The Rails Initialization Process

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

After reading this guide, you will know:

How to use bin/rails server.
The timeline of Rails' initialization sequence.
Where different files are required by the boot sequence.
How the Rails::Server interface is defined and used.

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

Let's start to boot and initialize the app. A Rails application is usually started by running bin/rails console or bin/rails server.

1.1. `bin/rails`

This file is as follows:

#!/usr/bin/env ruby
APP_PATH = File.expand_path("../config/application", __dir__)
require_relative "../config/boot"
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:

ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
require "bundler/setup" # Set up gems listed in the Gemfile.
require "bootsnap/setup" # Speed up boot time by caching expensive operations.

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, then bundler/setup is required. The require is used by Bundler to configure the load path for your Gemfile's dependencies.

1.3. `rails/commands.rb`

Once config/boot.rb has finished, the next file that is required is rails/commands, which helps in expanding aliases. In the current case, the ARGV array simply contains server which will be passed over:

require "rails/command"
aliases = {
 "g" => "generate",
 "d" => "destroy",
 "c" => "console",
 "s" => "server",
 "db" => "dbconsole",
 "r" => "runner",
 "t" => "test"
}
command = ARGV.shift
command = aliases[command] || command
Rails::Command.invoke command, ARGV

If we had used s rather than server, Rails would have used the aliases defined here to find the matching command.

1.4. `rails/command.rb`

When one types a Rails command, invoke tries to lookup a command for the given namespace and executes the command if found.

If Rails doesn't recognize the command, it hands the reins over to Rake to run a task of the same name.

As shown, Rails::Command displays the help output automatically if the namespace is empty.

module Rails
 module Command
 class << self
 def invoke(full_namespace, args = [], **config)
 args = ["--help"] if rails_new_with_no_path?(args)
 full_namespace = full_namespace.to_s
 namespace, command_name = split_namespace(full_namespace)
 command = find_by_namespace(namespace, command_name)
 with_argv(args) do
 if command && command.all_commands[command_name]
 command.perform(command_name, args, config)
 else
 invoke_rake(full_namespace, args, config)
 end
 end
 rescue UnrecognizedCommandError => error
 if error.name == full_namespace && command && command_name == full_namespace
 command.perform("help", [], config)
 else
 puts error.detailed_message
 end
 exit(1)
 end
 end
 end
end

With the server command, Rails will further run the following code:

module Rails
 module Command
 class ServerCommand < Base # :nodoc:
 def perform
 set_application_directory!
 prepare_restart
 Rails::Server.new(server_options).tap do |server|
 # Require application after server sets environment to propagate
 # the --environment option.
 require APP_PATH
 Dir.chdir(Rails.application.root)
 if server.serveable?
 print_boot_information(server.server, server.served_url)
 after_stop_callback = -> { say "Exiting" unless options[:daemon] }
 server.start(after_stop_callback)
 else
 say rack_server_suggestion(options[:using])
 end
 end
 end
 end
 end
end

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 config.ru file isn't found. This then starts up the Rails::Server class.

1.5. `actionpack/lib/action_dispatch.rb`

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

1.6. `rails/commands/server/server_command.rb`

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

module Rails
 class Server < Rackup::Server
 def initialize(options = nil)
 @default_options = options || {}
 super(@default_options)
 set_environment
 end
 end
end

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

1.7. Rackup: `lib/rackup/server.rb`

Rackup::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 Rackup::Server simply sets several variables:

module Rackup
 class Server
 def initialize(options = nil)
 @ignore_options = []
 if options
 @use_default_options = false
 @options = options
 @app = options[:app] if options[:app]
 else
 @use_default_options = true
 @options = parse_options(ARGV)
 end
 end
 end
end

In this case, return value of Rails::Command::ServerCommand#server_options will be assigned to options. When lines inside if statement is evaluated, a couple of instance variables will be set.

server_options method in Rails::Command::ServerCommand is defined as follows:

module Rails
 module Command
 class ServerCommand < Base # :nodoc:
 no_commands do
 def server_options
 {
 user_supplied_options: user_supplied_options,
 server: options[:using],
 log_stdout: log_to_stdout?,
 Port: port,
 Host: host,
 DoNotReverseLookup: true,
 config: options[:config],
 environment: environment,
 daemonize: options[:daemon],
 pid: pid,
 caching: options[:dev_caching],
 restart_cmd: restart_command,
 early_hints: early_hints
 }
 end
 end
 end
 end
end

The value will be assigned to instance variable @options.

