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Caching with Rails: An Overview

This guide is an introduction to speeding up your Rails application with caching.

Caching means to store content generated during the request-response cycle and to reuse it when responding to similar requests.

Caching is often the most effective way to boost an application's performance. Through caching, web sites running on a single server with a single database can sustain a load of thousands of concurrent users.

Rails provides a set of caching features out of the box. This guide will teach you the scope and purpose of each one of them. Master these techniques and your Rails applications can serve millions of views without exorbitant response times or server bills.

After reading this guide, you will know:

1 Basic Caching

This is an introduction to three types of caching techniques: page, action and fragment caching. By default Rails provides fragment caching. In order to use page and action caching you will need to add actionpack-page_caching and actionpack-action_caching to your Gemfile.

By default, caching is only enabled in your production environment. To play around with caching locally you'll want to enable caching in your local environment by setting config.action_controller.perform_caching to true in the relevant config/environments/*.rb file:

config.action_controller.perform_caching = true

Changing the value of config.action_controller.perform_caching will only have an effect on the caching provided by the Action Controller component. For instance, it will not impact low-level caching, that we address below.

1.1 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 having to go through the entire Rails stack. While this is super fast it can't be applied to every situation (such as pages that need authentication). Also, because the webserver is serving a file directly from the filesystem you will need to implement cache expiration.

Page Caching has been removed from Rails 4. See the actionpack-page_caching gem.

1.2 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.

Action Caching has been removed from Rails 4. See the actionpack-action_caching gem. See DHH's key-based cache expiration overview for the newly-preferred method.

1.3 Fragment Caching

Dynamic web applications usually build pages with a variety of components not all of which have the same caching characteristics. When different parts of the page need to be cached and expired separately you can use 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.

For example, if you wanted to cache each product on a page, you could use this code:

<% @products.each do |product| %>
  <% cache product do %>
    <%= render product %>
  <% end %>
<% end %>

When your application receives its first request to this page, Rails will write a new cache entry with a unique key. A key looks something like this:

views/products/1-201505056193031061005000/bea67108094918eeba42cd4a6e786901

The number in the middle is the product_id followed by the timestamp value in the updated_at attribute of the product record. Rails uses the timestamp value to make sure it is not serving stale data. If the value of updated_at has changed, a new key will be generated. Then Rails will write a new cache to that key, and the old cache written to the old key will never be used again. This is called key-based expiration.

Cache fragments will also be expired when the view fragment changes (e.g., the HTML in the view changes). The string of characters at the end of the key is a template tree digest. It is an md5 hash computed based on the contents of the view fragment you are caching. If you change the view fragment, the md5 hash will change, expiring the existing file.

Cache stores like Memcached will automatically delete old cache files.

If you want to cache a fragment under certain conditions, you can use cache_if or cache_unless:

<% cache_if admin?, product do %>
  <%= render product %>
<% end %>

1.3.1 Collection caching

The render helper can also cache individual templates rendered for a collection. It can even one up the previous example with each by reading all cache templates at once instead of one by one. This is done automatically if the template rendered by the collection includes a cache call. Take a collection that renders a products/_product.html.erb partial for each element:

render products

If products/_product.html.erb starts with a cache call like so:

<% cache product do %>
  <%= product.name %>
<% end %>

All the cached templates from previous renders will be fetched at once with much greater speed. There's more info on how to make your templates eligible for collection caching.

1.4 Russian Doll Caching

You may want to nest cached fragments inside other cached fragments. This is called Russian doll caching.

The advantage of Russian doll caching is that if a single product is updated, all the other inner fragments can be reused when regenerating the outer fragment.

As explained in the previous section, a cached file will expire if the value of updated_at changes for a record on which the cached file directly depends. However, this will not expire any cache the fragment is nested within.

For example, take the following view:

<% cache product do %>
  <%= render product.games %>
<% end %>

Which in turn renders this view:

<% cache game do %>
  <%= render game %>
<% end %>

If any attribute of game is changed, the updated_at value will be set to the current time, thereby expiring the cache. However, because updated_at will not be changed for the product object, that cache will not be expired and your app will serve stale data. To fix this, we tie the models together with the touch method:

class Product < ActiveRecord::Base
  has_many :games
end

class Game < ActiveRecord::Base
  belongs_to :product, touch: true
end

With touch set to true, any action which changes updated_at for a game record will also change it for the associated product, thereby expiring the cache.

1.5 Managing dependencies

In order to correctly invalidate the cache, you need to properly define the caching dependencies. Rails is clever enough to handle common cases so you don't have to specify anything. However, sometimes, when you're dealing with custom helpers for instance, you need to explicitly define them.

1.5.1 Implicit dependencies

Most template dependencies can be derived from calls to render in the template itself. Here are some examples of render calls that ActionView::Digestor knows how to decode:

render partial: "comments/comment", collection: commentable.comments
render "comments/comments"
render 'comments/comments'
render('comments/comments')

render "header" => render("comments/header")

render(@topic)         => render("topics/topic")
render(topics)         => render("topics/topic")
render(message.topics) => render("topics/topic")

On the other hand, some calls need to be changed to make caching work properly. For instance, if you're passing a custom collection, you'll need to change:

render @project.documents.where(published: true)

to:

render partial: "documents/document", collection: @project.documents.where(published: true)

1.5.2 Explicit dependencies

Sometimes you'll have template dependencies that can't be derived at all. This is typically the case when rendering happens in helpers. Here's an example:

<%= render_sortable_todolists @project.todolists %>

You'll need to use a special comment format to call those out:

<%# Template Dependency: todolists/todolist %>
<%= render_sortable_todolists @project.todolists %>

In some cases, like a single table inheritance setup, you might have a bunch of explicit dependencies. Instead of writing every template out, you can use a wildcard to match any template in a directory:

