**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON http://guides.rubyonrails.org.** 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 `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 `rails server` to boot your app. NOTE: Paths in this guide are relative to Rails or a Rails application unless otherwise specified. TIP: If you want to follow along while browsing the Rails [source code](https://github.com/rails/rails), we recommend that you use the `t` key binding to open the file finder inside GitHub and find files quickly. Launch! ------- Let's start to boot and initialize the app. A Rails application is usually started by running `rails console` or `rails server`. ### `railties/exe/rails` The `rails` in the command `rails server` is a ruby executable in your load path. This executable contains the following lines: ```ruby version = ">= 0" load Gem.bin_path('railties', 'rails', version) ``` If you try out this command in a Rails console, you would see that this loads `railties/exe/rails`. A part of the file `railties/exe/rails.rb` has the following code: ```ruby require "rails/cli" ``` The file `railties/lib/rails/cli` in turn calls `Rails::AppLoader.exec_app`. ### `railties/lib/rails/app_loader.rb` The primary goal of the function `exec_app` is to execute your app's `bin/rails`. If the current directory does not have a `bin/rails`, it will navigate upwards until it finds a `bin/rails` executable. Thus one can invoke a `rails` command from anywhere inside a rails application. For `rails server` the equivalent of the following command is executed: ```bash $ exec ruby bin/rails server ``` ### `bin/rails` This file is as follows: ```ruby #!/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. ### `config/boot.rb` `config/boot.rb` contains: ```ruby ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../Gemfile', __dir__) require 'bundler/setup' # Set up gems listed in the 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, then `bundler/setup` is required. The require is used by Bundler to configure the load path for your Gemfile's dependencies. A standard Rails application depends on several gems, specifically: * actioncable * actionmailer * actionpack * actionview * activejob * activemodel * activerecord * activestorage * activesupport * arel * builder * bundler * erubi * i18n * mail * mime-types * rack * rack-cache * rack-mount * rack-test * rails * railties * rake * sqlite3 * thor * tzinfo ### `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: ```ruby require_relative "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. ### `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 `args` are empty. ```ruby module Rails::Command class << self def invoke(namespace, args = [], **config) namespace = namespace.to_s namespace = "help" if namespace.blank? || HELP_MAPPINGS.include?(namespace) namespace = "version" if %w( -v --version ).include? namespace if command = find_by_namespace(namespace) command.perform(namespace, args, config) else find_by_namespace("rake").perform(namespace, args, config) end end end end ``` With the `server` command, Rails will further run the following code: ```ruby module Rails module Command class ServerCommand < Base # :nodoc: def perform set_application_directory! Rails::Server.new.tap do |server| # Require application after server sets environment to propagate # the --environment option. require APP_PATH Dir.chdir(Rails.application.root) server.start 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. ### `actionpack/lib/action_dispatch.rb` Action Dispatch is the routing component of the Rails framework. It adds functionality like routing, session, and common middlewares. ### `rails/commands/server/server_command.rb` The `Rails::Server` class is defined in this file by inheriting from `Rack::Server`. When `Rails::Server.new` is called, this calls the `initialize` method in `rails/commands/server/server_command.rb`: ```ruby def initialize(*) super set_environment end ``` Firstly, `super` is called which calls the `initialize` method on `Rack::Server`. ### 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: ```ruby def initialize(options = nil) @options = options @app = options[:app] if options && options[:app] end ``` 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/server_command.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: ```ruby def set_environment ENV["RAILS_ENV"] ||= options[:environment] end ``` In fact, the `options` method here does quite a lot. This method is defined in `Rack::Server` like this: ```ruby def options @options ||= parse_options(ARGV) end ``` Then `parse_options` is defined like this: ```ruby def parse_options(args) options = default_options # Don't evaluate CGI ISINDEX parameters. # http://www.meb.uni-bonn.de/docs/cgi/cl.html 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] options end ``` With the `default_options` set to this: ```ruby def default_options super.merge( Port: ENV.fetch("PORT", 3000).to_i, Host: ENV.fetch("HOST", "localhost").dup, DoNotReverseLookup: true, environment: (ENV["RAILS_ENV"] || ENV["RACK_ENV"] || "development").dup, daemonize: false, caching: nil, pid: Options::DEFAULT_PID_PATH, restart_cmd: restart_command) end ``` 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`: ```ruby def opt_parser Options.new end ``` The class **is** defined in `Rack::Server`, but is overwritten in `Rails::Server` to take different arguments. Its `parse!` method looks like this: ```ruby def parse!