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diff --git a/railties/guides/source/caching_with_rails.textile b/railties/guides/source/caching_with_rails.textile new file mode 100644 index 0000000000..940466ecb3 --- /dev/null +++ b/railties/guides/source/caching_with_rails.textile @@ -0,0 +1,429 @@ +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. + +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 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. + +<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, 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 + +<ruby> +class ProductsController < ActionController + + caches_page :index + + def index; end + +end +</ruby> + +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: + +<ruby> +class ProductsController < ActionController + + caches_page :list + + def list; 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. + +[More: caching paginated results? more examples? Walk-through of page caching?] + +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, 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: + +<ruby> +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 +</ruby> + +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?] + +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.find(: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(:key => ['all_available_products', @latest_product.created_at].join(':')) 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(:key => ['all_available_products', @latest_product.created_at].join(':')) +</ruby> + +[More: more examples? description of fragment keys and expiration, etc? pagination?] + +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 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: + +<ruby> +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 +</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, :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 +</ruby> + +[More: more examples? better sweepers?] + +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 used the cached result set as opposed to running the query against the +database again. + +For example: + +<ruby> +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 +</ruby> + +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. + +h4. 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. + +<ruby> +ActionController::Base.cache_store = :memory_store +</ruby> + +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. + + +<ruby> +ActionController::Base.cache_store = :file_store, "/path/to/cache/directory" +</ruby> + +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. + +<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. + +<ruby> +ActionController::Base.cache_store = :mem_cache_store, "localhost" +</ruby> + +5) 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+ + +h3. 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: + +<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 => @article + 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. |