h2. 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.

After reading this guide, you should be able to use and configure:

* Page, action, and fragment caching
* Sweepers
* Alternative cache stores
* Conditional GET support

endprologue.

h3. 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 start playing with testing you'll want to ensure that
+config.action_controller.perform_caching+ is set
to +true+ if you're running in development mode. This flag is normally set in the
corresponding config/environments/*.rb and caching is disabled by default
 for development and test, and enabled for production.

<ruby>
config.action_controller.perform_caching = true
</ruby>

h4. Page Caching

Page caching is a Rails mechanism which allows the request for a generated
page to be fulfilled by the webserver (i.e. apache or nginx), 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 an +index+ action that lists all
the products

<ruby>
class ProductsController < ActionController

  caches_page :list

  def list
    @products = Products.all
  end

end
</ruby>

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

By default, the page cache directory is set to +Rails.public_path+ (which is
usually set to the +public+ folder) 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+ extension 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 controller like this:

<ruby>
class ProductsController < ActionController

  caches_page :list

  def list
    @products = Products.all
  end

  def create
    expire_page :action => :list
  end

end
</ruby>

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.

Note: Page caching ignores all parameters. For example +/products?page=1+ will be written out to the filesystem as +products.html+ with no reference to the +page+ parameter. Thus, if someone requests +/products?page=2+ later, they will get the cached first page. Be careful when page caching GET parameters in the URL!

h4. 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. This allows
authentication and other restriction to be run 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 call actions on +ProductsController+.

<ruby>
class ProductsController < ActionController

  before_filter :authenticate
  caches_action :index

  def index
    @products = Product.all
  end

  def create
    expire_action :action => :index
  end

end
</ruby>

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.

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.  

Finally, if you are using memcached, you can also pass +:expires_in+. In fact,
all parameters not used by +caches_action+ are sent to the underlying cache
store. 

h4. 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:

<ruby>
<% Order.find_recent.each do |o| %>
  <%= o.buyer.name %> bought <% o.product.name %>
<% end %>

<% cache do %>
  All available products:
  <% Product.all.each do |p| %>
    <%= link_to p.name, product_url(p) %>
  <% end %>
<% end %>
</ruby>

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:

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

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

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

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:

<ruby>
<% cache('all_available_products') do %>
  All available products:
<% end %>
</ruby>

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

<ruby>
expire_fragment('all_available_products')
</ruby>

h4. Sweepers

Cache sweeping is a mechanism which allows you to get around having a ton of
+expire_{page,action,fragment}+ calls in your code.  It does this by moving all the work
required to expire cached content into a +ActionController::Caching::Sweeper+
class.  This class 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 in
an around or after filter.

Continuing with our Product controller example, we could rewrite it with a
sweeper like this:

<ruby>
class ProductSweeper < 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(product)
    # Expire the index page now that we added a new product
    expire_page(:controller => 'products', :action => 'index')

    # Expire a fragment
    expire_fragment('all_available_products')
  end
end
</ruby>

You may notice that the actual product gets passed to the sweeper, so if we
were caching the edit action for each product, we could add a expire method
which specifies the page we want to expire:

<ruby>
  expire_action(:controller => 'products', :action => 'edit', :id => product)
</ruby>

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:

<ruby>
class ProductsController < ActionController

  before_filter :authenticate
  caches_action :index
  cache_sweeper :product_sweeper

  def index
    @products = Product.all
  end

end
</ruby>

h4. 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 use the cached result set as opposed to running the query against the
database again.

For example:

<ruby>
class ProductsController < ActionController

  def index
    # Run a find query
    @products = Product.all

    ...

    # Run the same query again
    @products = Product.all
  end

end
</ruby>

The second time the same query is run against the database, it's not actually
going to hit the database.  The first time the result is returned from the query
it is stored in the query cache (in memory) and the second time it's pulled from memory.

However, it's important to note that 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.  If you'd like to store query results in a more
persistent fashion, you can in Rails by using low level caching.

h4. Cache stores

Rails (as of 2.1) provides different stores for the cached data created by action and
fragment caches. Page caches are always stored on disk.

Rails 2.1 and above provide +ActiveSupport::Cache::Store+ which can be used to
cache strings. Some cache store implementations, like MemoryStore, are able to
cache arbitrary Ruby objects, but don't count on every cache store to be able
to do that.

The default cache stores provided with Rails include:

1) ActiveSupport::Cache::MemoryStore: A cache store implementation which stores
everything into memory in the same process. If you're running multiple Ruby on
Rails server processes (which is the case if you're using mongrel_cluster or
Phusion Passenger), then this means that your Rails server process instances
won't be able to share cache data with each other. If your application never
performs manual cache item expiry (e.g. when you‘re using generational cache
keys), then using +MemoryStore+ is ok. Otherwise, consider carefully whether you
should be using this cache store.  

