Global Dynamic Supervisor in a cluster

Questions / Help
1

I posted this question on stackoverflow but didn’t get any response:
I have a unique issue that I have not had a need to address in elxir.

I need to use the dynamic supervisor to start (n) amount of children dynamicly in a clustered environment. I am using libcluster to manage the clustering and use the global process registry to lookup the dynamic supervisor pid… Here is what is happening:

global: Name conflict terminating {:packer_supervisor, #PID<31555.1430.0>}

Here is the code for the supervisor:

defmodule EcompackingCore.PackerSupervisor do
  use DynamicSupervisor
  require Logger

  def start_link() do
    DynamicSupervisor.start_link(__MODULE__, :ok, name: {:global, :packer_supervisor})
  end

  def init(:ok) do
    Logger.info("Starting Packer Supervisor")
    DynamicSupervisor.init(strategy: :one_for_one)
  end

  def add_packer(badge_id, packer_name) do
    child_spec = {EcompackingCore.Packer, {badge_id, packer_name}}
    DynamicSupervisor.start_child(:global.whereis_name(:packer_supervisor), child_spec)
  end

  def remove_packer(packer_pid) do
    DynamicSupervisor.terminate_child(:global.whereis_name(:packer_supervisor), packer_pid)
  end

  def children do
    DynamicSupervisor.which_children(:global.whereis_name(:packer_supervisor))
  end

  def count_children do
    DynamicSupervisor.count_children(:global.whereis_name(:packer_supervisor))
  end

end

The issue seems to be that the supervisor is started on both nodes. What would be the best way to handle this? I really need the supervisor to be dynamic so I can manage the worker modules effectively. Possibly a different registry?

Thanks for your help.

2 Likes
2

Hello,

You have a conflict because the process is already started. Instead of having

  def start_link() do
    DynamicSupervisor.start_link(__MODULE__, :ok, name: {:global, :packer_supervisor})
  end

you could have something like:

  :global.trans({__MODULE__ , :packer_supervisor}, fn ->
    case DynamicSupervisor.start_link(__MODULE__, :ok, name: {:global, :packer_supervisor})
      {:ok, pid} ->  {:ok, pid}
      {:error, {:already_started, pid}} ->
        Process.link(pid)
        {:ok, pid}
      error ->  error
end

:global.trans will ensure you don t have concurrency issue while starting the process. It won t crash as it handles the case where the process is already started.

2 Likes
3

Hey, I was wondering wouldn’t the Process.link(pid) in your example mean that this process will remain alive and linked with the original process with the same name, hence having two identical processes?

4

Move your registration to the init function and use this function call
https://erlang.org/doc/man/global.html#register_name-3

Then for the resolve function, you can use random_exit_name/3 if you want it to exit or the notify version if you want to keep the process alive as a backup.
https://erlang.org/doc/man/global.html#random_exit_name-3

5

Process.link is. linked from the caller (parent)

for example B use Process.link and you start_link in A then start B in A too so A is the caller if A die B will die too