Supervisor

As erlang needs to be reliable system for applications, so built-in to the OTP (Open Telecom Platform) library is the idea of supervision for processes.

You can think of a process as a small self-contained program. Supervisor is a process which watches over other processes (referred to as child processes).

We can take advantage of this functionality by creating something called a supervision tree. We will take a look at what that means.

First let’s look at how we can set up a Supervisor. In our myapp.ex file located in our lib directory - we are going to add the following snippet.

defmodule Myapp do
  use Application
  use Supervisor

  def start(_type, _args) do
    Supervisor.start_link([{NewApp.Router, []}], strategy: :one_for_one)
  end
end

When our supervisor starts it will call a function start_link. This function takes two arguments - a list of tuples, and a strategy for how to restart child processes when they fail. Each tuple contains the name of the Child Process, and any initial arguments that will get passed into the start_link function of that respective module.

The children are the processes that the Supervisor will watch. The Supervisor will iterate over every child module and find its child_spec/ which defines how the child will be started, stopped, and restarted. Usually this is with a start_link/1 function. We will look at the child_spec soon.

In the options argument, we also need to specify a strategy. This tells the supervisor what to do if a child process fails.

In this case we are using the :one_for_one strategy. With this process, if the child process terminates, it will be restarted.

Now let’s create our router that will handle our requests.

Start application on Boot

Since we want our application to start at run time, we need to add a configuration to our mix.exs file.

We will add the following to our application

  mod: {Myapp, []}

So our application function now looks like:

  def application do
    # Specify extra applications you'll use from Erlang/Elixir
    [
      extra_applications: [:logger, :cowboy, :plug],
      mod: {Myapp, []}
    ]
  end

Notice that we specify the name of our App as what we pass to the mod option. This is our application callback module. So when our application starts, the module we pass to mod will be started