After super has finished in Rackup::Server, we jump back to rails/commands/server/server_command.rb. At this point, set_environment is called within the context of the Rails::Server object.

module Rails
 module Server
 def set_environment
 ENV["RAILS_ENV"] ||= options[:environment]
 end
 end
end

After initialize has finished, we jump back into the server command where APP_PATH (which was set earlier) is required.

1.8. `config/application`

When require APP_PATH is executed, config/application.rb is loaded (recall that APP_PATH is defined in bin/rails). This file exists in your application and it's free for you to change based on your needs.

1.9. `Rails::Server#start`

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

module Rails
 class Server < ::Rackup::Server
 def start(after_stop_callback = nil)
 trap(:INT) { exit }
 create_tmp_directories
 setup_dev_caching
 log_to_stdout if options[:log_stdout]
 super()
 # ...
 end
 private
 def setup_dev_caching
 if options[:environment] == "development"
 Rails::DevCaching.enable_by_argument(options[:caching])
 end
 end
 def create_tmp_directories
 %w(cache pids sockets).each do |dir_to_make|
 FileUtils.mkdir_p(File.join(Rails.root, "tmp", dir_to_make))
 end
 end
 def log_to_stdout
 wrapped_app # touch the app so the logger is set up
 console = ActiveSupport::Logger.new(STDOUT)
 console.formatter = Rails.logger.formatter
 console.level = Rails.logger.level
 unless ActiveSupport::Logger.logger_outputs_to?(Rails.logger, STDERR, STDOUT)
 Rails.logger.broadcast_to(console)
 end
 end
 end
end

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, and tmp/sockets directories. It then enables caching in development if bin/rails server is called with --dev-caching. Finally, it 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 Rackup::Server.start which begins its definition as follows:

module Rackup
 class Server
 def start(&block)
 if options[:warn]
 $-w = true
 end
 if includes = options[:include]
 $LOAD_PATH.unshift(*includes)
 end
 Array(options[:require]).each do |library|
 require library
 end
 if options[:debug]
 $DEBUG = true
 require "pp"
 p options[:server]
 pp wrapped_app
 pp app
 end
 check_pid! if options[:pid]
 # Touch the wrapped app, so that the config.ru is loaded before
 # daemonization (i.e. before chdir, etc).
 handle_profiling(options[:heapfile], options[:profile_mode], options[:profile_file]) do
 wrapped_app
 end
 daemonize_app if options[:daemonize]
 write_pid if options[:pid]
 trap(:INT) do
 if server.respond_to?(:shutdown)
 server.shutdown
 else
 exit
 end
 end
 server.run(wrapped_app, **options, &block)
 end
 end
end

The interesting part for a Rails app is the last line, server.run. 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 memoized by now).

module Rackup
 class Server
 def wrapped_app
 @wrapped_app ||= build_app app
 end
 end
end

The app method here is defined like so:

module Rackup
 class Server
 def app
 @app ||= options[:builder] ? build_app_from_string : build_app_and_options_from_config
 end
 # ...
 private
 def build_app_and_options_from_config
 if !::File.exist? options[:config]
 abort "configuration #{options[:config]} not found"
 end
 Rack::Builder.parse_file(self.options[:config])
 end
 def build_app_from_string
 Rack::Builder.new_from_string(self.options[:builder])
 end
 end
end

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

# This file is used by Rack-based servers to start the application.
require_relative "config/environment"
run Rails.application
Rails.application.load_server

