Caching with Rails: An overview

Everyone caches. This guide will teach you what you need to know about -avoiding that expensive round-trip to your database and returning what you -need to return to those hungry web clients in the shortest time possible.

1. Basic Caching

This is an introduction to the three types of caching techniques that Rails -provides by default without the use of any third party plugins.

To get started make sure config.action_controller.perform_caching is set -to true for your environment. This flag is normally set in the -corresponding config/environments/*.rb and caching is disabled by default -there for development and test, and enabled for production.

config.action_controller.perform_caching = true

1.1. Page Caching

Page caching is a Rails mechanism which allows the request for a generated -page to be fulfilled by the webserver, without ever having to go through the -Rails stack at all. Obviously, this is super-fast. Unfortunately, it can’t be -applied to every situation (such as pages that need authentication) and since -the webserver is literally just serving a file from the filesystem, cache -expiration is an issue that needs to be dealt with.

So, how do you enable this super-fast cache behavior? Simple, let’s say you -have a controller called ProductsController and a list action that lists all -the products

class ProductsController < ActionController
-
-  caches_page :index
-
-  def index; end
-
-end

The first time anyone requests products/index, Rails will generate a file -called index.html and the webserver will then look for that file before it -passes the next request for products/index to your Rails application.

By default, the page cache directory is set to Rails.public_path (which is -usually set to RAILS_ROOT + "/public") and this can be configured by -changing the configuration setting config.action_controller.page_cache_directory. -Changing the default from /public helps avoid naming conflicts, since you may -want to put other static html in /public, but changing this will require web -server reconfiguration to let the web server know where to serve the cached -files from.

The Page Caching mechanism will automatically add a .html exxtension to -requests for pages that do not have an extension to make it easy for the -webserver to find those pages and this can be configured by changing the -configuration setting config.action_controller.page_cache_extension.

In order to expire this page when a new product is added we could extend our -example controler like this:

class ProductsController < ActionController
-
-  caches_page :list
-
-  def list; end
-
-  def create
-    expire_page :action => :list
-  end
-
-end

If you want a more complicated expiration scheme, you can use cache sweepers -to expire cached objects when things change. This is covered in the section on Sweepers.

1.2. Action Caching

One of the issues with Page Caching is that you cannot use it for pages that -require to restrict access somehow. This is where Action Caching comes in. -Action Caching works like Page Caching except for the fact that the incoming -web request does go from the webserver to the Rails stack and Action Pack so -that before filters can be run on it before the cache is served, so that -authentication and other restrictions can be used while still serving the -result of the output from a cached copy.

Clearing the cache works in the exact same way as with Page Caching.

Let’s say you only wanted authenticated users to edit or create a Product -object, but still cache those pages:

class ProductsController < ActionController
-
-  before_filter :authenticate, :only => [ :edit, :create ]
-  caches_page :list
-  caches_action :edit
-
-  def list; end
-
-  def create
-    expire_page :action => :list
-    expire_action :action => :edit
-  end
-
-  def edit; end
-
-end

And you can also use :if (or :unless) to pass a Proc that specifies when the -action should be cached. Also, you can use :layout => false to cache without -layout so that dynamic information in the layout such as logged in user info -or the number of items in the cart can be left uncached. This feature is -available as of Rails 2.2.

[More: more examples? Walk-through of Action Caching from request to response? - Description of Rake tasks to clear cached files? Show example of - subdomain caching? Talk about :cache_path, :if and assing blocks/Procs - to expire_action?]

1.3. Fragment Caching

Life would be perfect if we could get away with caching the entire contents of -a page or action and serving it out to the world. Unfortunately, dynamic web -applications usually build pages with a variety of components not all of which -have the same caching characteristics. In order to address such a dynamically -created page where different parts of the page need to be cached and expired -differently Rails provides a mechanism called Fragment Caching.

Fragment Caching allows a fragment of view logic to be wrapped in a cache -block and served out of the cache store when the next request comes in.