(args) args, options = args.dup, {} option_parser(options).parse! args options[:log_stdout] = options[:daemonize].blank? && (options[:environment] || Rails.env) == "development" options[:server] = args.shift options end ``` 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 the server command where `APP_PATH` (which was set earlier) is required. ### `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. ### `Rails::Server#start` After `config/application` is loaded, `server.start` is called. This method is defined like this: ```ruby def start print_boot_information trap(:INT) { exit } create_tmp_directories setup_dev_caching log_to_stdout if options[:log_stdout] super ... end private def print_boot_information ... puts "=> Run `rails server -h` for more startup options" 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 setup_dev_caching if options[:environment] == "development" Rails::DevCaching.enable_by_argument(options[:caching]) 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, STDOUT) Rails.logger.extend(ActiveSupport::Logger.broadcast(console)) end end ``` 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`, and `tmp/sockets` directories. It then enables caching in development if `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 `Rack::Server.start` which begins its definition like this: ```ruby def start &blk if options[:warn] $-w = true end if includes = options[:include] $LOAD_PATH.unshift(*includes) end if library = options[:require] 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). wrapped_app 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, &blk 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). ```ruby @wrapped_app ||= build_app app ``` The `app` method here is defined like so: ```ruby 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 app, options = Rack::Builder.parse_file(self.options[:config], opt_parser) self.options.merge! options app end def build_app_from_string Rack::Builder.new_from_string(self.options[:builder]) end ``` The `options[:config]` value defaults to `config.ru` which contains this: ```ruby # This file is used by Rack-based servers to start the application. require_relative 'config/environment' run <%= app_const %> ``` The `Rack::Builder.parse_file` method here takes the content from this `config.ru` file and parses it using this code: ```ruby app = new_from_string cfgfile, config ... def self.new_from_string(builder_script, file="(rackup)") eval "Rack::Builder.new {\n" + builder_script + "\n}.to_app", TOPLEVEL_BINDING, file, 0 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: ```ruby require_relative 'config/environment' ``` ### `config/environment.rb` This file is the common file required by `config.ru` (`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`: ```ruby require_relative 'application' ``` ### `config/application.rb` This file requires `config/boot.rb`: ```ruby require_relative 'boot' ``` 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! Loading Rails ------------- The next line in `config/application.rb` is: ```ruby require 'rails/all' ``` ### `railties/lib/rails/all.rb` This file is responsible for requiring all the individual frameworks of Rails: ```ruby require "rails" %w( active_record/railtie action_controller/railtie action_view/railtie action_mailer/railtie active_job/railtie action_cable/engine active_storage/engine rails/test_unit/railtie sprockets/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. ### 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`. ### `railties/lib/rails/application.rb` The `initialize!` method looks like this: ```ruby def initialize!(group=:default) #:nodoc: raise "Application has been already initialized." if @initialized run_initializers(group, self) @initialized = true self end ``` As you can see, you can only initialize an app once. The initializers are run through the `run_initializers` method which is defined in `railties/lib/rails/initializable.rb`: ```ruby 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 `finishers`. After this is done we go back to `Rack::Server`. ### Rack: lib/rack/server.rb Last time we left when the `app` method was being defined: ```ruby 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 app, options = Rack::Builder.parse_file(self.options[:config], opt_parser) self.options.merge! options app end def build_app_from_string Rack::Builder.new_from_string(self.options[:builder]) end ``` At this point `app` is the Rails app itself (a middleware), and what happens next is Rack will call all the provided middlewares: ```ruby def build_app(app) middleware[options[:environment]].reverse_each do |middleware| middleware = middleware.call(self) if middleware.respond_to?(:call) next unless middleware klass = middleware.shift app = klass.new(app, *middleware) end app end ``` Remember, `build_app` was called (by `wrapped_app`) in the last line of `Server#start`. Here's how it looked like when we left: ```ruby server.run wrapped_app, options, &blk ``` 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: ```ruby ... DEFAULT_OPTIONS = { :Host => '0.0.0.0', :Port => 8080, :Threads => '0:16', :Verbose => false } def self.run(app, options = {}) options = DEFAULT_OPTIONS.merge(options) if options[:Verbose] app = Rack::CommonLogger.new(app, STDOUT) end if options[:environment] ENV['RACK_ENV'] = options[:environment].to_s end server = ::Puma::Server.new(app) min, max = options[:Threads].split(':', 2) puts "Puma #{::Puma::Const::PUMA_VERSION} starting..." puts "* Min threads: #{min}, max threads: #{max}" puts "* Environment: #{ENV['RACK_ENV']}" puts "* Listening on tcp://#{options[:Host]}:#{options[:Port]}" server.add_tcp_listener options[:Host], options[:Port] server.min_threads = min server.max_threads = max yield server if block_given? begin server.run.join rescue Interrupt puts "* Gracefully stopping, waiting for requests to finish" server.stop(true) puts "* Goodbye!" 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.