Category: Learned the Hard Way

You’ll like it if you, like me, prefer to learn by try and error and read too few docs. (Unlike me, you should read more because Elixirs docs are terrific). All this is in docs; it just happened I learned this the hard way instead.

If you prefer to read docs first, I guess you may wonder what pitfalls you’ve avoided. Here they are!

Singleton GenServer: Short track to the orphanage

Posted on by Dima Ermilov

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.…

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Category: Learned the Hard Way