In my journey to really understand how a proper Elixir application should be written I am exercising through Benjamin Tan Wei Hao's excelent The Little Elixir & OTP Guidebook . If you're just getting started, this is a no-brainer: buy and study this guidebook. Yes, it will help if you already read Dave Thomas' Programming Elixir book first. In my Ex Manga Downloadr Part 2 article I explored adding better process pool control using the excelent and robust Poolboy library. One of the guidebook main exercises is to build a simpler version of Poolboy in pure Elixir (Poolboy is written in good, old, Erlang). This main goal of this article is to introduce what Fault Tolerance in Erlang/Elixir mean and it is also an excuse for me to show off Erlang's observer: