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 Base.perform_caching is set to true for your environment.

Base.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 ProductController and a list action that lists all the products

class ProductController < ActionController

  cache_page :list

  def list; end

end

The first time anyone requestsion products/list, Rails will generate a file called list.html and the webserver will then look for that file before it passes the next request for products/list 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 Base.cache_public_directory

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 Base.page_cache_extension

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

class ProductController < ActionController

  cache_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 ProductController < ActionController

  before_filter :authenticate, :only => [ :edit, :create ]
  cache_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_path 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 => 'producst', :action => 'recent', :action_suffix => 'all_products)

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 Post model

  # If our sweeper detects that a Post was created call this
  def after_create(product)
          expire_cache_for(product)
  end

  # If our sweeper detects that a Post was updated call this
  def after_update(product)
          expire_cache_for(product)
  end

  # If our sweeper detects that a Post 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 ProductController < ActionController

  before_filter :authenticate, :only => [ :edit, :create ]
  cache_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 ProductController < ActionController

  before_filter :authenticate, :only => [ :edit, :create ]
  cache_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. Requires the ruby-memcache library: gem install ruby-memcache.

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")

2. 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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