How Rails handles status codes

Posted by Ilija Eftimov on May 27, 2016

Recently, I have been building an API as part of my day job. Rails is a great framework to build APIs in, and it has been a joy so far. When building the responses of the API, it’s paramount to understand what HTTP statuses you should utilize, which will in return help the consumers by providing more meaningful responses.

Sure, you could always have a status property in the response JSON, which will be a human-readable status code. But, I would like you to think about HTTP status codes as a nice card on a present, with your beautiful handwriting and best wishes to whom the gift goes to. HTTP status code don’t just add more semantical correctness, but they also speak about the nature of the response.

Rails and statuses

Ruby on Rails, being a great framework, adds a very nice layer of abstraction of the status codes, and allows the developers to easily use custom status codes in their responses. I am sure, at some point you have seen something like:

render json: @resource, status: 201 # Created

While this might be sometimes redundant, the render method in Rails allows you to specify HTTP status codes for the response. But also, being very developer friendly, Rails allows you to do the same with “human readable” status codes:

render json: @resource, status: :created

The two code samples are identical, they specify the HTTP 201 status code to the response. But, what are the available Rails status codes (as symbols) that you can use? And how does Rails actually do this?

Status codes

The HTTP status codes list is a quite stable and static list. Although sometimes status codes are added to the list, once you learn it, keeping up to date is rather easy. The latest addition to this list is HTTP 451 - Unavailable For Legal Reasons. You can see the RFC where it was proposed here.

So, how does Rails knows how to create all of these symbols, so we can use them in our render methods? Well, what’s very interesting is that Rails actually doesn’t do much about this. It relies on Rack, which does all of this magic. In an irb session, type:

>> Rack::Utils::HTTP_STATUS_CODES

{100=>"Continue",
 101=>"Switching Protocols",
 102=>"Processing",
 200=>"OK",
 201=>"Created",
 202=>"Accepted",
 203=>"Non-Authoritative Information",
 204=>"No Content",
 205=>"Reset Content",
 206=>"Partial Content",
 207=>"Multi-Status",
 208=>"Already Reported",
 226=>"IM Used",
 300=>"Multiple Choices",
 301=>"Moved Permanently",
 302=>"Found",
 303=>"See Other",
 304=>"Not Modified",
 305=>"Use Proxy",
 307=>"Temporary Redirect",
 308=>"Permanent Redirect",
 400=>"Bad Request",
 401=>"Unauthorized",
 402=>"Payment Required",
 403=>"Forbidden",
 404=>"Not Found",
 405=>"Method Not Allowed",
 406=>"Not Acceptable",
 407=>"Proxy Authentication Required",
 408=>"Request Timeout",
 409=>"Conflict",
 410=>"Gone",
 411=>"Length Required",
 412=>"Precondition Failed",
 413=>"Payload Too Large",
 414=>"URI Too Long",
 415=>"Unsupported Media Type",
 416=>"Range Not Satisfiable",
 417=>"Expectation Failed",
 422=>"Unprocessable Entity",
 423=>"Locked",
 424=>"Failed Dependency",
 426=>"Upgrade Required",
 428=>"Precondition Required",
 429=>"Too Many Requests",
 431=>"Request Header Fields Too Large",
 500=>"Internal Server Error",
 501=>"Not Implemented",
 502=>"Bad Gateway",
 503=>"Service Unavailable",
 504=>"Gateway Timeout",
 505=>"HTTP Version Not Supported",
 506=>"Variant Also Negotiates",
 507=>"Insufficient Storage",
 508=>"Loop Detected",
 510=>"Not Extended",
 511=>"Network Authentication Required"}

As you can see, the Rack::Utils::HTTP_STATUS_CODES is a Hash that has all of the HTTP status codes, with the corresponding messages. If you open the Rack::Utils documentation you can see how these are programmatically created, including the code that does the conversion. The status codes are pulled from this CSV file and are parsed using this piece of code:

ruby -ne 'm = /^(\d{3}),(?!Unassigned|\(Unused\))([^,]+)/.match($_) and puts "#{m[1]} => \x27#{m[2].strip}\x27,"'

Great. But, how do we get the symbolized versions of the status messages? Well, Rack does that for us as well. In an irb session, try this:

