@Fire-Dragon-DoL Have you seen the approach outlined in Application Layering - A Pattern for Extensible Elixir Application Design?
I have used a similar design where a behaviour is defined for each Umbrella app’s public API module and the implementation module is configured via the Mix environment. For :dev and :prod it uses the default implementation, for :test it uses a Mox mock.
This allows the tests within one app to stub responses for calls to any other app. You can also use Mox’s stub_with/2 function to forward calls to the mock on to the actual implementation if necessary. This can be a useful approach where you can mock some calls, but allow other calls to be handled as normal.
