Using poolboy

poolboy_sample.ex

Mix.install([
  {:poolboy, ''}
])

defmodule Pool do
  @pool_name Macro.underscore(__MODULE__)
        |> String.to_atom()

  def child_spec(opts) do
    :poolboy.child_spec(
      __MODULE__,
      poolboy_config,
      opts
    )
  end

  def test do
    :poolboy.transaction(@pool_name, fn pid ->
      GenServer.call(pid, :test)
    end)
  end

  defp poolboy_config do
    [
      name: {:local, @pool_name},
      worker_module: Sample
    ]
  end
end

defmodule Sample do
  @moduledoc """
  Documentation for `Sample`.
  """

  use GenServer

  def start_link(opts) do
    IO.inspect(opts, label: "start_link arg")
    GenServer.start_link(__MODULE__, %{}, opts)
  end

  @impl true
  def init(init_arg) do
    {:ok, init_arg}
  end

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

children = [Pool]
opts = [strategy: :one_for_one, name: Sample.Supervisor]

Supervisor.start_link(children, opts)