Elixir :invalid_child_spec for supervised process. Can't figure out why

Question

I'm working on implementing a supervisor for the first time and I'm running into issues that I can't figure out from the documentation. Specifically, when I try to start my process using SlowRamp.flood I get {:error, {:invalid_child_spec, []}}.

This is a very simple application and was made using mix new slow_ramp --sup.

The main file in ./lib/slow_ramp.ex is:

defmodule SlowRamp do
  use Application

  # See http://elixir-lang.org/docs/stable/elixir/Application.html
  # for more information on OTP Applications
  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    children = [
      worker(SlowRamp.Flood, [])
    ]

    # See http://elixir-lang.org/docs/stable/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: SlowRamp.Supervisor]
    Supervisor.start_link(children, opts)
  end

  def flood do
    Supervisor.start_child(SlowRamp.Supervisor, [])
  end
end

My child function / file is in ./lib/SlowRamp/flood.ex and looks like this:

defmodule SlowRamp.Flood do
  def start_link do
    Task.start_link(fn -> start end)
  end

  defp start do
    receive do
      {:start, host, caller} ->
        send caller, System.cmd("cmd", ["opt"])
    end
  end
end

Any help would be very much appreciated. Thanks!

Answer 1

The problem is on

Supervisor.start_child(SlowRamp.Supervisor, [])

You need a valid child specification, like:

def flood do
  import Supervisor.Spec
  Supervisor.start_child(SlowRamp.Supervisor, worker(SlowRamp.Flood, [], [id: :foo]))
end

Thats the reason its telling that the child spec is invalid