I have an umbrella app setup as described the docs, and all is compiling nicely.
One question that I'm left with is how does the Phoenix app in this scenario interact with GPIO? elixir_ale will only compile on the target device, and I can't reference the Nerves project via the Phoenix app.
Scenario: I click the proverbial button to toggle a light source. The web request will hit the controller; how to I reference the underlying Nerves application to interact with the GPIO?
You'll want to create some flavor of message broker that acts as an intermediary. The OTP applications that you run on a Nerves-based device are running on the same BEAM so all of the regular messaging features and patterns are available to you.
You can use the pub-sub mechanism in Phoenix channels to effectively broker events between a JS frontend and the BEAM applications running on your Nerves-based device. There are Phoenix channel client implementations available such as PhoenixChannelClient that you can use from any OTP application for this purpose.
If that's too heavy for what you're doing, a simpler alternative would be to globally register the name of a GenServer in your Nerves app and then send messages to it directly from your Phoenix controller.
A third solution that might be worth considering that is somewhere in between the two above on the level of investment/complexity: if you like the idea of the pub-sub model but you still want processes to only communicate internally within the BEAM, you can use pg2 to emulate pub-sub topics and subscriptions by using named process groups. Interested subscriber processes can join globally named groups which publisher processes can then query for the list of members and then proceed to send them messages.