Background

I am trying to ensure an application’s dependency is started before the main application is, whilst also having said dependency as part of the Supervision Tree (as a child).

Problem

Currently I have this in application.ex:

    children = [
      Telemetry,
      {Phoenix.PubSub, name: PubSub},
      Endpoint,
      MyDependency
    ]

    {:ok, things} = MyDependency.list_things()

    IO.inpsect(things, label: "THINGS")
    opts = [strategy: :one_for_one, name: WebInterface.Supervisor]
    Supervisor.start_link(children, opts)
  end

However, MyDependency.list_things() crashes with:

[notice] Application web_interface exited: exited in: WebInterface.Application.start(:normal, [])
    ** (EXIT) exited in: GenServer.call(MyDependency.Runtime.Worker, :list_things, 5000)
        ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started

I understand this happens because the MyDependency dependency is not started, and thus I cannot call it.

Research

I tried adding this dependency to extra_applications, but to no avail:

def application do
    [
      mod: {WebInterface.Application, []},
      extra_applications: [:logger, :runtime_tools, :my_dependency]
    ]
  end

I also read the docs for the Application start function, hoping to find some way to defer the start of my main app to a later stage, but I couldn’t find it:

Questions

How can I fix this?