>> Rack::Utils::SYMBOL_TO_STATUS_CODE
{:continue=>100,
 :switching_protocols=>101,
 :processing=>102,
 :ok=>200,
 :created=>201,
 :accepted=>202,
 :non_authoritative_information=>203,
 :no_content=>204,
 :reset_content=>205,
 :partial_content=>206,
 :multi_status=>207,
 :already_reported=>208,
 :im_used=>226,
 :multiple_choices=>300,
 :moved_permanently=>301,
 :found=>302,
 :see_other=>303,
 :not_modified=>304,
 :use_proxy=>305,
 :temporary_redirect=>307,
 :permanent_redirect=>308,
 :bad_request=>400,
 :unauthorized=>401,
 :payment_required=>402,
 :forbidden=>403,
 :not_found=>404,
 :method_not_allowed=>405,
 :not_acceptable=>406,
 :proxy_authentication_required=>407,
 :request_timeout=>408,
 :conflict=>409,
 :gone=>410,
 :length_required=>411,
 :precondition_failed=>412,
 :payload_too_large=>413,
 :uri_too_long=>414,
 :unsupported_media_type=>415,
 :range_not_satisfiable=>416,
 :expectation_failed=>417,
 :unprocessable_entity=>422,
 :locked=>423,
 :failed_dependency=>424,
 :upgrade_required=>426,
 :precondition_required=>428,
 :too_many_requests=>429,
 :request_header_fields_too_large=>431,
 :internal_server_error=>500,
 :not_implemented=>501,
 :bad_gateway=>502,
 :service_unavailable=>503,
 :gateway_timeout=>504,
 :http_version_not_supported=>505,
 :variant_also_negotiates=>506,
 :insufficient_storage=>507,
 :loop_detected=>508,
 :not_extended=>510,
 :network_authentication_required=>511}

As you can see, the Rack::Utils::SYMBOL_TO_STATUS_CODE constant contains all of the status codes as symbols. The documentation also shows the actual conversion, from HTTP_STATUS_CODES to SYMBOL_TO_STATUS_CODE:

Hash[*HTTP_STATUS_CODES.map { |code, message|
  [message.downcase.gsub(/\s|-|'/, '_').to_sym, code]
}.flatten]

Having all of this documentation in place, is rather easy to see how everything falls into place. Rails being a Rack app, can utilize everything that Rack provides, and this is just an example. But, how does Rails actually plugs this into its runtime and has the ability to understand what is the status code?

A bit deeper

Rails’ source code, although very well documented and well structured, can still be overwhelming to someone that hasn’t spent enough time digging through. If you are a Ruby developer, I encourage you to spend some time around Rails’ code - you will learn a lot, I promise you!

So, how does Rails know what status code to apply to the response, if you provide just the symbolized version on the status? Well, let’s start with Rails’ class hierarchy, which is quite nice.

On the surface, or what we usually see as developers, the rendering is done in the controllers. If you open any of your applications’ ApplicationController, you will see something like:

class ApplicationController < ActionController::Base
  ...
end

This means that all of our controllers are subclasses of the ActionController::Base class. If you open the source code of the ActionController::Base class, you will see some very interesting things, that can be quite informative. But for our use case, we are interested in the following:

MODULES = [
  AbstractController::Rendering,
  AbstractController::Translation,
  AbstractController::AssetPaths,

  Helpers,
  UrlFor,
  Redirecting,
  ActionView::Layouts,
  Rendering,
  Renderers::All,
  ConditionalGet,
  EtagWithTemplateDigest,
  Caching,
  MimeResponds,
  ImplicitRender,
  StrongParameters,

  Cookies,
  Flash,
  FormBuilder,
  RequestForgeryProtection,
  ForceSSL,
  Streaming,
  DataStreaming,
  HttpAuthentication::Basic::ControllerMethods,
  HttpAuthentication::Digest::ControllerMethods,
  HttpAuthentication::Token::ControllerMethods,

  # Before callbacks should also be executed the earliest as possible, so
  # also include them at the bottom.
  AbstractController::Callbacks,

  # Append rescue at the bottom to wrap as much as possible.
  Rescue,

  # Add instrumentations hooks at the bottom, to ensure they instrument
  # all the methods properly.
  Instrumentation,

  # Params wrapper should come before instrumentation so they are
  # properly showed in logs
  ParamsWrapper
]

MODULES.each do |mod|
  include mod
end

This piece of code, actually includes all of these modules in the ActionController::Base class, which in return gives us all of the functionlities that our controllers have. Just look at the module names, like Redirecting, ActionView::Layouts, Rendering, Cookies, Flash and so on. Very self-explanatory, right?

Well, the classes that we are interested in are AbstractController::Rendering and ActionController::Rendering. The first one is an abstract class, kind of like an interface, and the second one is the default implementation class, which contains all of the rendering mechanisms that Rails uses to render your response. When we call render in our controllers, we are executing the ActionController::Rendering#render method, which does couple of things for us: it normalizes the arguments, the options and then builds the body of the response. When it normalizes the arguments, deep inside the _normalize_render method, it contains these lines of code:

if options[:status]
  options[:status] = Rack::Utils.status_code(options[:status])
end

This piece of code, takes the :status option from our render call, and translates the symbol to an actual HTTP code. If we go back to Rack::Utils documentation, we can see the actual implementation of the status_code method:

# File 'lib/rack/utils.rb', line 573

def status_code(status)
  if status.is_a?(Symbol)
    SYMBOL_TO_STATUS_CODE[status] || 500
  else
    status.to_i
  end
end

Here’s how your status: :created gets translated to a status: 201. As you can see, although Rails’ source is sometimes hard to digest, it’s very well done and navigating through it is not that hard.

