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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.
- Using
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.
Launch!
A Rails application is usually started with the command rails server
.
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 requires rails/cli
.
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".
Rails Initialization
Only now we finally start the real initialization process, beginning
with script/rails
.
TIP: If you execute script/rails
directly from your Rails app you will
skip executing all the code that we've just described.
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.
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)
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.
actionpack/lib/action_dispatch.rb
Action Dispatch is the routing component of the Rails framework. It adds functionalities like routing, session, and common middlewares.
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
.
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.
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 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 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 <%= 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!
Loading Rails
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/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 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 is all being defined here.
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
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
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 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.