**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON http://guides.rubyonrails.org.**


Using Rails for API-only Applications
=====================================

In this guide you will learn:

* What Rails provides for API-only applications
* How to configure Rails to start without any browser features
* How to decide which middlewares you will want to include
* How to decide which modules to use in your controller

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

What is an API app?
-------------------

Traditionally, when people said that they used Rails as an "API", they meant
providing a programmatically accessible API alongside their web application.
For example, GitHub provides [an API](http://developer.github.com) that you
can use from your own custom clients.

With the advent of client-side frameworks, more developers are using Rails to
build a back-end that is shared between their web application and other native
applications.

For example, Twitter uses its [public API](https://dev.twitter.com) in its web
application, which is built as a static site that consumes JSON resources.

Instead of using Rails to generate dynamic HTML that will communicate with the
server through forms and links, many developers are treating their web application
as just another client, delivered as static HTML, CSS and JavaScript  consuming
a simple JSON API.

This guide covers building a Rails application that serves JSON resources to an
API client **or** a client-side framework.

Why use Rails for JSON APIs?
----------------------------

The first question a lot of people have when thinking about building a JSON API
using Rails is: "isn't using Rails to spit out some JSON overkill? Shouldn't I
just use something like Sinatra?".

For very simple APIs, this may be true. However, even in very HTML-heavy
applications, most of an application's logic is actually outside of the view
layer.

The reason most people use Rails is that it provides a set of defaults that
allows us to get up and running quickly without having to make a lot of trivial
decisions.

Let's take a look at some of the things that Rails provides out of the box that are
still applicable to API applications.

Handled at the middleware layer:

- Reloading: Rails applications support transparent reloading. This works even if
  your application gets big and restarting the server for every request becomes
  non-viable.
- Development Mode: Rails applications come with smart defaults for development,
  making development pleasant without compromising production-time performance.
- Test Mode: Ditto development mode.
- Logging: Rails applications log every request, with a level of verbosity
  appropriate for the current mode. Rails logs in development include information
  about the request environment, database queries, and basic performance
  information.
- Security: Rails detects and thwarts [IP spoofing
  attacks](http://en.wikipedia.org/wiki/IP_address_spoofing) and handles
  cryptographic signatures in a [timing
  attack](http://en.wikipedia.org/wiki/Timing_attack) aware way. Don't know what
  an IP spoofing attack or a timing attack is? Exactly.
- Parameter Parsing: Want to specify your parameters as JSON instead of as a
  URL-encoded String? No problem. Rails will decode the JSON for you and make
  it available in `params`. Want to use nested URL-encoded parameters? That
  works too.
- Conditional GETs: Rails handles conditional `GET`, (`ETag` and `Last-Modified`),
  processing request headers and returning the correct response headers and status
  code. All you need to do is use the
  [`stale?`](http://api.rubyonrails.org/classes/ActionController/ConditionalGet.html#method-i-stale-3F)
  check in your controller, and Rails will handle all of the HTTP details for you.
- Caching: If you use `dirty?` with public cache control, Rails will automatically
  cache your responses. You can easily configure the cache store.
- HEAD requests: Rails will transparently convert `HEAD` requests into `GET` ones,
  and return just the headers on the way out. This makes `HEAD` work reliably in
  all Rails APIs.

While you could obviously build these up in terms of existing Rack middlewares,
this list demonstrates that the default Rails middleware stack provides a lot
of value, even if you're "just generating JSON".

Handled at the Action Pack layer:

- Resourceful Routing: If you're building a RESTful JSON API, you want to be
  using the Rails router. Clean and conventional mapping from HTTP to controllers
  means not having to spend time thinking about how to model your API in terms
  of HTTP.
- URL Generation: The flip side of routing is URL generation. A good API based
  on HTTP includes URLs (see [the GitHub gist API](http://developer.github.com/v3/gists/)
  for an example).
- Header and Redirection Responses: `head :no_content` and
  `redirect_to user_url(current_user)` come in handy. Sure, you could manually
  add the response headers, but why?
- Caching: Rails provides page, action and fragment caching. Fragment caching
  is especially helpful when building up a nested JSON object.
- Basic, Digest and Token Authentication: Rails comes with out-of-the-box support
  for three kinds of HTTP authentication.
- Instrumentation: Rails has an instrumentation API that will trigger registered
  handlers for a variety of events, such as action processing, sending a file or
  data, redirection, and database queries. The payload of each event comes with
  relevant information (for the action processing event, the payload includes
  the controller, action, parameters, request format, request method and the
  request's full path).
- Generators: This may be passé for advanced Rails users, but it can be nice to
  generate a resource and get your model, controller, test stubs, and routes
  created for you in a single command.
- Plugins: Many third-party libraries come with support for Rails that reduce
  or eliminate the cost of setting up and gluing together the library and the
  web framework. This includes things like overriding default generators, adding
  rake tasks, and honoring Rails choices (like the logger and cache back-end).

