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The Rails Initialization Process

This guide explains the internals of the initialization process in Rails as of Rails 4. It is an extremely in-depth guide and recommended for advanced Rails developers.

After reading this guide, you will know:

This guide goes through every method call that is required to boot up the Ruby on Rails stack for a default Rails 4 application, explaining each part in detail along the way. For this guide, we will be focusing on what happens when you execute +rails server+ to boot your app.

Paths in this guide are relative to Rails or a Rails application unless otherwise specified.

If you want to follow along while browsing the Rails source code, we recommend that you use the t key binding to open the file finder inside GitHub and find files quickly.

1 Launch!

Now we finally boot and initialize the app. It all starts with your app's bin/rails executable. A Rails application is usually started by running rails console or rails server.

1.1 bin/rails

This file is as follows:

#!/usr/bin/env ruby
APP_PATH = File.expand_path('../../config/application',  __FILE__)
require File.expand_path('../../config/boot',  __FILE__)
require 'rails/commands'

The APP_PATH constant will be used later in rails/commands. The config/boot file referenced here is the config/boot.rb file in our application which is responsible for loading Bundler and setting it up.

1.2 config/boot.rb

config/boot.rb contains:

# Set up gems listed in the Gemfile.
ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile', __FILE__)

require 'bundler/setup' if File.exist?(ENV['BUNDLE_GEMFILE'])

In a standard Rails application, there's a Gemfile which declares all dependencies of the application. config/boot.rb sets ENV['BUNDLE_GEMFILE'] to the location of this file. If the Gemfile exists, bundler/setup is then required.

A standard Rails application depends on several gems, specifically:

  • abstract
  • actionmailer
  • actionpack
  • activemodel
  • activerecord
  • activesupport
  • arel
  • builder
  • bundler
  • erubis
  • i18n
  • mail
  • mime-types
  • polyglot
  • rack
  • rack-cache
  • rack-mount
  • rack-test
  • rails
  • railties
  • rake
  • sqlite3-ruby
  • thor
  • treetop
  • tzinfo

1.3 rails/commands.rb

Once config/boot.rb has finished, the next file that is required is rails/commands which will execute a command based on the arguments passed in. In this case, the ARGV array simply contains server which is extracted into the command variable using these lines:

ARGV << '--help' if ARGV.empty?

aliases = {
  "g"  => "generate",
  "d"  => "destroy",
  "c"  => "console",
  "s"  => "server",
  "db" => "dbconsole",
  "r"  => "runner"

command = ARGV.shift
command = aliases[command] || command

As you can see, an empty ARGV list will make Rails show the help snippet.

If we used s rather than server, Rails will use the aliases defined in the file and match them to their respective commands. With the server command, Rails will run this code:

when 'server'
  # Change to the application's path if there is no file in current dir.
  # This allows us to run `rails server` from other directories, but still get
  # the main and properly set the tmp directory.
  Dir.chdir(File.expand_path('../../', APP_PATH)) unless File.exist?(File.expand_path(""))

  require 'rails/commands/server' do |server|
    # We need to require application after the server sets environment,
    # otherwise the --environment option given to the server won't propagate.
    require APP_PATH

This file will change into the Rails root directory (a path two directories up from APP_PATH which points at config/application.rb), but only if the file isn't found. This then requires rails/commands/server which sets up the Rails::Server class.

require 'fileutils'
require 'optparse'
require 'action_dispatch'

module Rails
  class Server < ::Rack::Server

fileutils and optparse are standard Ruby libraries which provide helper functions for working with files and parsing options.

1.4 actionpack/lib/action_dispatch.rb

Action Dispatch is the routing component of the Rails framework. It adds functionality like routing, session, and common middlewares.

1.5 rails/commands/server.rb

The Rails::Server class is defined in this file by inheriting from Rack::Server. When is called, this calls the initialize method in rails/commands/server.rb:

def initialize(*)

Firstly, super is called which calls the initialize method on Rack::Server.

1.6 Rack: lib/rack/server.rb

Rack::Server is responsible for providing a common server interface for all Rack-based applications, which Rails is now a part of.

The initialize method in Rack::Server simply sets a couple of variables:

def initialize(options = nil)
  @options = options
  @app = options[:app] if options && options[:app]

In this case, options will be nil so nothing happens in this method.

