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diff --git a/guides/source/caching_with_rails.md b/guides/source/caching_with_rails.md index 773102400a..cbcd053950 100644 --- a/guides/source/caching_with_rails.md +++ b/guides/source/caching_with_rails.md @@ -5,8 +5,8 @@ This guide will teach you what you need to know about avoiding that expensive ro After reading this guide, you will know: -* Page, action, and fragment caching. -* Sweepers. +* Page and action caching (moved to separate gems as of Rails 4). +* Fragment caching. * Alternative cache stores. * Conditional GET support. @@ -28,103 +28,15 @@ config.action_controller.perform_caching = true ### 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. +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. -To enable page caching, you need to use the `caches_page` method. - -```ruby -class ProductsController < ActionController - - caches_page :index - - def index - @products = Product.all - end -end -``` - -Let's say you have a controller called `ProductsController` and an `index` action that lists all the products. 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.default_static_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 :index - - def index - @products = Product.all - end - - def create - expire_page action: :index - end - -end -``` - -By default, page caching automatically gzips files (for example, to `products.html.gz` if user requests `/products`) to reduce the size of data transmitted (web servers are typically configured to use a moderate compression ratio as a compromise, but since precompilation happens once, compression ratio is maximum). - -Nginx is able to serve compressed content directly from disk by enabling `gzip_static`: - -```nginx -location / { - gzip_static on; # to serve pre-gzipped version -} -``` - -You can disable gzipping by setting `:gzip` option to false (for example, if action returns image): - -```ruby -caches_page :image, gzip: false -``` - -Or, you can set custom gzip compression level (level names are taken from `Zlib` constants): - -```ruby -caches_page :image, gzip: :best_speed -``` - -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. A workaround for this limitation is to include the parameters in the page's path, e.g. `/products/page/1`. - -INFO: Page caching runs in an after filter. Thus, invalid requests won't generate spurious cache entries as long as you halt them. Typically, a redirection in some before filter that checks request preconditions does the job. +INFO: Page Caching has been removed from Rails 4. See the [actionpack-page_caching gem](https://github.com/rails/actionpack-page_caching). See [DHH's key-based cache expiration overview](http://signalvnoise.com/posts/3113-how-key-based-cache-expiration-works) for the newly-preferred method. ### Action Caching Page Caching cannot be used for actions that have before filters - for example, pages that require authentication. This is where Action Caching comes in. Action Caching works like Page Caching except the incoming web request hits the Rails stack so that before filters can be run on it before the cache is served. This allows authentication and other restrictions to be run while still serving the result of the output from a cached copy. -Clearing the cache works in a similar way to Page Caching, except you use `expire_action` instead of `expire_page`. - -Let's say you only wanted authenticated users to call actions on `ProductsController`. - -```ruby -class ProductsController < ActionController - - before_action :authenticate - caches_action :index - - def index - @products = Product.all - end - - def create - expire_action action: :index - end - -end -``` - -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 or Ehcache, you can also pass `:expires_in`. In fact, all parameters not used by `caches_action` are sent to the underlying cache store. - -INFO: Action caching runs in an after filter. Thus, invalid requests won't generate spurious cache entries as long as you halt them. Typically, a redirection in some before filter that checks request preconditions does the job. +INFO: Action Caching has been removed from Rails 4. See the [actionpack-action_caching gem](https://github.com/rails/actionpack-action_caching). See [DHH's key-based cache expiration overview](http://signalvnoise.com/posts/3113-how-key-based-cache-expiration-works) for the newly-preferred method. ### Fragment Caching @@ -173,6 +85,80 @@ This fragment is then available to all actions in the `ProductsController` using ```ruby expire_fragment('all_available_products') ``` +If you want to avoid expiring the fragment manually, whenever an action updates a product, you can define a helper method: + +```ruby +module ProductsHelper + def cache_key_for_products + count = Product.count + max_updated_at = Product.maximum(:updated_at).try(:utc).try(:to_s, :number) + "products/all-#{count}-#{max_updated_at}" + end +end +``` + +This method generates a cache key that depends on all products and can be used in the view: + +```erb +<% cache(cache_key_for_products) do %> + All available products: +<% end %> +``` + +If you want to cache a fragment under certain condition you can use `cache_if` or `cache_unless` + +```erb +<% cache_if (condition, cache_key_for_products) do %> + All available products: +<% end %> +``` + +You can also use an Active Record model as the cache key: + +```erb +<% Product.all.each do |p| %> + <% cache(p) do %> + <%= link_to p.name, product_url(p) %> + <% end %> +<% end %> +``` + +Behind the scenes, a method called `cache_key` will be invoked on the model and it returns a string like `products/23-20130109142513`. The cache key includes the model name, the id and finally the updated_at timestamp. Thus it will automatically generate a new fragment when the product is updated because the key changes. + +You can also combine the two schemes which is called "Russian Doll Caching": + +```erb +<% cache(cache_key_for_products) do %> + All available products: + <% Product.all.each do |p| %> + <% cache(p) do %> + <%= link_to p.name, product_url(p) %> + <% end %> + <% end %> +<% end %> +``` + +It's called "Russian Doll Caching" because it nests multiple fragments. The advantage is that if a single product is updated, all the other inner fragments can be reused when regenerating the outer fragment. + +### Low-Level Caching + +Sometimes you need to