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

module Rack
 class Builder
 def self.load_file(path, **options)
 # ...
 new_from_string(config, path, **options)
 end
 # ...
 def self.new_from_string(builder_script, path = "(rackup)", **options)
 builder = self.new(**options)
 # We want to build a variant of TOPLEVEL_BINDING with self as a Rack::Builder instance.
 # We cannot use instance_eval(String) as that would resolve constants differently.
 binding = BUILDER_TOPLEVEL_BINDING.call(builder)
 eval(builder_script, binding, path)
 builder.to_app
 end
 end
end

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 config.ru is the first to run:

require_relative "config/environment"

1.10. `config/environment.rb`

This file is the common file required by config.ru (bin/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:

require_relative "application"

1.11. `config/application.rb`

This file requires config/boot.rb:

require_relative "boot"

But only if it hasn't been required before, which would be the case in bin/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"
%w(
 active_record/railtie
 active_storage/engine
 action_controller/railtie
 action_view/railtie
 action_mailer/railtie
 active_job/railtie
 action_cable/engine
 action_mailbox/engine
 action_text/engine
 rails/test_unit/railtie
).each do |railtie|
 begin
 require railtie
 rescue LoadError
 end
end

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`

The rest of config/application.rb defines the configuration for the Rails::Application which will be used once the application is fully initialized. When config/application.rb has finished loading Rails and defined the application namespace, we go back to config/environment.rb. Here, the application is initialized with Rails.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
 self
end

You can only initialize an app once. The Railtie initializers are run through the run_initializers method which is defined in railties/lib/rails/initializable.rb:

def run_initializers(group = :default, *args)
 return if instance_variable_defined?(:@ran)
 initializers.tsort_each do |initializer|
 initializer.run(*args) if initializer.belongs_to?(group)
 end
 @ran = true
end

The run_initializers code itself is tricky. What Rails is doing here is traversing all the class ancestors looking for those that respond to an initializers method. It then sorts the ancestors by name, and runs them. For example, the Engine class will make all the engines available by providing an initializers method on them.

The Rails::Application class, as defined in railties/lib/rails/application.rb defines bootstrap, railtie, and finisher initializers. The bootstrap initializers prepare the application (like initializing the logger) while the finisher initializers (like building the middleware stack) are run last. The railtie initializers are the initializers which have been defined on the Rails::Application itself and are run between the bootstrap and finisher.

Do not confuse Railtie initializers overall with the load_config_initializers initializer instance or its associated config initializers in config/initializers.

After this is done we go back to Rackup::Server.

2.4. Rack: lib/rack/server.rb

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

module Rackup
 class Server
 def app
 @app ||= options[:builder] ? build_app_from_string : build_app_and_options_from_config
 end
 # ...
 private
 def build_app_and_options_from_config
 if !::File.exist? options[:config]
 abort "configuration #{options[:config]} not found"
 end
 Rack::Builder.parse_file(self.options[:config])
 end
 def build_app_from_string
 Rack::Builder.new_from_string(self.options[:builder])
 end
 end
end

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

module Rackup
 class Server
 private
 def build_app(app)
 middleware[options[:environment]].reverse_each do |middleware|
 middleware = middleware.call(self) if middleware.respond_to?(:call)
 next unless middleware
 klass, *args = middleware
 app = klass.new(app, *args)
 end
 app
 end
 end
end

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

server.run(wrapped_app, **options, &block)

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

module Rack
 module Handler
 module Puma
 # ...
 def self.run(app, options = {})
 conf = self.config(app, options)
 log_writer = options.delete(:Silent) ? ::Puma::LogWriter.strings : ::Puma::LogWriter.stdio
 launcher = ::Puma::Launcher.new(conf, log_writer: log_writer, events: @events)
 yield launcher if block_given?
 begin
 launcher.run
 rescue Interrupt
 puts "* Gracefully stopping, waiting for requests to finish"
 launcher.stop
 puts "* Goodbye!"
 end
 end
 # ...
 end
 end
end

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.