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:
How to use rails server
.
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.
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, we recommend that
you use the t
key binding to open the file finder inside
GitHub and find files quickly.
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
.
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.
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.
A standard Rails application depends on several gems, specifically:
abstract
actionmailer
actionpack
activemodel
activerecord
activesupport
arel
builder
bundler
erubis
i18n
mime-types
polyglot
rack
rack-cache
rack-mount
rack-test
rails
railties
rake
sqlite3-ruby
thor
treetop
tzinfo
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 `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 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 Dir.chdir(Rails.application.root) server.start 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 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.
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.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.rb
:
def initialize(*) super set_environment end
Firstly, super
is called which calls the
initialize
method on Rack::Server
.
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.
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.
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)) 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 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" 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 <%Q 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__)
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
.
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!
The next line in config/application.rb
is:
require 'rails/all'
railties/lib/rails/all.rb
¶ ↑This file is responsible for requiring all the individual frameworks of Rails:
require "rails" %w( active_record action_controller action_mailer rails/test_unit sprockets ).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 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.
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
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
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 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.