As an example, if you wanted to show all the orders placed on your website -in real time and didn’t want to cache that part of the page, but did want -to cache the part of the page which lists all products available, you -could use this piece of code:

<% Order.find_recent.each do |o| %>
-  <%= o.buyer.name %> bought <% o.product.name %>
-<% end %>
-
-<% cache do %>
-  All available products:
-  <% Product.find(:all).each do |p| %>
-    <%= link_to p.name, product_url(p) %>
-  <% end %>
-<% end %>

The cache block in our example will bind to the action that called it and is -written out to the same place as the Action Cache, which means that if you -want to cache multiple fragments per action, you should provide an action_suffix to the cache call:

<% cache(:action => 'recent', :action_suffix => 'all_products') do %>
-  All available products:

and you can expire it using the expire_fragment method, like so:

expire_fragment(:controller => 'products', :action => 'recent', :action_suffix => 'all_products)

If you don’t want the cache block to bind to the action that called it, You can -also use globally keyed fragments by calling the cache method with a key, like -so:

<% cache(:key => ['all_available_products', @latest_product.created_at].join(':')) do %>
-  All available products:

This fragment is then available to all actions in the ProductsController using -the key and can be expired the same way:

expire_fragment(:key => ['all_available_products', @latest_product.created_at].join(':'))

1.4. Sweepers

Cache sweeping is a mechanism which allows you to get around having a ton of -expire_{page,action,fragment} calls in your code by moving all the work -required to expire cached content into a ActionController::Caching::Sweeper -class that is an Observer and looks for changes to an object via callbacks, -and when a change occurs it expires the caches associated with that object n -an around or after filter.

Continuing with our Product controller example, we could rewrite it with a -sweeper such as the following:

class StoreSweeper < ActionController::Caching::Sweeper
-  observe Product # This sweeper is going to keep an eye on the Product model
-
-  # If our sweeper detects that a Product was created call this
-  def after_create(product)
-          expire_cache_for(product)
-  end
-
-  # If our sweeper detects that a Product was updated call this
-  def after_update(product)
-          expire_cache_for(product)
-  end
-
-  # If our sweeper detects that a Product was deleted call this
-  def after_destroy(product)
-          expire_cache_for(product)
-  end
-
-  private
-  def expire_cache_for(record)
-    # Expire the list page now that we added a new product
-    expire_page(:controller => '#{record}', :action => 'list')
-
-    # Expire a fragment
-    expire_fragment(:controller => '#{record}', :action => 'recent', :action_suffix => 'all_products')
-  end
-end

Then we add it to our controller to tell it to call the sweeper when certain -actions are called. So, if we wanted to expire the cached content for the -list and edit actions when the create action was called, we could do the -following:

class ProductsController < ActionController
-
-  before_filter :authenticate, :only => [ :edit, :create ]
-  caches_page :list
-  caches_action :edit
-  cache_sweeper :store_sweeper, :only => [ :create ]
-
-  def list; end
-
-  def create
-    expire_page :action => :list
-    expire_action :action => :edit
-  end
-
-  def edit; end
-
-end

1.5. SQL Caching

Query caching is a Rails feature that caches the result set returned by each -query so that if Rails encounters the same query again for that request, it -will used the cached result set as opposed to running the query against the -database again.

For example:

class ProductsController < ActionController
-
-  before_filter :authenticate, :only => [ :edit, :create ]
-  caches_page :list
-  caches_action :edit
-  cache_sweeper :store_sweeper, :only => [ :create ]
-
-  def list
-    # Run a find query
-    Product.find(:all)
-
-    ...
-
-    # Run the same query again
-    Product.find(:all)
-  end
-
-  def create
-    expire_page :action => :list
-    expire_action :action => :edit
-  end
-
-  def edit; end
-
-end

In the list action above, the result set returned by the first -Product.find(:all) will be cached and will be used to avoid querying the -database again the second time that finder is called.

