AlanMcCann

Supervisor.start_child error :undef?

I’m having trouble getting Supervisor.start_child to work and I get the following error when running these commands (the supervisor is started automatically. Any help is appreciated. I’m likely missing something simple.

import Supervisor.Spec
sup_pid = sup_pid = Process.whereis(App.SessionSupervisor)
Supervisor.start_child(sup_pid,["a"])

{:error,
   {:EXIT,
    {:undef,
      [{App.Session, :start_link, [["a"]], []},
      {:supervisor, :do_start_child_i, 3, [file: 'supervisor.erl', line: 374]},
      {:supervisor, :handle_call, 3, [file: 'supervisor.erl', line: 399]},
      {:gen_server, :try_handle_call, 4, [file: 'gen_server.erl', line: 629]},
      {:gen_server, :handle_msg, 5, [file: 'gen_server.erl', line: 661]},
      {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 240]}]}}}

session_supervisor.ex

defmodule App.SessionSupervisor do
  use Supervisor

  def start_link do
    Supervisor.start_link(__MODULE__, :ok, name: App.SessionSupervisor)
  end

  def init(:ok) do
    children = [worker(App.Session, [], restart: :transient)]
    worker(App.Session, [], restart: :transient)

    supervise(children, strategy: :simple_one_for_one)
  end
end

session.ex

defmodule App.Session do

  use GenServer

  @doc """
  Starts a new session.
  """
  def start_link(uuid, opts \\ []) do
    GenServer.start_link(__MODULE__, %{uuid: uuid})
  end

… rest of the code.

7 comments

#supervisor

0 5688 7

2020-11-12 14:29:48 UTC

Most Liked

scouten

Hi Alan, bumped into this just a few hours ago myself. The docs on this are terrible at telling you what you should provide. (Thankfully, that is a rarity!)

In my case, I had printed a start_link/1 function, but I needed to provide a start_link/2 function. Took me a while to get that right.

In your code, in see three different models name prefixes: App, Jaeger, and Jeager. Perhaps this is as simple as a typo?

Post #2

AlanMcCann

Fix found - formatting the params for start_link as

["uuid", []]

"uuid", []

allows it to properly started a linked child process

>iex -S mix
Erlang/OTP 18 [erts-7.3] [source-d2a6d81] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]

Interactive Elixir (1.3.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> sup_pid = Process.whereis(App.SessionSupervisor)
#PID<0.304.0>
iex(2)> Supervisor.count_children(sup_pid)
%{active: 0, specs: 1, supervisors: 0, workers: 0}
iex(3)> {:ok, pid} = Supervisor.start_child(sup_pid, ["uuid", []])
{:ok, #PID<0.311.0>}
iex(4)> Supervisor.count_children(sup_pid)
%{active: 1, specs: 1, supervisors: 0, workers: 1}

Thanks @NobbZ and @scouten for offering good pointers.

Post #6

Last Post!

vinibrl

For those who might run into this problem, in my case it was a typo in the module name.

child_spec = %{
  id: WrongModuleName, # << typo here!
  start: {WrongModuleName, :start_link, [arg]},
  restart: :transient
}

case DynamicSupervisor.start_child(__MODULE__, child_spec) do
  {:error, {:already_started, pid}} -> {:ok, pid}
  {:ok, pid} -> {:ok, pid}
end

Post #8