h2. 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. * Using +rails server+ * Using Passenger endprologue. 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 how the two most common methods (+rails server+ and Passenger) boot a Rails application. NOTE: Paths in this guide are relative to Rails or a Rails application unless otherwise specified. h3. Launch! A Rails application is usually started with the command +rails server+. h4. +bin/rails+ The actual +rails+ command is kept in _bin/rails_: #!/usr/bin/env ruby if File.exists?(File.join(File.expand_path('../../..', __FILE__), '.git')) railties_path = File.expand_path('../../lib', __FILE__) $:.unshift(railties_path) end require "rails/cli" This file will first attempt to push the +railties/lib+ directory if present, and then require +rails/cli+. h4. +railties/lib/rails/cli.rb+ This file looks like this: require 'rbconfig' require 'rails/script_rails_loader' # If we are inside a Rails application this method performs an exec and thus # the rest of this script is not run. Rails::ScriptRailsLoader.exec_script_rails! require 'rails/ruby_version_check' Signal.trap("INT") { puts; exit(1) } if ARGV.first == 'plugin' ARGV.shift require 'rails/commands/plugin_new' else require 'rails/commands/application' end The +rbconfig+ file from the Ruby standard library provides us with the +RbConfig+ class which contains detailed information about the Ruby environment, including how Ruby was compiled. We can see this in use in +railties/lib/rails/script_rails_loader+. require 'pathname' module Rails module ScriptRailsLoader RUBY = File.join(*RbConfig::CONFIG.values_at("bindir", "ruby_install_name")) + RbConfig::CONFIG["EXEEXT"] SCRIPT_RAILS = File.join('script', 'rails') ... end end The +rails/script_rails_loader+ file uses +RbConfig::Config+ to obtain the +bin_dir+ and +ruby_install_name+ values for the configuration which together form the path to the Ruby interpreter. The +RbConfig::CONFIG["EXEEXT"]+ will suffix this path with ".exe" if the script is running on Windows. This constant is used later on in +exec_script_rails!+. As for the +SCRIPT_RAILS+ constant, we'll see that when we get to the +in_rails_application?+ method. Back in +rails/cli+, the next line is this: Rails::ScriptRailsLoader.exec_script_rails! This method is defined in +rails/script_rails_loader+: def self.exec_script_rails! cwd = Dir.pwd return unless in_rails_application? || in_rails_application_subdirectory? exec RUBY, SCRIPT_RAILS, *ARGV if in_rails_application? Dir.chdir("..") do # Recurse in a chdir block: if the search fails we want to be sure # the application is generated in the original working directory. exec_script_rails! unless cwd == Dir.pwd end rescue SystemCallError # could not chdir, no problem just return end This method will first check if the current working directory (+cwd+) is a Rails application or a subdirectory of one. This is determined by the +in_rails_application?+ method: def self.in_rails_application? File.exists?(SCRIPT_RAILS) end The +SCRIPT_RAILS+ constant defined earlier is used here, with +File.exists?+ checking for its presence in the current directory. If this method returns +false+ then +in_rails_application_subdirectory?+ will be used: def self.in_rails_application_subdirectory?(path = Pathname.new(Dir.pwd)) File.exists?(File.join(path, SCRIPT_RAILS)) || !path.root? && in_rails_application_subdirectory?(path.parent) end This climbs the directory tree until it reaches a path which contains a +script/rails+ file. If a directory containing this file is reached then this line will run: exec RUBY, SCRIPT_RAILS, *ARGV if in_rails_application? This is effectively the same as running +ruby script/rails [arguments]+, where +[arguments]+ at this point in time is simply "server". TIP: If you execute +script/rails+ directly from your Rails app you will avoid executing the code that we just described. h4. +script/rails+ This file is as follows: 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. h4. +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.exists?(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. The gems that a Rails 4 application depends on are as follows: TODO: change these when the Rails 4 release is near. * abstract (1.0.0) * actionmailer (4.0.0.beta) * actionpack (4.0.0.beta) * activemodel (4.0.0.beta) * activerecord (4.0.0.beta) * activesupport (4.0.0.beta) * arel (2.0.7) * builder (3.0.0) * bundler (1.0.6) * erubis (2.6.6) * i18n (0.5.0) * mail (2.2.12) * mime-types (1.16) * polyglot (0.3.1) * rack (1.2.1) * rack-cache (0.5.3) * rack-mount (0.6.13) * rack-test (0.5.6) * rails (4.0.0.beta) * railties (4.0.0.beta) * rake (0.8.7) * sqlite3-ruby (1.3.2) * thor (0.14.6) * treetop (1.4.9) * tzinfo (0.3.23) h4. +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 TIP: 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 config.ru file in current dir. # This allows us to run script/rails server from other directories, but still get # the main config.ru and properly set the tmp directory. Dir.chdir(File.expand_path('../../', APP_PATH)) unless File.exists?