+MemoryStore+ is not only able to store strings, but also arbitrary Ruby objects.

+MemoryStore+ is not thread-safe. Use +SynchronizedMemoryStore+ instead if you
need thread-safety.
                                      

<ruby>
ActionController::Base.cache_store = :memory_store
</ruby>

2) ActiveSupport::Cache::FileStore: 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. If /tmp/cache does not
exist, the default store becomes MemoryStore.


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

3) ActiveSupport::Cache::DRbStore: 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.


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

4) MemCached store: Works like DRbStore, but uses Danga's MemCache instead.
Rails uses the bundled memcached-client gem by default. This is currently the
most popular cache store for production websites.

Special features:
 * Clustering and load balancing. One can specify multiple memcached servers,
   and MemCacheStore will load balance between all available servers. If a
   server goes down, then MemCacheStore will ignore it until it goes back
   online.
 * Time-based expiry support. See +write+ and the +:expires_in+ option.
 * Per-request in memory cache for all communication with the MemCache server(s).

It also accepts a hash of additional options:

 * +:namespace+- specifies a string that will automatically be prepended to keys when accessing the memcached store.
 * +:readonly+- a boolean value that when set to true will make the store read-only, with an error raised on any attempt to write.
 * +:multithread+ - a boolean value that adds thread safety to read/write operations - it is unlikely you'll need to use this option as the Rails threadsafe! method offers the same functionality.

The read and write methods of the MemCacheStore accept an options hash too.
When reading you can specify +:raw => true+ to prevent the object being marshaled
(by default this is false which means the raw value in the cache is passed to
+Marshal.load+ before being returned to you.)

When writing to the cache it is also possible to specify +:raw => true+ means
the value is not passed to +Marshal.dump+ before being stored in the cache (by
default this is false). 

The write method also accepts an +:unless_exist+ flag which determines whether
the memcached add (when true) or set (when false) method is used to store the
item in the cache and an +:expires_in+ option that specifies the time-to-live
for the cached item in seconds.


<ruby>
ActionController::Base.cache_store = :mem_cache_store, "localhost"
</ruby>

5) ActiveSupport::Cache::SynchronizedMemoryStore: Like ActiveSupport::Cache::MemoryStore but thread-safe.


<ruby>
ActionController::Base.cache_store = :synchronized_memory_store
</ruby>

6) ActiveSupport::Cache::CompressedMemCacheStore: Works just like the regular
MemCacheStore but uses GZip to decompress/compress on read/write.


<ruby>
ActionController::Base.cache_store = :compressed_mem_cache_store, "localhost"
</ruby>

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


<ruby>
ActionController::Base.cache_store = MyOwnStore.new("parameter")
</ruby>

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

In addition to all of this, Rails also adds the +ActiveRecord::Base#cache_key+
method that generates a key using the class name, +id+ and +updated_at+ timestamp (if available).

You can access these cache stores at a low level for storing queries and other objects. Here's an example:

<ruby>
Rails.cache.read("city")   # => nil
Rails.cache.write("city", "Duckburgh")
Rails.cache.read("city")   # => "Duckburgh"
</ruby>

h3. Conditional GET support

Conditional GETs are a feature of the HTTP specification 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:

<ruby>
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
</ruby>

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:

<ruby>
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 => @product
  end
end
</ruby>

h3. 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.

Also the new "Cache money":http://github.com/nkallen/cache-money/tree/master plugin is supposed to be mad cool. 

h3. References

* "Scaling Rails Screencasts":http://railslab.newrelic.com/scaling-rails
* "RailsEnvy, Rails Caching Tutorial, Part 1":http://www.railsenvy.com/2007/2/28/rails-caching-tutorial
* "RailsEnvy, Rails Caching Tutorial, Part 2":http://www.railsenvy.com/2007/3/20/ruby-on-rails-caching-tutorial-part-2
* "ActiveSupport::Cache documentation":http://api.rubyonrails.org/classes/ActiveSupport/Cache.html
* "Rails 2.1 integrated caching tutorial":http://thewebfellas.com/blog/2008/6/9/rails-2-1-now-with-better-integrated-caching


h3. Changelog
"Lighthouse ticket":http://rails.lighthouseapp.com/projects/16213-rails-guides/tickets/10-guide-to-caching

April     26, 2009: Clean up typos in submitted patch
April      1, 2009: Made a bunch of small fixes
February  22, 2009: Beefed up the section on cache_stores
December  27, 2008: Typo fixes
November  23, 2008: Incremental updates with various suggested changes and formatting cleanup
September 15, 2008: Initial version by Aditya Chadha