Getting a sibling process in Elixir

Question

I have an Elixir/Erlang process tree:

parent (Supervisor)
├── child1 (GenServer)
└── child2 (GenServer)

child1 (a DB client) has information that child2 needs to use. What's a good way to pass a reference from the Supervisor process to child2 so that child2 will always have a valid reference to child1? Do I just need to restructure my app so that child2 is supervised by child1?

Answer 1

The simplest way would probably be for child1 and child2 to be registered under local aliases. You can do that while starting your GenServer by passing the name option:

defmodule Child1 do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end
end

In this case a process backed by the Child1 module will be registered locally under an alias Child1. Then if you want to send it messages you can use that alias instead of a PID.

defmodule Child1 do
  # ...

  def get_data do
    GenServer.call(__MODULE__, :get_data)
  end

  def handle_call(:get_data, _from, state) do
    {:reply, extract_data_from_state(state), state}
  end
end

If you want a more complex solution where for example many different processes of the same type may be registered, take a look at the gproc library