Of course, the Rails boot process also glues together all registered components.
For example, the Rails boot process is what uses your `config/database.yml` file
when configuring Active Record.

**The short version is**: you may not have thought about which parts of Rails
are still applicable even if you remove the view layer, but the answer turns out
to be "most of it".

The Basic Configuration
-----------------------

If you're building a Rails application that will be an API server first and
foremost, you can start with a more limited subset of Rails and add in features
as needed.

You can generate a new api Rails app:

```bash
$ rails new my_api --api
```

This will do three main things for you:

- Configure your application to start with a more limited set of middlewares
  than normal. Specifically, it will not include any middleware primarily useful
  for browser applications (like cookies support) by default.
- Make `ApplicationController` inherit from `ActionController::API` instead of
  `ActionController::Base`. As with middlewares, this will leave out any Action
  Controller modules that provide functionalities primarily used by browser
  applications.
- Configure the generators to skip generating views, helpers and assets when
  you generate a new resource.

If you want to take an existing application and make it an API one, read the
following steps.

In `config/application.rb` add the following line at the top of the `Application`
class definition:

```ruby
config.api_only = true
```

Optionally, in `config/environments/development.rb` add the following line
to render error responses using the API format (JSON by default) when it
is a local request:

```ruby
config.debug_exception_response_format = :api
```

Finally, inside `app/controllers/application_controller.rb`, instead of:

```ruby
class ApplicationController < ActionController::Base
end
```

do:

```ruby
class ApplicationController < ActionController::API
end
```

Choosing Middlewares
--------------------

An API application comes with the following middlewares by default:

- `Rack::Sendfile`
- `ActionDispatch::Static`
- `Rack::Lock`
- `ActiveSupport::Cache::Strategy::LocalCache::Middleware`
- `ActionDispatch::RequestId`
- `Rails::Rack::Logger`
- `Rack::Runtime`
- `ActionDispatch::ShowExceptions`
- `ActionDispatch::DebugExceptions`
- `ActionDispatch::RemoteIp`
- `ActionDispatch::Reloader`
- `ActionDispatch::Callbacks`
- `Rack::Head`
- `Rack::ConditionalGet`
- `Rack::ETag`

See the [internal middlewares](rails_on_rack.html#internal-middleware-stack)
section of the Rack guide for further information on them.

Other plugins, including Active Record, may add additional middlewares. In
general, these middlewares are agnostic to the type of application you are
building, and make sense in an API-only Rails application.

You can get a list of all middlewares in your application via:

```bash
$ rails middleware
```

### Using the Cache Middleware

By default, Rails will add a middleware that provides a cache store based on
the configuration of your application (memcache by default). This means that
the built-in HTTP cache will rely on it.

For instance, using the `stale?` method:

```ruby
def show
  @post = Post.find(params[:id])

  if stale?(last_modified: @post.updated_at)
    render json: @post
  end
end
```

The call to `stale?` will compare the `If-Modified-Since` header in the request
with `@post.updated_at`. If the header is newer than the last modified, this
action will return a "304 Not Modified" response. Otherwise, it will render the
response and include a `Last-Modified` header in it.

Normally, this mechanism is used on a per-client basis. The cache middleware
allows us to share this caching mechanism across clients. We can enable
cross-client caching in the call to `stale?`:

```ruby
def show
  @post = Post.find(params[:id])

  if stale?(last_modified: @post.updated_at, public: true)
    render json: @post
  end
end
```

This means that the cache middleware will store off the `Last-Modified` value
for a URL in the Rails cache, and add an `If-Modified-Since` header to any
subsequent inbound requests for the same URL.

Think of it as page caching using HTTP semantics.

NOTE: This middleware is always outside of the `Rack::Lock` mutex, even in
single-threaded applications.

### Using Rack::Sendfile

When you use the `send_file` method inside a Rails controller, it sets the
`X-Sendfile` header. `Rack::Sendfile` is responsible for actually sending the
file.

If your front-end server supports accelerated file sending, `Rack::Sendfile`
will offload the actual file sending work to the front-end server.

