Start Application dependencies before main Applciation is started

Questions / Help
1

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:
https://hexdocs.pm/elixir/Application.html#c:start/2

Questions

How can I fix this?

2

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.

3

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.

4

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}
    ]
  end

Yes, this is the case.

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

5

Shouldn’t it be:

Supervisor.start_link(children, opts)   
{:ok, things} = MyDependency.list_things()
1 Like
6

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

7 
with {:ok, pid} <- Supervisor.start_link(children, opts) do
  {:ok, things} = MyDependency.list_things()
  {:ok, pid}
end

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

2 Likes