More than one Supervisor in Elixir Application

Question

I want to register 2 root supervisors in my Phoenix Application, are there any reasons to not to do this?

Example is below

defmodule MyApp.Application do
  use Application
  import Supervisor.Spec

  def start(_type, _args) do
    children_sv1 = [
      supervisor(MyApp.Repo, []),
      ... # other workers
    ]

    children_sv2 = [
      supervisor(MyApp.Endpoint, []),
      ... # other workers
    ]

    opts1 = [strategy: :one_for_one, name: MyApp.Supervisor1]
    opts2 = [strategy: :one_for_one, name: MyApp.Supervisor2]

    Supervisor.start_link(children_sv1, opts)
    Supervisor.start_link(children_sv2, opts)
  end
end

Answer 1

One reason not to do this is that your particular application is itself being supervised as part of an OTP-wide supervision tree. The return value for start is used by the top-level Application supervisor to supervise your particular app.

If you explicitly assign results to the supervisor calls above, you'll see that you're dropping information:

{:ok, sup1_pid} = Supervisor.start_link(children_sv1, opts)
{:ok, sup2_pid} = Supervisor.start_link(children_sv2, opts)
{:ok, sup2_pid}

This means that while the first supervisor will be linked to whatever process is starting your app (e.g. the top-level Application supervisor), it will not appear in the output of functions that look at supervision trees, like Supervisor.count_children. During normal operation, this shouldn't be a big deal, but if anything goes wrong, you yourself might have a hard time dissecting the issue, and OTP tools that depend on a proper supervision hierarchy might behave strangely in the face of this setup. Gracefully stopping the application might or might not be a problem.

It's always a safer bet to specify your entire supervision tree as a proper tree -- you'll end up with a much more predictable application. If you need granular control over which branches of the tree are indepenent and how they should be restarted, child specifications are your best friend.