Publish/subscribe on your beamlet's shared message bus.

A process subscribes to a topic string, and every message broadcast on that topic is delivered to it as an ordinary process message. A LiveView subscribes in mount/3 and handles the message in handle_info/2; a controller action broadcasts:

def mount(_params, _session, socket) do
  if connected?(socket), do: Host.PubSub.subscribe("notes:updated")
  {:ok, assign(socket, notes: Notes.all())}
end

def handle_info({:note_saved, note}, socket) do
  {:noreply, update(socket, :notes, &[note | &1])}
end

def create(conn, params) do
  note = Notes.save(params)
  Host.PubSub.broadcast("notes:updated", {:note_saved, note})
  json(conn, %{ok: true})
end

Topics are one shared namespace across every agent and module on your beamlet, so prefix them with your domain, e.g. "notes:updated". Subscriptions belong to the calling process and end with it.