Start Application dependencies before main Applciation is started


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).


Currently I have this in application.ex:

    children = [
      {Phoenix.PubSub, name: PubSub},

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

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

However, MyDependency.list_things() crashes with:

[notice] Application web_interface exited: exited in: WebInterface.Application.start(:normal, [])
    ** (EXIT) exited in:, :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.


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]

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:


How can I fix this?

Where does :my_dependency come from. Unless it’s an elixir core or otp application it should generally be listed in your mix.exs deps. :extra_applications is for those elixir core/otp applications you might depend on.

How will this work if MyDependency starts a named process? The process will start when the dependency starts and then you’ll try to start it again in your supervision tree.

It is an umbrella library. Yes, it does have a GenServer but it is not an OTP application.
I have it in my mix deps function as well:

  defp deps do
      # Phoenix
      {:phoenix, "~> 1.7.2"},
      {:phoenix_html, "~> 3.3"},
      {:phoenix_live_reload, "~> 1.2", only: :dev},
      {:phoenix_live_view, "~> 0.18.16"},
      {:floki, "~> 0.34"},
      {:esbuild, "~> 0.7", runtime: Mix.env() == :dev},
      {:tailwind, "~> 0.2.0", runtime: Mix.env() == :dev},
      {:telemetry_metrics, "~> 0.6"},
      {:telemetry_poller, "~> 1.0"},
      {:jason, "~> 1.2"},
      {:plug_cowboy, "~> 2.5"},

      # Umbrella deps
      {:manager, in_umbrella: true}

Yes, this is the case.

I am guessing the issue here is that the process is not yet started (judging from the error)

Shouldn’t it be:

Supervisor.start_link(children, opts)   
{:ok, things} = MyDependency.list_things()
1 Like

The start function/callback must return a link to a Supervisor. That won’t be the case if I follow your suggestion.

with {:ok, pid} <- Supervisor.start_link(children, opts) do
  {:ok, things} = MyDependency.list_things()
  {:ok, pid}

You need to start the supervisor (and therefore its children) before you can expect the children to run.