What about exceptions?

So now we know how Rails does the expected rendering, which was created by us, developers. But, what happens when Rails hits an exception of some sort, but it still has to recover from it and return a proper HTTP status code to the client?

Although at first you might expect some sort of a rescue block in ActionController::Metal class, this is not the case. Open any Rails app you have on your computer, and run in it’s root directory:

➜  rake middleware

use Rack::Sendfile
use ActionDispatch::Static
use ActionDispatch::LoadInterlock
use ActiveSupport::Cache::Strategy::LocalCache::Middleware
use Rack::Runtime
use ActionDispatch::RequestId
use Rails::Rack::Logger
*use ActionDispatch::ShowExceptions*
use ActionDispatch::DebugExceptions
use ActionDispatch::RemoteIp
use ActionDispatch::Reloader
use ActionDispatch::Callbacks
use ActiveRecord::Migration::CheckPending
use ActiveRecord::ConnectionAdapters::ConnectionManagement
use ActiveRecord::QueryCache
use Rack::Head
use Rack::ConditionalGet
use Rack::ETag
use ActionView::Digestor::PerRequestDigestCacheExpiry
run YourApp::Application.routes

Actually, the exceptions are handled in Rails’ middleware, more specifically in the ActionDispatch::ShowExceptions middleware class. It wraps a group of exceptions and maps them to a specific status code. This is the code that does the mapping:

module ActionDispatch
  class ExceptionWrapper
    cattr_accessor :rescue_responses
    @@rescue_responses = Hash.new(:internal_server_error)
    @@rescue_responses.merge!(
      'ActionController::RoutingError'              => :not_found,
      'AbstractController::ActionNotFound'          => :not_found,
      'ActionController::MethodNotAllowed'          => :method_not_allowed,
      'ActionController::UnknownHttpMethod'         => :method_not_allowed,
      'ActionController::NotImplemented'            => :not_implemented,
      'ActionController::UnknownFormat'             => :not_acceptable,
      'ActionController::InvalidAuthenticityToken'  => :unprocessable_entity,
      'ActionController::InvalidCrossOriginRequest' => :unprocessable_entity,
      'ActionDispatch::ParamsParser::ParseError'    => :bad_request,
      'ActionController::BadRequest'                => :bad_request,
      'ActionController::ParameterMissing'          => :bad_request,
      'Rack::Utils::ParameterTypeError'             => :bad_request,
      'Rack::Utils::InvalidParameterError'          => :bad_request
    )
    ...
  end
end

All of the exceptions that can occur in the request/response lifecycle are mapped to a specific status code, which will be returned by the middleware if an exception is rescued. You can see this functionality in ActionDispatch::ShowExceptions#call method, which invokes the render_exception method if an Exception is rescued:

def call(env)
  request = ActionDispatch::Request.new env
  @app.call(env)
rescue Exception => exception
  if request.show_exceptions?
    render_exception(request, exception)
  else
    raise exception
  end
end
def render_exception(request, exception)
  backtrace_cleaner = request.get_header 'action_dispatch.backtrace_cleaner'
  wrapper = ExceptionWrapper.new(backtrace_cleaner, exception)
  status  = wrapper.status_code
  request.set_header "action_dispatch.exception", wrapper.exception
  request.set_header "action_dispatch.original_path", request.path_info
  request.path_info = "/#{status}"
  response = @exceptions_app.call(request.env)
  response[1]['X-Cascade'] == 'pass' ? pass_response(status) : response
rescue Exception => failsafe_error
  $stderr.puts "Error during failsafe response: #{failsafe_error}\n  #{failsafe_error.backtrace * "\n  "}"
  FAILSAFE_RESPONSE
end

Now, when the exception is being rendered, the ExceptionWrapper comes into the picture. It will wrap the exception, and return an appropriate status code, which is translated from the symbolized variant, to the number version, using:

def status_code
  self.class.status_code_for_exception(@exception.class.name)
end

The status_code method invokes the status_code_for_exception method, which converts the status code:

def self.status_code_for_exception(class_name)
  Rack::Utils.status_code(@@rescue_responses[class_name])
end

And that’s about it. After that, the middleware stack is being executed in order to return the new response, with the exception and the proper HTTP status code attached.

After all

Although all of this Rails “magic” might be a bit much to digest at first, Rails’ source code does quite a good job of explaining what is going on. And as you can see, there’s always something more than meets the eye. If you need to remember anything from this blogpost is that Rails does a very good (and interesting) job at handling HTTP status codes, to make your app’s responses semantically correct and client friendly.

If you got to this point - thanks so much for reading. I know it was a bit of a long journey, and I hope it was informative for you. Looking forward to reading your comments!


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