517 lines
19 KiB
Plaintext
517 lines
19 KiB
Plaintext
h2. Caching with Rails: An overview
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Everyone caches. This guide will teach you what you need to know about
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avoiding that expensive round-trip to your database and returning what you
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need to return to those hungry web clients in the shortest time possible.
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endprologue.
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h3. Basic Caching
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This is an introduction to the three types of caching techniques that Rails
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provides by default without the use of any third party plugins.
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To get started make sure +config.action_controller.perform_caching+ is set
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to +true+ for your environment. This flag is normally set in the
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corresponding config/environments/*.rb and caching is disabled by default
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there for development and test, and enabled for production.
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<ruby>
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config.action_controller.perform_caching = true
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</ruby>
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h4. Page Caching
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Page caching is a Rails mechanism which allows the request for a generated
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page to be fulfilled by the webserver, without ever having to go through the
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Rails stack at all. Obviously, this is super-fast. Unfortunately, it can't be
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applied to every situation (such as pages that need authentication) and since
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the webserver is literally just serving a file from the filesystem, cache
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expiration is an issue that needs to be dealt with.
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So, how do you enable this super-fast cache behavior? Simple, let's say you
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have a controller called ProductsController and a 'list' action that lists all
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the products
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<ruby>
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class ProductsController < ActionController
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caches_page :index
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def index; end
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end
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</ruby>
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The first time anyone requests products/index, Rails will generate a file
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called +index.html+ and the webserver will then look for that file before it
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passes the next request for products/index to your Rails application.
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By default, the page cache directory is set to Rails.public_path (which is
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usually set to +RAILS_ROOT + "/public"+) and this can be configured by
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changing the configuration setting +config.action_controller.page_cache_directory+.
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Changing the default from /public helps avoid naming conflicts, since you may
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want to put other static html in /public, but changing this will require web
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server reconfiguration to let the web server know where to serve the cached
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files from.
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The Page Caching mechanism will automatically add a +.html+ extension to
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requests for pages that do not have an extension to make it easy for the
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webserver to find those pages and this can be configured by changing the
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configuration setting +config.action_controller.page_cache_extension+.
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In order to expire this page when a new product is added we could extend our
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example controller like this:
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<ruby>
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class ProductsController < ActionController
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caches_page :list
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def list; end
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def create
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expire_page :action => :list
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end
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end
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</ruby>
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If you want a more complicated expiration scheme, you can use cache sweepers
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to expire cached objects when things change. This is covered in the section on Sweepers.
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Note: Page caching ignores all parameters, so /products/list?page=1 will be written out to the filesystem as /products/list.html and if someone requests /products/list?page=2, they will be returned the same result as page=1, so be careful when page caching GET parameters in the URL!
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h4. Action Caching
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One of the issues with Page Caching is that you cannot use it for pages that
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require to restrict access somehow. This is where Action Caching comes in.
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Action Caching works like Page Caching except for the fact that the incoming
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web request does go from the webserver to the Rails stack and Action Pack so
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that before filters can be run on it before the cache is served, so that
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authentication and other restrictions can be used while still serving the
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result of the output from a cached copy.
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Clearing the cache works in the exact same way as with Page Caching.
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Let's say you only wanted authenticated users to edit or create a Product
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object, but still cache those pages:
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<ruby>
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class ProductsController < ActionController
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before_filter :authenticate, :only => [ :edit, :create ]
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caches_page :list
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caches_action :edit
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def list; end
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def create
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expire_page :action => :list
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expire_action :action => :edit
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end
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def edit; end
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end
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</ruby>
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And you can also use +:if+ (or +:unless+) to pass a Proc that specifies when the
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action should be cached. Also, you can use +:layout => false+ to cache without
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layout so that dynamic information in the layout such as logged in user info
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or the number of items in the cart can be left uncached. This feature is
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available as of Rails 2.2.
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You can modify the default action cache path by passing a +:cache_path+ option.
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This will be passed directly to ActionCachePath.path_for. This is handy for
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actions with multiple possible routes that should be cached differently. If
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a block is given, it is called with the current controller instance.
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Finally, if you are using memcached, you can also pass +:expires_in+. In fact,
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all parameters not used by caches_action are sent to the underlying cache
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store.