After super has finished in Rack::Server, we jump back to rails/commands/server.rb. At this point, set_environment is called within the context of the Rails::Server object and this method doesn't appear to do much at first glance:

def set_environment
  ENV["RAILS_ENV"] ||= options[:environment]

In fact, the options method here does quite a lot. This method is defined in Rack::Server like this:

def options
  @options ||= parse_options(ARGV)

Then parse_options is defined like this:

def parse_options(args)
  options = default_options

  # Don't evaluate CGI ISINDEX parameters.
  args.clear if ENV.include?("REQUEST_METHOD")

  options.merge! opt_parser.parse! args
  options[:config] = ::File.expand_path(options[:config])
  ENV["RACK_ENV"] = options[:environment]

With the default_options set to this:

def default_options
    :environment => ENV['RACK_ENV'] || "development",
    :pid         => nil,
    :Port        => 9292,
    :Host        => "",
    :AccessLog   => [],
    :config      => ""

There is no REQUEST_METHOD key in ENV so we can skip over that line. The next line merges in the options from opt_parser which is defined plainly in Rack::Server

def opt_parser

The class is defined in Rack::Server, but is overwritten in Rails::Server to take different arguments. Its parse! method begins like this:

def parse!(args)
  args, options = args.dup, {}

  opt_parser = do |opts|
    opts.banner = "Usage: rails server [mongrel, thin, etc] [options]"
    opts.on("-p", "--port=port", Integer,
            "Runs Rails on the specified port.", "Default: 3000") { |v| options[:Port] = v }

This method will set up keys for the options which Rails will then be able to use to determine how its server should run. After initialize has finished, we jump back into rails/server where APP_PATH (which was set earlier) is required.

1.7 config/application

When require APP_PATH is executed, config/application.rb is loaded. This file exists in your app and it's free for you to change based on your needs.

1.8 Rails::Server#start

After config/application is loaded, server.start is called. This method is defined like this:

def start
  url = "#{options[:SSLEnable] ? 'https' : 'http'}://#{options[:Host]}:#{options[:Port]}"
  puts "=> Booting #{ActiveSupport::Inflector.demodulize(server)}"
  puts "=> Rails #{Rails.version} application starting in #{Rails.env} on #{url}"
  puts "=> Run `rails server -h` for more startup options"
  trap(:INT) { exit }
  puts "=> Ctrl-C to shutdown server" unless options[:daemonize]

  #Create required tmp directories if not found
  %w(cache pids sessions sockets).each do |dir_to_make|
    FileUtils.mkdir_p(Rails.root.join('tmp', dir_to_make))

  unless options[:daemonize]
    wrapped_app # touch the app so the logger is set up

    console =$stdout)
    console.formatter = Rails.logger.formatter


  # The '-h' option calls exit before @options is set.
  # If we call 'options' with it unset, we get double help banners.
  puts 'Exiting' unless @options && options[:daemonize]

This is where the first output of the Rails initialization happens. This method creates a trap for INT signals, so if you CTRL-C the server, it will exit the process. As we can see from the code here, it will create the tmp/cache, tmp/pids, tmp/sessions and tmp/sockets directories. It then calls wrapped_app which is responsible for creating the Rack app, before creating and assigning an instance of ActiveSupport::Logger.

The super method will call Rack::Server.start which begins its definition like this:

def start &blk
  if options[:warn]
    $-w = true

  if includes = options[:include]

  if library = options[:require]
    require library

  if options[:debug]
    $DEBUG = true
    require 'pp'
    p options[:server]
    pp wrapped_app
    pp app

  check_pid! if options[:pid]

  # Touch the wrapped app, so that the is loaded before
  # daemonization (i.e. before chdir, etc).

  daemonize_app if options[:daemonize]

  write_pid if options[:pid]

  trap(:INT) do
    if server.respond_to?(:shutdown)
  end wrapped_app, options, &blk

The interesting part for a Rails app is the last line, Here we encounter the wrapped_app method again, which this time we're going to explore more (even though it was executed before, and thus memorized by now).

