Ensure that only one instance of a given (supervised) process exists in the cluster.

global_register.ex

defmodule Party.Clock do
  use GenServer

  def start_link do
    case GenServer.start_link(__MODULE__, [], name: {:global, __MODULE__}) do
      {:ok, pid} ->
        {:ok, pid}
      {:error, {:already_started, pid}} ->
        IO.puts("Already started!!")
        Process.link(pid)
        {:ok, pid}
    end
  end
end

defmodule Party.Clock.Supervisor do
  use Supervisor
  def start_link do
    Supervisor.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init(_) do
    children = [supervisor(Party.Clock, [])]
    supervise(children, strategy: :one_for_one)
  end
end