Question

I am not understand why config.ru file in rails3. Please explain why is there?

Answer 1

rails server does the basic job of creating a Rack::Builder object and starting the webserver. This is Rails’ equivalent of Rack’s rackup script.

Here’s how rails server creates an instance of Rack::Builder

app = Rack::Builder.new {
  use Rails::Rack::LogTailer unless options[:detach]
  use Rails::Rack::Debugger if options[:debugger]
  use ActionDispatch::Static
  run ActionController::Dispatcher.new
}.to_app

To use rackup instead of Rails’ rails server, you can put the following inside config.ru of your Rails application’s root directory:

# Rails.root/config.ru
require "config/environment"

use Rails::Rack::LogTailer
use ActionDispatch::Static
run ActionController::Dispatcher.new

And start the server:

$ rackup config.ru

To find out more about different rackup options:

$ rackup --help

To know about more rakup please have a look Ruby on Rails guide