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h4. Fragment Caching
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Life would be perfect if we could get away with caching the entire contents of
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a page or action and serving it out to the world. Unfortunately, dynamic web
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applications usually build pages with a variety of components not all of which
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have the same caching characteristics. In order to address such a dynamically
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created page where different parts of the page need to be cached and expired
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differently Rails provides a mechanism called Fragment Caching.
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Fragment Caching allows a fragment of view logic to be wrapped in a cache
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block and served out of the cache store when the next request comes in.
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As an example, if you wanted to show all the orders placed on your website
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in real time and didn't want to cache that part of the page, but did want
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to cache the part of the page which lists all products available, you
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could use this piece of code:
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<ruby>
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<% Order.find_recent.each do |o| %>
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<%= o.buyer.name %> bought <% o.product.name %>
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<% end %>
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<% cache do %>
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All available products:
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<% Product.find(:all).each do |p| %>
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<%= link_to p.name, product_url(p) %>
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<% end %>
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<% end %>
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</ruby>
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The cache block in our example will bind to the action that called it and is
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written out to the same place as the Action Cache, which means that if you
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want to cache multiple fragments per action, you should provide an +action_suffix+ to the cache call:
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<ruby>
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<% cache(:action => 'recent', :action_suffix => 'all_products') do %>
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All available products:
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</ruby>
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and you can expire it using the +expire_fragment+ method, like so:
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<ruby>
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expire_fragment(:controller => 'products', :action => 'recent', :action_suffix => 'all_products)
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</ruby>
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If you don't want the cache block to bind to the action that called it, You can
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also use globally keyed fragments by calling the cache method with a key, like
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so:
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<ruby>
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<% cache(:key => ['all_available_products', @latest_product.created_at].join(':')) do %>
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All available products:
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<% end %>
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</ruby>
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This fragment is then available to all actions in the ProductsController using
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the key and can be expired the same way:
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<ruby>
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expire_fragment(:key => ['all_available_products', @latest_product.created_at].join(':'))
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</ruby>
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h4. Sweepers
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Cache sweeping is a mechanism which allows you to get around having a ton of
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expire_{page,action,fragment} calls in your code by moving all the work
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required to expire cached content into a +ActionController::Caching::Sweeper+
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class that is an Observer and looks for changes to an object via callbacks,
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and when a change occurs it expires the caches associated with that object n
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an around or after filter.
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Continuing with our Product controller example, we could rewrite it with a
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sweeper such as the following:
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<ruby>
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class StoreSweeper < ActionController::Caching::Sweeper
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observe Product # This sweeper is going to keep an eye on the Product model
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# If our sweeper detects that a Product was created call this
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def after_create(product)
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expire_cache_for(product)
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end
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# If our sweeper detects that a Product was updated call this
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def after_update(product)
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expire_cache_for(product)
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end
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# If our sweeper detects that a Product was deleted call this
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def after_destroy(product)
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expire_cache_for(product)
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end
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private
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def expire_cache_for(record)
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# Expire the list page now that we added a new product
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expire_page(:controller => '#{record}', :action => 'list')
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# Expire a fragment
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expire_fragment(:controller => '#{record}', :action => 'recent', :action_suffix => 'all_products')
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end
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end
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</ruby>
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Then we add it to our controller to tell it to call the sweeper when certain
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actions are called. So, if we wanted to expire the cached content for the
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list and edit actions when the create action was called, we could do the
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following:
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<ruby>
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class ProductsController < ActionController
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before_filter :authenticate, :only => [ :edit, :create ]
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caches_page :list
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caches_action :edit
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cache_sweeper :store_sweeper, :only => [ :create ]
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def list; end
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def create
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expire_page :action => :list
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expire_action :action => :edit
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end
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def edit; end
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end
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</ruby>
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h4. SQL Caching
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Query caching is a Rails feature that caches the result set returned by each
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query so that if Rails encounters the same query again for that request, it
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will used the cached result set as opposed to running the query against the
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database again.
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For example:
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<ruby>
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class ProductsController < ActionController
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before_filter :authenticate, :only => [ :edit, :create ]
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caches_page :list
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caches_action :edit
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cache_sweeper :store_sweeper, :only => [ :create ]
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def list
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# Run a find query
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Product.find(:all)
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...
