Rewrite bits and pieces to mention the command infrastructure.
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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, 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:
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:
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:
$ exec ruby bin/rails server
bin/rails
This file is as follows:
#!/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:
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:
- actionmailer
- actionpack
- actionview
- activemodel
- activerecord
- activesupport
- activejob
- arel
- builder
- bundler
- erubis
- i18n
- 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:
require "rails/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 executing 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.
module Rails::Command
class << self
def invoke(namespace, args = [], **config)
namespace = namespace.to_s
namespace = "help" if namespace.blank? || Thor::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:
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
:
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/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:
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://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:
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
:
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:
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:
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:
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 ||= 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:
# 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:
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:
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
:
require_relative 'application'
config/application.rb
This file requires config/boot.rb
:
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:
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/railtie
action_controller/railtie
action_view/railtie
action_mailer/railtie
active_job/railtie
action_cable/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:
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 ||= 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:
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 Puma, here's what
the run
method would look like:
...
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.