Hello,

I have an Elixir app where a process depends on another process to be alive and send messages back. This second process is mainly a database connection and may occasionally fail. I created an example app below, which mimics the structure. ModuleA my worker process, ModuleB is the database connection:

defmodule Test.ModuleA do
  use GenServer

  def start_link(opts \\ []) do
    name = Keyword.get(opts, :name, __MODULE__)
    GenServer.start_link(__MODULE__, nil, name: name)
  end

  def init(_) do
    IO.puts("#{__MODULE__} starting")

    {:ok, nil}
  end
end

defmodule Test.ModuleB do
  use GenServer

  def start_link(opts \\ []) do
    name = Keyword.get(opts, :name, __MODULE__)
    GenServer.start_link(__MODULE__, nil, name: name)
  end

  def init(_) do
    IO.puts("#{__MODULE__} starting")

    Process.send_after(self(), :crash, 5000)

    {:ok, nil}
  end

  def handle_info(:crash, _state) do
    raise "boom"
  end
end

Using the application’s main supervisor like this everything works as expected, both processes are restarted when the DB connection (ModuleB) crashes:

def start(_type, _args) do
  children = [
    # Starts a worker by calling: Test.Worker.start_link(arg)
    # {Test.Worker, arg}

    Test.ModuleB,
    Test.ModuleA
  ]

  # See https://hexdocs.pm/elixir/Supervisor.html
  # for other strategies and supported options
  opts = [strategy: :rest_for_one, name: Test.Supervisor]
  Supervisor.start_link(children, opts)
end

But because the ModuleB is not really part of my application but of an external package we use, I don’t really can supervise it in my supervisor. Therefore my idea was to link ModuleA to ModuleB like so (and also changed my supervisors strategy back to :one_for_one):

defmodule Test.ModuleA do
  use GenServer

  def start_link(opts \\ []) do
    name = Keyword.get(opts, :name, __MODULE__)
    GenServer.start_link(__MODULE__, nil, name: name)
  end

  def init(_) do
    IO.puts("#{__MODULE__} starting")

    Test.ModuleB
    |> Process.whereis()
    |> Process.link()

    {:ok, nil}
  end
end

However this results in a very weird problem where my app completely crashes after two simulated connection problems:

What’s going on? Did I misunderstood process links and/or supervisor config?