Adding a new worker to the app supervisor dynamically?

Questions / Help
1

When you have a app that starts the toplevel app supervisor, is there a way to add new workers to it dynamically (while app is running).

This is to avoid having to start/stop the app.

2

Yes You can.

https://hexdocs.pm/elixir/DynamicSupervisor.html

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4

Seems like this works? But will it hold up?

defmodule Application do
  use Application

  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    supervisor = Supervisor.start_link(
      [{DynamicSupervisor, strategy: :one_for_one, name: Supervisor}], 
      strategy: :one_for_one)
    {:ok, _} = DynamicSupervisor.start_child(Supervisor, %{id: QLockTest, start: {QLock, :start_link, [[0], QLockTest]}})
    {:ok, _} = DynamicSupervisor.start_child(Supervisor, %{id: QLockTest2, start: {QLock, :start_link, [[0], QLockTest2]}})

    supervisor
  end
end
5

I think this is what you need https://github.com/slashdotdash/til/blob/master/elixir/dynamic-supervisor-start-children.md

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6

Im wondering if the code I pasted is the same effect, it seems to work in a quick test.

But I am guessing the children wont survive the app_down/up?

Carrying it out into its own module solves that?

7

It is not clear what your use case is…

Dynamic Supervisor should have a module, and be placed in the supervision tree for a start/stop on demand workers.

And to survive the app down/up You would need to know how… maybe retrieve data from db?!

8

The usecase is to replace the main application supervisor with a dynamicsupervisor (or similar) to achieve the effect of being able to add workers to it on demand without having to restart the app.

9

You don’t need to overwrite main supervisor for this… just put a dynamic supervisor in it and let it manage start/stop of dynamic worker.

A supervision tree can be very large, and have many nested level, but for sure I would not touch the main Supervisor :slight_smile:

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