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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:

* Fragment and Russian doll caching.
* How to manage the caching dependencies.
* Alternative cache stores.
* Conditional GET support.

--------------------------------------------------------------------------------

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:

```ruby
config.action_controller.perform_caching = true
```

NOTE: 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](#low-level-caching).

### 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.

INFO: Page Caching has been removed from Rails 4. See the [actionpack-page_caching gem](https://github.com/rails/actionpack-page_caching).

### 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.

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

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:

```html+erb
<% @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.

TIP: 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`:

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

#### 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 by passing `cached: true` when rendering the collection:

```html+erb
<%= render partial: 'products/product', collection: @products, cached: true %>
```

All cached templates from previous renders will be fetched at once with much
greater speed. Additionally, the templates that haven't yet been cached will be
written to cache and multi fetched on the next render.


### 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:

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

Which in turn renders this view:

```erb
<% 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:

```ruby
class Product < ApplicationRecord
  has_many :games
end

class Game < ApplicationRecord
  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.

### 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.

#### 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:

```ruby
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:

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

to:

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

#### 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:

```html+erb
<%= render_sortable_todolists @project.todolists %>
```

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

```html+erb
<%# 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:

```html+erb
<%# 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:

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

#### 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:

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

### 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 < ApplicationRecord
  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 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.

### 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:

```ruby
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.

Cache Stores
------------

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

### 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:

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

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

You can access the cache by calling `Rails.cache`.

### 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.

#### 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.

```ruby
config.cache_store = MyCacheStore.new
```

### 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.

```ruby
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.

### 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.

```ruby
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.

### 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.

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

### 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.

```ruby
config.cache_store = :null_store
```

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.

```ruby
# 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.

Conditional GET support
-----------------------

Conditional GETs are a feature of the HTTP specification that provide a way for web servers to tell browsers that the response to a GET request hasn't changed since the last request and can be safely pulled from the browser cache.

They work by using the `HTTP_IF_NONE_MATCH` and `HTTP_IF_MODIFIED_SINCE` headers to pass back and forth both a unique content identifier and the timestamp of when the content was last changed. If the browser makes a request where the content identifier (etag) or last modified since timestamp matches the server's version then the server only needs to send back an empty response with a not modified status.

It is the server's (i.e. our) responsibility to look for a last modified timestamp and the if-none-match header and determine whether or not to send back the full response. With conditional-get support in Rails this is a pretty easy task:

```ruby
class ProductsController < ApplicationController

  def show
    @product = Product.find(params[:id])

    # If the request is stale according to the given timestamp and etag value
    # (i.e. it needs to be processed again) then execute this block
    if stale?(last_modified: @product.updated_at.utc, etag: @product.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`:

```ruby
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`:

```ruby
class ProductsController < ApplicationController

  # This will automatically send back a :not_modified if the request is fresh,
  # and will render the default template (product.*) if it's stale.

  def show
    @product = Product.find(params[:id])
    fresh_when last_modified: @product.published_at.utc, etag: @product
  end
end
```

### Strong v/s Weak ETags

Rails generates weak ETags by default. Weak ETags allow semantically equivalent
responses to have the same ETags, even if their bodies do not match exactly.
This is useful when we don't want the page to be regenerated for minor changes in
response body.

Weak ETags have a leading `W/` to differentiate them from strong ETags.

```
  W/"618bbc92e2d35ea1945008b42799b0e7" → Weak ETag
  "618bbc92e2d35ea1945008b42799b0e7" → Strong ETag
```

Unlike weak ETag, strong ETag implies that response should be exactly the same
and byte by byte identical. Useful when doing Range requests within a
large video or PDF file. Some CDNs support only strong ETags, like Akamai.
If you absolutely need to generate a strong ETag, it can be done as follows.

```ruby
  class ProductsController < ApplicationController
    def show
      @product = Product.find(params[:id])
      fresh_when last_modified: @product.published_at.utc, strong_etag: @product
    end
  end
```

You can also set the strong ETag directly on the response.

```ruby
  response.strong_etag = response.body # => "618bbc92e2d35ea1945008b42799b0e7"
```

References
----------

* [DHH's article on key-based expiration](https://signalvnoise.com/posts/3113-how-key-based-cache-expiration-works)
* [Ryan Bates' Railscast on cache digests](http://railscasts.com/episodes/387-cache-digests)
