As mentioned there is no supervision for or of applications. If the root process of an application terminates the application will terminate. There’s neither an attempt made to restart the application, nor to restart the root process (the pid returned from Application.start/2 is the root process).
What happens as a result of the application terminating depends on its restart type. If :permanent the whole vm will shut down, with :transient the same will happen only if the exit reason of the root process wasn’t :normal otherwise it behaves like :temporary, which means the application termination is reported, but nothing else happens. Application — Elixir v1.20.2
The default for your dependencies and all your apps in an umbrella is :permanent, but you can customize the type in the release settings.
Shoehorn builds on top of those things by defaulting all applications to :temporary unless explicitly mentioned as an :init application, making them permanent and ordering them to start as early as possible.
It also integrates with the application termination reporting to allow you to restart applications. I’d however strongly suggest to exhaust other solutions before going with that one.