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# Run the same query again
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Product.find(:all)
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end
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def create
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expire_page :action => :list
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expire_action :action => :edit
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end
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def edit; end
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end
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</ruby>
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In the 'list' action above, the result set returned by the first
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Product.find(:all) will be cached and will be used to avoid querying the
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database again the second time that finder is called.
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Query caches are created at the start of an action and destroyed at the end of
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that action and thus persist only for the duration of the action.
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h4. Cache stores
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Rails (as of 2.1) provides different stores for the cached data for action and
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fragment caches. Page caches are always stored on disk.
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Rails 2.1 and above provide ActiveSupport::Cache::Store which can be used to
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cache strings. Some cache store implementations, like MemoryStore, are able to
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cache arbitrary Ruby objects, but don't count on every cache store to be able
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to do that.
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The default cache stores provided include:
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1) ActiveSupport::Cache::MemoryStore: A cache store implementation which stores
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everything into memory in the same process. If you're running multiple Ruby on
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Rails server processes (which is the case if you're using mongrel_cluster or
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Phusion Passenger), then this means that your Rails server process instances
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won't be able to share cache data with each other. If your application never
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performs manual cache item expiry (e.g. when you‘re using generational cache
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keys), then using +MemoryStore+ is ok. Otherwise, consider carefully whether you
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should be using this cache store.
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+MemoryStore+ is not only able to store strings, but also arbitrary Ruby objects.
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+MemoryStore+ is not thread-safe. Use +SynchronizedMemoryStore+ instead if you
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need thread-safety.
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<ruby>
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ActionController::Base.cache_store = :memory_store
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</ruby>
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2) ActiveSupport::Cache::FileStore: Cached data is stored on the disk, this is
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the default store and the default path for this store is: /tmp/cache. Works
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well for all types of environments and allows all processes running from the
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same application directory to access the cached content. If /tmp/cache does not
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exist, the default store becomes MemoryStore.
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<ruby>
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ActionController::Base.cache_store = :file_store, "/path/to/cache/directory"
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</ruby>
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3) ActiveSupport::Cache::DRbStore: Cached data is stored in a separate shared
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DRb process that all servers communicate with. This works for all environments
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and only keeps one cache around for all processes, but requires that you run
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and manage a separate DRb process.
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<ruby>
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ActionController::Base.cache_store = :drb_store, "druby://localhost:9192"
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</ruby>
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4) MemCached store: Works like DRbStore, but uses Danga's MemCache instead.
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Rails uses the bundled memcached-client gem by default. This is currently the
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most popular cache store for production websites.
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Special features:
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* Clustering and load balancing. One can specify multiple memcached servers,
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and MemCacheStore will load balance between all available servers. If a
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server goes down, then MemCacheStore will ignore it until it goes back
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online.
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* Time-based expiry support. See write and the +:expires_in+ option.
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* Per-request in memory cache for all communication with the MemCache server(s).
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It also accepts a hash of additional options:
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* +:namespace+- specifies a string that will automatically be prepended to keys when accessing the memcached store.
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* +:readonly+- a boolean value that when set to true will make the store read-only, with an error raised on any attempt to write.
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* +: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.
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The read and write methods of the MemCacheStore accept an options hash too.
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When reading you can specify +:raw => true+ to prevent the object being
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marshaled
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(by default this is false which means the raw value in the cache is passed to
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Marshal.load before being returned to you.)
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When writing to the cache it is also possible to specify +:raw => true+ means
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the value is not passed to Marshal.dump before being stored in the cache (by
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default this is false).
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The write method also accepts an +:unless_exist+ flag which determines whether
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the memcached add (when true) or set (when false) method is used to store the
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item in the cache and an +:expires_in+ option that specifies the time-to-live
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for the cached item in seconds.
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<ruby>
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ActionController::Base.cache_store = :mem_cache_store, "localhost"
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</ruby>
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5) ActiveSupport::Cache::SynchronizedMemoryStore: Like ActiveSupport::Cache::MemoryStore but thread-safe.
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<ruby>
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ActionController::Base.cache_store = :synchronized_memory_store
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</ruby>
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6) ActiveSupport::Cache::CompressedMemCacheStore: Works just like the regular
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MemCacheStore but uses GZip to decompress/compress on read/write.
