Not only that, but it is generally discouraged to use the config for libraries (see the infobox at Config — Elixir v1.16.2).
I personally consider one’s “internal” dependencies to be an exception, and so this is my approach to runtime config for such libraries:
in the main app’s runtime.exs:
[SubApp1.RuntimeConfig]
|> Enum.each(&apply(&1, :config))
in the dependency (at lib/runtime_config.exs):
defmodule SubApp1.RuntimeConfig do
def config do
import Config
config :sub_app1,
key: System.get_env("SUB_APP1_key", "default_val")
