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This guide goes through every method call that isrequired to boot up the Ruby on Rails stack for a default Railsapplication, explaining each part in detail along the way. For thisguide, we will be focusing on what happens when you execute bin/rails serverto boot your app.

Paths in this guide are relative to Rails or a Rails application unless otherwise specified.

Facebook Lite App Version History and Changelog Facebook Lite 244.0.0.6.117 APK (Latest Version APK) Facebook Lite 243.0.0.8.115 APK (Old Version APK). Open page Download Facebook Lite 1.0.0.0.0-2 for Android - OldVersion.com. The Rails Initialization ProcessThis 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 bin/rails server. The timeline of Rails' initialization sequence. Where different files are required by the boot sequence.

If you want to follow along while browsing the Rails sourcecode, we recommend that you use the tkey binding to open the file finder inside GitHub and find filesquickly.

1 Launch!

Let's start to boot and initialize the app. A Rails application is usuallystarted by running bin/rails console or bin/rails server.

1.1 `bin/rails`

This file is as follows:

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.

1.2 `config/boot.rb`

config/boot.rb contains:

In a standard Rails application, there's a Gemfile which declares alldependencies of the application. config/boot.rb setsENV['BUNDLE_GEMFILE'] to the location of this file. If the Gemfileexists, then bundler/setup is required. The require is used by Bundler toconfigure the load path for your Gemfile's dependencies.

A standard Rails application depends on several gems, specifically:

actioncable
actionmailer
actionpack
actionview
activejob
activemodel
activerecord
activestorage
activesupport
actionmailbox
actiontext
arel
builder
bundler
erubi
i18n
mail
mime-types
rack
rack-test
rails
railties
rake
sqlite3
thor
tzinfo

1.3 `rails/commands.rb`

Once config/boot.rb has finished, the next file that is required israils/commands, which helps in expanding aliases. In the current case, theARGV array simply contains server which will be passed over:

If we had used s rather than server, Rails would have used the aliasesdefined here to find the matching command.

1.4 `rails/command.rb`

When one types a Rails command, invoke tries to lookup a command for the givennamespace and executes the command if found.

If Rails doesn't recognize the command, it hands the reins over to Raketo run a task of the same name.

As shown, Rails::Command displays the help output automatically if the namespaceis empty.

With the server command, Rails will further run the following code:

This file will change into the Rails root directory (a path two directories upfrom APP_PATH which points at config/application.rb), but only if theconfig.ru file isn't found. This then starts up the Rails::Server class.

1.5 `actionpack/lib/action_dispatch.rb`

Action Dispatch is the routing component of the Rails framework.It adds functionality like routing, session, and common middlewares.

1.6 `rails/commands/server/server_command.rb`

The Rails::Server class is defined in this file by inheriting fromRack::Server. When Rails::Server.new is called, this calls the initializemethod in rails/commands/server/server_command.rb:

Firstly, super is called which calls the initialize method on Rack::Server.

1.7 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 several variables:

In this case, return value of Rails::Command::ServerCommand#server_options will be assigned to options.When lines inside if statement is evaluated, a couple of instance variables will be set.

server_options method in Rails::Command::ServerCommand is defined as follows:

The value will be assigned to instance variable @options.

After super has finished in Rack::Server, we jump back torails/commands/server/server_command.rb. At this point, set_environmentis called within the context of the Rails::Server object.

After initialize has finished, we jump back into the server commandwhere APP_PATH (which was set earlier) is required.

1.8 `config/application`

When require APP_PATH is executed, config/application.rb is loaded (recallthat APP_PATH is defined in bin/rails). This file exists in your applicationand it's free for you to change based on your needs.

1.9 `Rails::Server#start`

After config/application is loaded, server.start is called. This method isdefined like this:

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 developmentif bin/rails server is called with --dev-caching. Finally, it calls wrapped_app which isresponsible for creating the Rack app, before creating and assigning an instanceof ActiveSupport::Logger.

The super method will call Rack::Server.start which begins its definition as follows:

The interesting part for a Rails app is the last line, server.run. Here we encounter the wrapped_app method again, which this timewe're going to explore more (even though it was executed before, andthus memoized by now).

The app method here is defined like so:

The options[:config] value defaults to config.ru which contains this:

The Rack::Builder.parse_file method here takes the content from this config.ru file and parses it using this code:

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:

1.10 `config/environment.rb`

This file is the common file required by config.ru (bin/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:

1.11 `config/application.rb`

This file requires config/boot.rb:

But only if it hasn't been required before, which would be the case in bin/rails serverbut wouldn't be the case with Passenger.

Then the fun begins!

2 Loading Rails

The next line in config/application.rb is:

2.1 `railties/lib/rails/all.rb`

This file is responsible for requiring all the individual frameworks of Rails:

This is where all the Rails frameworks are loaded and thus madeavailable to the application. We won't go into detail of what happensinside each of those frameworks, but you're encouraged to try andexplore 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.

2.2 Back to `config/environment.rb`

The rest of config/application.rb defines the configuration for theRails::Application which will be used once the application is fullyinitialized. When config/application.rb has finished loading Rails and definedthe application namespace, we go back to config/environment.rb. Here, theapplication is initialized with Rails.application.initialize!, which isdefined in rails/application.rb.

2.3 `railties/lib/rails/application.rb`

The initialize! method looks like this:

You can only initialize an app once. The Railtie initializersare run through the run_initializers method which is defined inrailties/lib/rails/initializable.rb:

The run_initializers code itself is tricky. What Rails is doing here istraversing all the class ancestors looking for those that respond to aninitializers method. It then sorts the ancestors by name, and runs them.For example, the Engine class will make all the engines available byproviding an initializers method on them.

The Rails::Application class, as defined in railties/lib/rails/application.rbdefines bootstrap, railtie, and finisher initializers. The bootstrap initializersprepare the application (like initializing the logger) while the finisherinitializers (like building the middleware stack) are run last. The railtieinitializers are the initializers which have been defined on the Rails::Applicationitself and are run between the bootstrap and finishers.

Note: Do not confuse Railtie initializers overall with the load_config_initializersinitializer instance or its associated config initializers in config/initializers.

After this is done we go back to Rack::Server.

2.4 Rack: lib/rack/server.rb

Last time we left when the app method was being defined:

At this point app is the Rails app itself (a middleware), and whathappens next is Rack will call all the provided middlewares:

Remember, build_app was called (by wrapped_app) in the last line of Rack::Server#start.Here's how it looked like when we left:

At this point, the implementation of server.run will depend on theserver you're using. For example, if you were using Puma, here's whatthe run method would look like:

We won't dig into the server configuration itself, but this isthe last piece of our journey in the Rails initialization process.

This high level overview will help you understand when your code isexecuted and how, and overall become a better Rails developer. If youstill want to know more, the Rails source code itself is probably thebest place to go next.

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