cache a particular value or query result, instead of caching view fragments. Rails caching mechanism works great for storing __any__ kind of information. + +The most efficient way to implement low-level caching is using the `Rails.cache.fetch` method. This method does both reading and writing to the cache. When passed only a single argument, the key is fetched and value from the cache is returned. If a block is passed, the result of the block will be cached to the given key and the result is returned. + +Consider the following example. An application has a `Product` model with an instance method that looks up the product’s price on a competing website. The data returned by this method would be perfect for low-level caching: + +```ruby +class Product < ActiveRecord::Base + def competing_price + Rails.cache.fetch("#{cache_key}/competing_price", expires_in: 12.hours) do + Competitor::API.find_price(id) + end + end +end +``` + +NOTE: Notice that in this example we used `cache_key` method, so the resulting cache-key will be something like `products/233-20140225082222765838000/competing_price`. `cache_key` generates a string based on the model’s `id` and `updated_at` attributes. This is a common convention and has the benefit of invalidating the cache whenever the product is updated. In general, when you use low-level caching for instance level information, you need to generate a cache key. ### SQL Caching @@ -181,7 +167,7 @@ Query caching is a Rails feature that caches the result set returned by each que For example: ```ruby -class ProductsController < ActionController +class ProductsController < ApplicationController def index # Run a find query @@ -196,14 +182,10 @@ class ProductsController < ActionController end ``` -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. - Cache Stores ------------ -Rails provides different stores for the cached data created by <b>action</b> and <b>fragment</b> caches. +Rails provides different stores for the cached data created by **action** and **fragment** caches. TIP: Page caches are always stored on disk. @@ -227,7 +209,7 @@ The main methods to call are `read`, `write`, `delete`, `exist?`, and `fetch`. T There are some common options used by all cache implementations. These can be passed to the constructor or the various methods to interact with entries. -* `:namespace` - This option can be used to create a namespace within the cache store. It is especially useful if your application shares a cache with other applications. The default value will include the application name and Rails environment. +* `:namespace` - This option can be used to create a namespace within the cache store. It is especially useful if your application shares a cache with other applications. * `:compress` - This option can be used to indicate that compression should be used in the cache. This can be useful for transferring large cache entries over a slow network. @@ -263,7 +245,7 @@ This is the default cache store implementation. ### ActiveSupport::Cache::MemCacheStore -This cache store uses Danga's `memcached` server to provide a centralized cache for your application. Rails uses the bundled `dalli` gem by default. This is currently the most popular cache store for production websites. It can be used to provide a single, shared cache cluster with very a high performance and redundancy. +This cache store uses Danga's `memcached` server to provide a centralized cache for your application. Rails uses the bundled `dalli` gem by default. This is currently the most popular cache store for production websites. It can be used to provide a single, shared cache cluster with very high performance and redundancy. When initializing the cache, you need to specify the addresses for all memcached servers in your cluster. If none is specified, it will assume memcached is running on the local host on the default port, but this is not an ideal set up for larger sites. @@ -283,7 +265,7 @@ config.cache_store = :ehcache_store When initializing the cache, you may use the `:ehcache_config` option to specify the Ehcache config file to use (where the default is "ehcache.xml" in your Rails config directory), and the :cache_name option to provide a custom name for your cache (the default is rails_cache). -In addition to the standard `:expires_in` option, the `write` method on this cache can also accept the additional `:unless_exist` option, which will cause the cache store to use Ehcache's `putIfAbsent` method instead of `put`, and therefore will not overwrite an existing entry. Additionally, the `write` method supports all of the properties exposed by the [Ehcache Element class](http://ehcache.org/apidocs/net/sf/ehcache/Element.html) , including: +In addition to the standard `:expires_in` option, the `write` method on this cache can also accept the additional `:unless_exist` option, which will cause the cache store to use Ehcache's `putIfAbsent` method instead of `put`, and therefore will not overwrite an existing entry. Additionally, the `write` method supports all of the properties exposed by the [Ehcache Element class](http://ehcache.org/apidocs/net/sf/ehcache/Element.html) , including: | Property | Argument Type | Description | | --------------------------- | ------------------- | ----------------------------------------------------------- | @@ -339,7 +321,7 @@ 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. +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: @@ -365,18 +347,23 @@ class ProductsController < ApplicationController end ``` -Instead of a options hash, you can also simply pass in a model, Rails will use the `updated_at` and `cache_key` methods for setting `last_modified` and `etag`: +Instead of an options hash, you can also simply pass in a model, Rails will use the `updated_at` and `cache_key` methods for setting `last_modified` and `etag`: ```ruby class ProductsController < ApplicationController def show @product = Product.find(params[:id]) - respond_with(@product) if stale?(@product) + + if stale?(@product) + respond_to do |wants| + # ... normal response processing + end + end end 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: +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 @@ -390,8 +377,3 @@ class ProductsController < ApplicationController end end ``` - -Further reading ---------------- - -* [Scaling Rails Screencasts](http://railslab.newrelic.com/scaling-rails) |