(File.expand_path("config.ru")) require 'rails/commands/server' Rails::Server.new.tap { |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 Dir.chdir(Rails.application.root) server.start } This file will change into the root of the directory (a path two directories back from +APP_PATH+ which points at +config/application.rb+), but only if the +config.ru+ 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. h4. +actionpack/lib/action_dispatch.rb+ Action Dispatch is the routing component of the Rails framework. Other than the rouing itself, it adds functionalities like routing, session, and common middlewares. Action Dispatch itself is also responsible for loading Active Support, Action Pack, Active Model, and Rack. h4. +rails/commands/server.rb+ The +Rails::Server+ class is defined in this file as inheriting from +Rack::Server+. When +Rails::Server.new+ is called, this calls the +initialize+ method in +rails/commands/server.rb+: def initialize(*) super set_environment end Firstly, +super+ is called which calls the +initialize+ method on +Rack::Server+. h4. 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] 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.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] end 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) end Then +parse_options+ is defined like this: def parse_options(args) options = default_options # Don't evaluate CGI ISINDEX parameters. # http://hoohoo.ncsa.uiuc.edu/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: def default_options { :environment => ENV['RACK_ENV'] || "development", :pid => nil, :Port => 9292, :Host => "0.0.0.0", :AccessLog => [], :config => "config.ru" } 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+ 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 begins like this: def parse!(args) args, options = args.dup, {} opt_parser = OptionParser.new 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. h4. +config/application+ When +require APP_PATH+ is executed, +config/application.rb+ is loaded. This is a file exists in your app and it's free for you to change based on your needs. Among other things, inside this file you load gems with bundler, and create your application namespace. h4. +Rails::Server#start+ After +congif/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 "=> Call with -d to detach" unless options[:daemonize] 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)) end unless options[:daemonize] wrapped_app # touch the app so the logger is set up console = ActiveSupport::Logger.new($stdout) console.formatter = Rails.logger.formatter Rails.logger.extend(ActiveSupport::Logger.broadcast(console)) end super ensure # 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] 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+, +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 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). @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" end app, options = Rack::Builder.parse_file(self.options[:config], opt_parser) self.options.merge! options app 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 ::File.expand_path('../config/environment', __FILE__) run <%= app_const %> The +Rack::Builder.parse_file+ method here takes the content from this +config.ru+ file and parses it using this code: app = eval "Rack::Builder.new {( " cfgfile "\n )}.to_app", TOPLEVEL_BINDING, config 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 ::File.expand_path('../config/environment', __FILE__) h4. +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+. h4. +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! h3. Loading Rails The next line in +config/application.rb+ is: require 'rails/all' h4. +railties/lib/rails/all.rb+ This file is responsible for requiring all the individual parts of Rails like so: require "rails" %w( active_record action_controller action_mailer rails/test_unit sprockets/rails ).each do |framework| begin require "#{framework}/railtie" rescue LoadError end end This is where all the Rails frameworks are loaded and thus made available to the application. We wont 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 is all bein defined here. h4. Back to +config/environment.rb+ When +config/application.rb+ has finished loading Rails, and defined your application namespace, you go back to +config/environment.rb+, where your application is initialized. For example, if you application was called +Blog+, here you would find +Blog::Application.initialize!+, which is defined in +rails/application.rb+ h4. +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 As you can see, you can only initialize an app once. This is also where the initializers are run. TODO: 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+ h4. 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" end app, options = Rack::Builder.parse_file(self.options[:config], opt_parser) self.options.merge! options app 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: 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: 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 Mongrel, here's what the +run+ method would look like: def self.run(app, options={}) server = ::Mongrel::HttpServer.new( options[:Host] || '0.0.0.0', 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] == ?/ server.register(path, Rack::Handler::Mongrel.new(appl)) end 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] == ?/ server.register(path, Rack::Handler::Mongrel.new(appl)) end else raise ArgumentError, "first argument should be a Hash or URLMap" end else server.register('/', Rack::Handler::Mongrel.new(app)) end yield server if block_given? server.run.join end We wont 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 you 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.