Elixir OTP — GenServer, Supervisor, and Application

defmodule Counter do
  use GenServer

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

  def increment do
    GenServer.call(__MODULE__, :increment)
  end

  def value do
    GenServer.call(__MODULE__, :value)
  end

  # Server callbacks
  @impl true
  def init(initial_value) do
    {:ok, initial_value}
  end

  @impl true
  def handle_call(:increment, _from, state) do
    {:reply, state + 1, state + 1}
  end

  @impl true
  def handle_call(:value, _from, state) do
    {:reply, state, state}
  end
end

# Usage
{:ok, _} = Counter.start_link(0)
Counter.increment()  # => 1
Counter.increment()  # => 2
Counter.value()       # => 2

defmodule MyApp.Supervisor do
  use Supervisor

  def start_link(init_arg) do
    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
  end

  @impl true
  def init(_init_arg) do
    children = [
      {Counter, [0]},
      # More children...
    ]

    Supervisor.init(children, strategy: :one_for_one)
  end
end

# Strategies:
# :one_for_one — restart only the failed child
# :one_for_all — restart all children
# :rest_for_one — restart the failed child and its dependents

defmodule MyApp.Application do
  use Application

  @impl true
  def start(_type, _args) do
    children = [
      MyApp.Supervisor,
    ]

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