@wrapped_app ||= build_app app

The app method here is defined like so:

def app
  @app ||= begin
    if !::File.exist? options[:config]
      abort "configuration #{options[:config]} not found"

    app, options = Rack::Builder.parse_file(self.options[:config], opt_parser)
    self.options.merge! options

The options[:config] value defaults to which contains this:

# This file is used by Rack-based servers to start the application.

require ::File.expand_path('../config/environment',  __FILE__)
run <%= app_const %>

The Rack::Builder.parse_file method here takes the content from this file and parses it using this code:

app = eval " {( " + cfgfile + "\n )}.to_app",

The initialize method of Rack::Builder will take the block here and execute it within an instance of Rack::Builder. This is where the majority of the initialization process of Rails happens. The require line for config/environment.rb in is the first to run:

require ::File.expand_path('../config/environment',  __FILE__)

1.9 config/environment.rb

This file is the common file required by (rails server) and Passenger. This is where these two ways to run the server meet; everything before this point has been Rack and Rails setup.

This file begins with requiring config/application.rb.

1.10 config/application.rb

This file requires config/boot.rb, but only if it hasn't been required before, which would be the case in rails server but wouldn't be the case with Passenger.

Then the fun begins!

2 Loading Rails

The next line in config/application.rb is:

require 'rails/all'

2.1 railties/lib/rails/all.rb

This file is responsible for requiring all the individual frameworks of Rails:

require "rails"

).each do |framework|
    require "#{framework}/railtie"
  rescue LoadError

This is where all the Rails frameworks are loaded and thus made available to the application. We won't go into detail of what happens inside each of those frameworks, but you're encouraged to try and explore them on your own.

For now, just keep in mind that common functionality like Rails engines, I18n and Rails configuration are all being defined here.

2.2 Back to config/environment.rb

When config/application.rb has finished loading Rails, and defined the application namespace, we go back to config/environment.rb, where the application is initialized. For example, if the application was called Blog, here we would find Blog::Application.initialize!, which is defined in rails/application.rb

2.3 railties/lib/rails/application.rb

The initialize! method looks like this:

def initialize!(group=:default) #:nodoc:
  raise "Application has been already initialized." if @initialized
  run_initializers(group, self)
  @initialized = true

As you can see, you can only initialize an app once. This is also where the initializers are run.

review this

The initializers code itself is tricky. What Rails is doing here is it traverses all the class ancestors looking for an initializers method, sorting them and running them. For example, the Engine class will make all the engines available by providing the initializers method.

After this is done we go back to Rack::Server

2.4 Rack: lib/rack/server.rb

Last time we left when the app method was being defined:

def app
  @app ||= begin
    if !::File.exist? options[:config]
      abort "configuration #{options[:config]} not found"

    app, options = Rack::Builder.parse_file(self.options[:config], opt_parser)
    self.options.merge! options

At this point app is the Rails app itself (a middleware), and what happens next is Rack will call all the provided middlewares:

def build_app(app)
  middleware[options[:environment]].reverse_each do |middleware|
    middleware = if middleware.respond_to?(:call)
    next unless middleware
    klass = middleware.shift
    app =, *middleware)

Remember, build_app was called (by wrapped_app) in the last line of Server#start. Here's how it looked like when we left: wrapped_app, options, &blk

At this point, the implementation of will depend on the server you're using. For example, if you were using Mongrel, here's what the run method would look like:

def, options={})
  server =
    options[:Host]           || '',
    options[:Port]           || 8080,
    options[:num_processors] || 950,
    options[:throttle]       || 0,
    options[:timeout]        || 60)
  # Acts like Rack::URLMap, utilizing Mongrel's own path finding methods.
  # Use is similar to #run, replacing the app argument with a hash of
  # { path=>app, ... } or an instance of Rack::URLMap.
  if options[:map]
    if app.is_a? Hash
      app.each do |path, appl|
        path = '/'+path unless path[0] == ?/
    elsif app.is_a? URLMap
      app.instance_variable_get(:@mapping).each do |(host, path, appl)|
       next if !host.nil? && !options[:Host].nil? && options[:Host] != host
       path = '/'+path unless path[0] == ?/
      raise ArgumentError, "first argument should be a Hash or URLMap"
  yield server  if block_given?

We won't dig into the server configuration itself, but this is the last piece of our journey in the Rails initialization process.

This high level overview will help you understand when your code is executed and how, and overall become a better Rails developer. If you still want to know more, the Rails source code itself is probably the best place to go next.


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