Query caches are created at the start of an action and destroyed at the end of -that action and thus persist only for the duration of the action.

1.6. Cache stores

Rails provides different stores for the cached data for action and fragment -caches. Page caches are always stored on disk.

The cache stores provided include:

1) Memory store: Cached data is stored in the memory allocated to the Rails - process, which is fine for WEBrick and for FCGI (if you - don’t care that each FCGI process holds its own fragment - store). It’s not suitable for CGI as the process is thrown - away at the end of each request. It can potentially also - take up a lot of memory since each process keeps all the - caches in memory.

ActionController::Base.cache_store = :memory_store

2) File store: Cached data is stored on the disk, this is the default store - and the default path for this store is: /tmp/cache. Works - well for all types of environments and allows all processes - running from the same application directory to access the - cached content.

ActionController::Base.cache_store = :file_store, "/path/to/cache/directory"

3) DRb store: Cached data is stored in a separate shared DRb process that all - servers communicate with. This works for all environments and - only keeps one cache around for all processes, but requires - that you run and manage a separate DRb process.

ActionController::Base.cache_store = :drb_store, "druby://localhost:9192"

4) MemCached store: Works like DRbStore, but uses Danga’s MemCache instead. - Rails uses the bundled memcached-client gem by default.

ActionController::Base.cache_store = :mem_cache_store, "localhost"

5) Custom store: You can define your own cache store (new in Rails 2.1)

ActionController::Base.cache_store = MyOwnStore.new("parameter")

Note: config.cache_store can be used in place of -ActionController::Base.cache_store in your Rails::Initializer.run block in -environment.rb

2. Conditional GET support

Conditional GETs are a facility of the HTTP spec that provide a way for web -servers to tell browsers that the response to a GET request hasn’t changed -since the last request and can be safely pulled from the browser cache.

They work by using the HTTP_IF_NONE_MATCH and HTTP_IF_MODIFIED_SINCE headers to -pass back and forth both a unique content identifier and the timestamp of when -the content was last changed. If the browser makes a request where the content -identifier (etag) or last modified since timestamp matches the server’s version -then the server only needs to send back an empty response with a not modified -status.

It is the server’s (i.e. our) responsibility to look for a last modified -timestamp and the if-none-match header and determine whether or not to send -back the full response. With conditional-get support in rails this is a pretty -easy task:

class ProductsController < ApplicationController
-
-  def show
-    @product = Product.find(params[:id])
-
-    # If the request is stale according to the given timestamp and etag value
-    # (i.e. it needs to be processed again) then execute this block
-    if stale?(:last_modified => @product.updated_at.utc, :etag => @product)
-      respond_to do |wants|
-        # ... normal response processing
-      end
-    end
-
-    # If the request is fresh (i.e. it's not modified) then you don't need to do
-    # anything. The default render checks for this using the parameters
-    # used in the previous call to stale? and will automatically send a
-    # :not_modified.  So that's it, you're done.
-end

If you don’t have any special response processing and are using the default -rendering mechanism (i.e. you’re not using respond_to or calling render -yourself) then you’ve got an easy helper in fresh_when:

class ProductsController < ApplicationController
-
-  # This will automatically send back a :not_modified if the request is fresh,
-  # and will render the default template (product.*) if it's stale.
-
-  def show
-    @product = Product.find(params[:id])
-    fresh_when :last_modified => @product.published_at.utc, :etag => @article
-  end
-end

3. Advanced Caching

Along with the built-in mechanisms outlined above, a number of excellent -plugins exist to help with finer grained control over caching. These include -Chris Wanstrath’s excellent cache_fu plugin (more info here: -http://errtheblog.com/posts/57-kickin-ass-w-cachefu) and Evan Weaver’s -interlock plugin (more info here: -http://blog.evanweaver.com/articles/2007/12/13/better-rails-caching/). Both -of these plugins play nice with memcached and are a must-see for anyone -seriously considering optimizing their caching needs.

- -

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