 OK' are cached
   (since the headers aren't cached in the original, all the clients would get
   is an empty '200 OK' response from subsequent requests)

5. Allow clients to provide their own implementation of the cache key for the actions, e.g.

	- application.rb
	
	  # Cache different pages for Admin and Users
      def action_fragment_key(options)
        url_for(options).split('://').last + "/#{admin? : 'admin' : 'user'}"
      end
      
   The options hash can be used to pass parameters in to override the current controller, and is
   used by the cache expiry code to expire an action from a sweeper or a different controller than
   the one the action is cached for.
    
6. Allow an action to specify a Time To Live for the cached item.  Set 'response.time_to_live' to
   the number of seconds before this cached item will be expired.  If not set, the default setting
   of 'never' will be used and the item will only be expired by using the regular action cache 
   expiry mechanism.
   
     def my_action
       @response.time_to_live = 10.minutes
       ...
     end

7.  If the ENABLE_X_SENDFILE environment variable is set, or the HTTP_ENABLE_X_SENDFILE request 
    header is set, and the fragment cache is set to the FileStore, then the Action Cache code 
    will not return the response body, but will set the X-Sendfile header in the response to 
    the filename of the cache entry that contains the body.
     
   Be sure your web server is has the X-Sendfile feature enabled, otherwise you'll just get
   empty responses!
     
   Check out the lighttpd documentation for how to use the X-Sendfile feature: http://lighttpd.net/
   
   To enable this, the ENABLE_X_SENDFILE environment variable must be set, *and* the FileStore fragment
   cache must be used.
   
   lighttpd.conf:
   	
	fastcgi.server = ( ".fcgi" =>
  		( "app" =>
    		( 	"min-procs" => 1, 
      			"max-procs" => 1,
				"allow-x-send-file" => "enable",
      			"socket"    => "/tmp/app.fcgi.socket",
      			"bin-path"  => "/path/to/app/public/dispatch.fcgi",
      			"bin-environment" => ( "RAILS_ENV" => "development", "ENABLE_X_SENDFILE" => "true" )
    		)
  		)
	)


   environment.rb:
     ActionController::Base.fragment_cache_store = :file_store, "/path/to/cache/directory"
   
   Note: The cache directory can be anywhere on your server that your web server user has read and write 
   access to.  This should *not* be in the Rails /public directory.		

8. Control whether caching occurs for an action at runtime instead of load time.  
   
   To control caching, add a method *cache_action?(action_name)* to your controller.  If this 
   method returns true, then the action cache will work as before.  If false, then caching will 
   not occur for this request.
   
   e.g.
   	
   	 class ApplicationController < ActionController::Base
   	 	...
   	 	
   	 	def cache_action?(action_name)
   	 	  !admin?
   	 	end
   
   		...
     end
	
	Note: The action must still be marked for caching by adding *caches_action :action* to the controller

9. If the ENABLE_X_ACCEL_REDIRECT request header is set, and the fragment cache is set to
   the FileStore, then the Action Cache code will not return the response body, but will set
   the X-Accel-Redirect header in the response to the filename of the cache entry that contains the 
   body.

   The nginx configuration must contain a 'location' section labeled 'cache', that points to the location
   you have configured for your Rails fragment cache, default is RAILS_ROOT/tmp/cache.  e.g:
   
     location /cache/ {
        internal;
        root   /path/to/rails/app/current/tmp;
     }

   To enable this, the ENABLE_X_SENDFILE environment variable must be set, *and* the FileStore fragment
   cache must be used.

   nginx.conf:
    location /cache/ {
      internal;
      root   /path/to/rails/app/current/tmp;
    }
    
    location / {
      proxy_set_header  X-Real-IP  $remote_addr;
      proxy_set_header  X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Host $http_host;
      proxy_set_header "ENABLE_X_ACCEL_REDIRECT" "true";
	   ...
	}      
   
   environment.rb:
     ActionController::Base.fragment_cache_store = :file_store, "/path/to/cache/directory"
   
   Note: The cache directory can be anywhere on your server that your web server user has read and write 
   access to.  This should *not* be in the Rails /public directory.		

10. A new method 'expire_all_actions' will clear out the entire action cache contents.

11. expire_action will now work with the custom generated action cache keys, so your cache
    expiry calls and sweepers will work correctly.
    
    The expire_action call implemented here will actually use the Regexp fragment expiry call,
    causing all matching cache items to be cleared.  For those of you using REST, and providing 
    HTML, JS and XML for the same action, all three will be expired when you expire one of them 
    with code like:
    
    	# Expires all formats of the action
    	expire_action :controller => 'foo', :action => 'bar'
	   
=== Performance

If a client requests an action whose output hasn't changed since their last request, the returning of
a 304 response instead of the full response greatly reduces the load on the server.

In my informal testing, with the X-Sendfile enabled, I was able to get about 20% more requests out of my
rails application, based on the requests-per-second displayed in the rails log.  This doesn't mean the 
request is faster, but that the work of delivering the content is offloaded to the web server from the
Rails app.