You can configure the name of the header that your front-end server uses for
this purpose using `config.action_dispatch.x_sendfile_header` in the appropriate
environment's configuration file.

You can learn more about how to use `Rack::Sendfile` with popular
front-ends in [the Rack::Sendfile
documentation](http://rubydoc.info/github/rack/rack/master/Rack/Sendfile).

Here are some values for popular servers, once they are configured, to support
accelerated file sending:

```ruby
# Apache and lighttpd
config.action_dispatch.x_sendfile_header = "X-Sendfile"

# Nginx
config.action_dispatch.x_sendfile_header = "X-Accel-Redirect"
```

Make sure to configure your server to support these options following the
instructions in the `Rack::Sendfile` documentation.

NOTE: The `Rack::Sendfile` middleware is always outside of the `Rack::Lock`
mutex, even in single-threaded applications.

### Using ActionDispatch::Request

`ActionDispatch::Request#params` will take parameters from the client in the JSON
format and make them available in your controller inside `params`.

To use this, your client will need to make a request with JSON-encoded parameters
and specify the `Content-Type` as `application/json`.

Here's an example in jQuery:

```javascript
jQuery.ajax({
  type: 'POST',
  url: '/people',
  dataType: 'json',
  contentType: 'application/json',
  data: JSON.stringify({ person: { firstName: "Yehuda", lastName: "Katz" } }),
  success: function(json) { }
});
```

`ActionDispatch::Request` will see the `Content-Type` and your parameters
will be:

```ruby
{ :person => { :firstName => "Yehuda", :lastName => "Katz" } }
```

### Other Middlewares

Rails ships with a number of other middlewares that you might want to use in an
API application, especially if one of your API clients is the browser:

- `Rack::MethodOverride`
- `ActionDispatch::Cookies`
- `ActionDispatch::Flash`
- For sessions management
    * `ActionDispatch::Session::CacheStore`
    * `ActionDispatch::Session::CookieStore`
    * `ActionDispatch::Session::MemCacheStore`

Any of these middlewares can be added via:

```ruby
config.middleware.use Rack::MethodOverride
```

### Removing Middlewares

If you don't want to use a middleware that is included by default in the API-only
middleware set, you can remove it with:

```ruby
config.middleware.delete ::Rack::Sendfile
```

Keep in mind that removing these middlewares will remove support for certain
features in Action Controller.

Choosing Controller Modules
---------------------------

An API application (using `ActionController::API`) comes with the following
controller modules by default:

- `ActionController::UrlFor`: Makes `url_for` and friends available.
- `ActionController::Redirecting`: Support for `redirect_to`.
- `ActionController::Rendering`: Basic support for rendering.
- `ActionController::Renderers::All`: Support for `render :json` and friends.
- `ActionController::ConditionalGet`: Support for `stale?`.
- `ActionController::ForceSSL`: Support for `force_ssl`.
- `ActionController::DataStreaming`: Support for `send_file` and `send_data`.
- `AbstractController::Callbacks`: Support for `before_action` and friends.
- `ActionController::Instrumentation`: Support for the instrumentation
  hooks defined by Action Controller (see [the instrumentation
  guide](active_support_instrumentation.html#action-controller)).
- `ActionController::Rescue`: Support for `rescue_from`.
- `ActionController::BasicImplicitRender`: Makes sure to return an empty response
  if there's not an explicit one.
- `ActionController::StrongParameters`: Support for parameters white-listing in
  combination with Active Model mass assignment.
- `ActionController::ParamsWrapper`: Wraps the parameters hash into a nested hash
  so you don't have to specify root elements sending POST requests for instance.

Other plugins may add additional modules. You can get a list of all modules
included into `ActionController::API` in the rails console:

```bash
$ bin/rails c
>> ActionController::API.ancestors - ActionController::Metal.ancestors
```

### Adding Other Modules

All Action Controller modules know about their dependent modules, so you can feel
free to include any modules into your controllers, and all dependencies will be
included and set up as well.

Some common modules you might want to add:

- `AbstractController::Translation`: Support for the `l` and `t` localization
  and translation methods.
- `ActionController::HttpAuthentication::Basic` (or `Digest` or `Token`): Support
  for basic, digest or token HTTP authentication.
- `AbstractController::Layouts`: Support for layouts when rendering.
- `ActionController::MimeResponds`: Support for `respond_to`.
- `ActionController::Cookies`: Support for `cookies`, which includes
  support for signed and encrypted cookies. This requires the cookies middleware.

The best place to add a module is in your `ApplicationController` but you can
also add modules to individual controllers.
