17 KiB
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.
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 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!
Let's start to boot and initialize the app. It all begins 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_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:
# 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, 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:
- 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 directory.
# 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
.
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 (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:
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 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__)
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
).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.
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
,
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. The 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 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:
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.