In a production application you usually have many servers, and each of those servers gets checked periodically to make sure they're still healthy and working as expected. When they are, requests can be routed at them by your load balancer. 

Come with me on a journey through the internals of Rails, as we trace a request from the web server to the controller action method, and follow the response back again.

Internationalization (dubbed as I18n as there are exactly eighteen characters between the first “I” and the last “n”) means creating an application that can be adapted to various languages easily, without the need to do complex changes.

Minitest is a testing suite for Ruby. It provides a complete suite of testing facilities supporting test-driven development (TDD), behavior-driven development (BDD), mocking, and benchmarking. It's small, fast, and it aims to make tests clean and readable.

When creating an app in Ruby on Rails, from the very beginning, on every single step of your developer's journey, you're going to use ActiveRecord methods. Lots of them, and frankly speaking, it's good to know them well (if not by heart).