<%# Template Dependency: events/* %>
<%= render_categorizable_events @person.events %>

As for collection caching, if the partial template doesn't start with a clean cache call, you can still benefit from collection caching by adding a special comment format anywhere in the template, like:

<%# Template Collection: notification %>
<% my_helper_that_calls_cache(some_arg, notification) do %>
  <%= notification.name %>
<% end %>

1.5.3 External dependencies

If you use a helper method, for example, inside a cached block and you then update that helper, you'll have to bump the cache as well. It doesn't really matter how you do it, but the md5 of the template file must change. One recommendation is to simply be explicit in a comment, like:

<%# Helper Dependency Updated: Jul 28, 2015 at 7pm %>
<%= some_helper_method(person) %>

1.6 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:

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

Notice that in this example we used the 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.

1.7 SQL Caching

Query caching is a Rails feature that caches the result set returned by each query. 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:

class ProductsController < ApplicationController

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

    ...

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

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 with low level caching.

2 Cache Stores

Rails provides different stores for the cached data (apart from SQL and page caching).

2.1 Configuration

You can set up your application's default cache store by setting the config.cache_store configuration option. Other parameters can be passed as arguments to the cache store's constructor:

config.cache_store = :memory_store, { size: 64.megabytes }

Alternatively, you can call ActionController::Base.cache_store outside of a configuration block.

You can access the cache by calling Rails.cache.

2.2 ActiveSupport::Cache::Store

This class provides the foundation for interacting with the cache in Rails. This is an abstract class and you cannot use it on its own. Rather you must use a concrete implementation of the class tied to a storage engine. Rails ships with several implementations documented below.

The main methods to call are read, write, delete, exist?, and fetch. The fetch method takes a block and will either return an existing value from the cache, or evaluate the block and write the result to the cache if no value exists.

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.

  • :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.

  • :compress_threshold - This option is used in conjunction with the :compress option to indicate a threshold under which cache entries should not be compressed. This defaults to 16 kilobytes.

  • :expires_in - This option sets an expiration time in seconds for the cache entry when it will be automatically removed from the cache.

  • :race_condition_ttl - This option is used in conjunction with the :expires_in option. It will prevent race conditions when cache entries expire by preventing multiple processes from simultaneously regenerating the same entry (also known as the dog pile effect). This option sets the number of seconds that an expired entry can be reused while a new value is being regenerated. It's a good practice to set this value if you use the :expires_in option.

2.2.1 Custom Cache Stores

You can create your own custom cache store by simply extending ActiveSupport::Cache::Store and implementing the appropriate methods. This way, you can swap in any number of caching technologies into your Rails application.

To use a custom cache store, simply set the cache store to a new instance of your custom class.

config.cache_store = MyCacheStore.new

2.3 ActiveSupport::Cache::MemoryStore

This cache store keeps entries in memory in the same Ruby process. The cache store has a bounded size specified by sending the :size option to the initializer (default is 32Mb). When the cache exceeds the allotted size, a cleanup will occur and the least recently used entries will be removed.

config.cache_store = :memory_store, { size: 64.megabytes }

If you're running multiple Ruby on Rails server processes (which is the case if you're using mongrel_cluster or Phusion Passenger), then your Rails server process instances won't be able to share cache data with each other. This cache store is not appropriate for large application deployments. However, it can work well for small, low traffic sites with only a couple of server processes, as well as development and test environments.

2.4 ActiveSupport::Cache::FileStore

This cache store uses the file system to store entries. The path to the directory where the store files will be stored must be specified when initializing the cache.

config.cache_store = :file_store, "/path/to/cache/directory"

With this cache store, multiple server processes on the same host can share a cache. The cache store is appropriate for low to medium traffic sites that are served off one or two hosts. Server processes running on different hosts could share a cache by using a shared file system, but that setup is not recommended.

As the cache will grow until the disk is full, it is recommended to periodically clear out old entries.

This is the default cache store implementation.

2.5 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 high performance and redundancy.

When initializing the cache, you need to specify the addresses for all memcached servers in your cluster. If none are specified, it will assume memcached is running on localhost on the default port, but this is not an ideal setup for larger sites.

The write and fetch methods on this cache accept two additional options that take advantage of features specific to memcached. You can specify :raw to send a value directly to the server with no serialization. The value must be a string or number. You can use memcached direct operations like increment and decrement only on raw values. You can also specify :unless_exist if you don't want memcached to overwrite an existing entry.

config.cache_store = :mem_cache_store, "cache-1.example.com", "cache-2.example.com"

2.6 ActiveSupport::Cache::NullStore

This cache store implementation is meant to be used only in development or test environments and it never stores anything. This can be very useful in development when you have code that interacts directly with Rails.cache but caching may interfere with being able to see the results of code changes. With this cache store, all fetch and read operations will result in a miss.

config.cache_store = :null_store

3 Cache Keys

The keys used in a cache can be any object that responds to either cache_key or to_param. You can implement the cache_key method on your classes if you need to generate custom keys. Active Record will generate keys based on the class name and record id.

You can use Hashes and Arrays of values as cache keys.

# This is a legal cache key
Rails.cache.read(site: "mysite", owners: [owner_1, owner_2])

The keys you use on Rails.cache will not be the same as those actually used with the storage engine. They may be modified with a namespace or altered to fit technology backend constraints. This means, for instance, that you can't save values with Rails.cache and then try to pull them out with the dalli gem. However, you also don't need to worry about exceeding the memcached size limit or violating syntax rules.

4 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:

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.cache_key)
      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
end

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:

class ProductsController < ApplicationController
  def show
    @product = Product.find(params[:id])

    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:

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

5 References

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