Singleton GenServer: Short track to the orphanage

While writing a GenServer as a singleton on a multi-node system, it’s easy to come up with something like this:

def start_link(_number, _opts \\ []) do
  case GenServer.start_link(__MODULE__, :ok, name: {:global, __MODULE__}) do
    {:ok, pid} ->
      {:ok, pid}

    {:error, {:already_started, pid}} ->
      {:ok, pid}
  end
end

It works perfectly until it doesn’t: when our singleton process goes down, the only supervision tree aware of this would be the one that started it. Every other node won’t get any message, so while the process is restarting, the caller will fail with Process is not alive. Even worse: if for some reason you are using the PID that is returned from start_link/2 in your code, your code on other nodes will never know the PID changed after the restart.

Luckily, it seems to be easily fixable with Process.link/1:

def start_link(_number, _opts \\ []) do
  case GenServer.start_link(__MODULE__, :ok, name: {:global, __MODULE__}) do
    {:ok, pid} ->
      {:ok, pid}

    {:error, {:already_started, pid}} ->
      # Now the process will send DOWN message to supervisors on every single node
      Process.link(pid)
      {:ok, pid}
  end
end