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Using Rails for API-only Applications

In this guide you will learn:

1 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 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 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.

2 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 and handles cryptographic signatures in a 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? 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 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".

3 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:

$ 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:

config.api_only = true

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

class ApplicationController < ActionController::Base


class ApplicationController < ActionController::API

4 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 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:

$ rake middleware

4.1 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:

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

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

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?:

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

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

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.

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

4.2 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.

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

# 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.

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

4.3 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:

  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:

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

4.4 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:

config.middleware.use Rack::MethodOverride

4.5 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:

config.middleware.delete ::Rack::Sendfile

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

5 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).
  • 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:

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

5.1 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.


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