Supervisor

Supervisors are a fundamental concept in OTP. They monitor and manage the lifecycle of processes, restarting them if they fail.

Create a application using mix:

mix new counter --sup

lib/counter/application.ex

defmodule Counter.Application do

  use Application

  def start(_type, _args) do
    children = [
      {Counter, 0}
    ]

    opts = [strategy: :one_for_one, name: Counter.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

put the counter code into lib/counter.ex

defmodule Counter do
  use GenServer
  
  def init(initial_value) do
    {:ok, initial_value}
  end

  # sync calls
  def handle_call(:get_count, _from, state) do
    {:reply, state, state}
  end

  # async calls
  def handle_cast(:incr, state) do
    {:noreply, state + 1}
  end

  # helper functions
  def incr, do: GenServer.cast(__MODULE__, :incr)
  def get_count, do: GenServer.call(__MODULE__, :get_count)

  def start_link(initial_value) do
    GenServer.start_link(__MODULE__, initial_value, name: __MODULE__)
  end
end

iex -S mix

iex(1)> Counter.get_count
0
iex(2)> Counter.incr
:ok
iex(3)> Counter.get_count
1