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<ruby>
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ActionController::Base.cache_store = :compressed_mem_cache_store, "localhost"
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</ruby>
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7) Custom store: You can define your own cache store (new in Rails 2.1)
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<ruby>
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ActionController::Base.cache_store = MyOwnStore.new("parameter")
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</ruby>
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+Note: config.cache_store can be used in place of
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ActionController::Base.cache_store in your Rails::Initializer.run block in
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environment.rb+
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In addition to all of this, Rails also adds the ActiveRecord::Base#cache_key
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method that generates a key using the class name, id and updated_at timestamp
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(if available).
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An example:
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<ruby>
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Rails.cache.read("city") # => nil
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Rails.cache.write("city", "Duckburgh")
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Rails.cache.read("city") # => "Duckburgh"
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</ruby>
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h3. Conditional GET support
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Conditional GETs are a facility of the HTTP spec that provide a way for web
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servers to tell browsers that the response to a GET request hasn't changed
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since the last request and can be safely pulled from the browser cache.
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They work by using the HTTP_IF_NONE_MATCH and HTTP_IF_MODIFIED_SINCE headers to
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pass back and forth both a unique content identifier and the timestamp of when
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the content was last changed. If the browser makes a request where the content
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identifier (etag) or last modified since timestamp matches the server’s version
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then the server only needs to send back an empty response with a not modified
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status.
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It is the server's (i.e. our) responsibility to look for a last modified
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timestamp and the if-none-match header and determine whether or not to send
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back the full response. With conditional-get support in rails this is a pretty
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easy task:
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<ruby>
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class ProductsController < ApplicationController
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def show
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@product = Product.find(params[:id])
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# If the request is stale according to the given timestamp and etag value
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# (i.e. it needs to be processed again) then execute this block
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if stale?(:last_modified => @product.updated_at.utc, :etag => @product)
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respond_to do |wants|
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# ... normal response processing
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end
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end
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# If the request is fresh (i.e. it's not modified) then you don't need to do
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# anything. The default render checks for this using the parameters
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# used in the previous call to stale? and will automatically send a
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# :not_modified. So that's it, you're done.
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end
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</ruby>
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If you don't have any special response processing and are using the default
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rendering mechanism (i.e. you're not using respond_to or calling render
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yourself) then you’ve got an easy helper in fresh_when:
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<ruby>
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class ProductsController < ApplicationController
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# This will automatically send back a :not_modified if the request is fresh,
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# and will render the default template (product.*) if it's stale.
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def show
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@product = Product.find(params[:id])
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fresh_when :last_modified => @product.published_at.utc, :etag => @article
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end
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end
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</ruby>
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h3. Advanced Caching
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Along with the built-in mechanisms outlined above, a number of excellent
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plugins exist to help with finer grained control over caching. These include
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Chris Wanstrath's excellent cache_fu plugin (more info "here": http://errtheblog.com/posts/57-kickin-ass-w-cachefu) and Evan Weaver's
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interlock plugin (more info "here": http://blog.evanweaver.com/articles/2007/12/13/better-rails-caching/). Both
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of these plugins play nice with memcached and are a must-see for anyone
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seriously considering optimizing their caching needs.
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Also the new "Cache money":http://github.com/nkallen/cache-money/tree/master plugin is supposed to be mad cool.
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h3. References
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* "RailsEnvy, Rails Caching Tutorial, Part 1":http://www.railsenvy.com/2007/2/28/rails-caching-tutorial
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* "RailsEnvy, Rails Caching Tutorial, Part 1":http://www.railsenvy.com/2007/3/20/ruby-on-rails-caching-tutorial-part-2
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* "ActiveSupport::Cache documentation":http://api.rubyonrails.org/classes/ActiveSupport/Cache.html
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* "Rails 2.1 integrated caching tutorial":http://thewebfellas.com/blog/2008/6/9/rails-2-1-now-with-better-integrated-caching
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h3. Changelog
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"Lighthouse ticket":http://rails.lighthouseapp.com/projects/16213-rails-guides/tickets/10-guide-to-caching
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* February 22, 2009: Beefed up the section on cache_stores
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* December 27, 2008: Typo fixes
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* November 23, 2008: Incremental updates with various suggested changes and formatting cleanup
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* September 15, 2008: Initial version by Aditya Chadha
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