chat.ex

defmodule Chat do
  use Application

  def start(_type, _args) do
    Chat.Supervisor.start_link
  end
end

server.ex

defmodule Chat.Server do
  use GenServer

  #client side
  def start_link do
    GenServer.start_link(__MODULE__, [], name: :chat_room)
  end

  def get_masgs do
    GenServer.call(:chat_room, :get_masgs)
  end

  def add_msg(msg) do
    GenServer.cast(:chat_room, {:add_msg, msg})
  end


  #server side/ callback finction
  def init(msgs) do
    {:ok, msgs}
  end

  def handle_call(:get_masgs, _from, msgs) do
    {:reply, msgs, msgs}
  end

  def handle_cast({:add_msg, msg}, msgs) do
    {:noreply, [msg | msgs]}
  end


end

supervisor.ex

defmodule Chat.Supervisor do
  use Supervisor

def start_link do
  Supervisor.start_link(__MODULE__, [])
end


def init(_) do
  children = [
    worker(Chat.Server, [])
  ]

  supervise(children, strategy: :one_for_one)
end

end