genserver.ex

defmodule Core.SyslogService do
  use GenServer

  require Logger

  def start_link(ref, socket, transport, opts \\ []) do
    Logger.debug "Trying to start_link"
    :proc_lib.start_link(__MODULE__, :init, [ref, socket, transport, opts])
  end


  def init(ref, socket, transport, opts \\ []) do
    Logger.debug "Got start link request"
    :ok = :proc_lib.init_ack({:ok, self()})
    Logger.debug "Start link request acked"
    Process.send(self(), :accept_ack, [])
    Logger.debug "Sent message to self to start ranch acking"
    {:ok, %{
      ref: ref,
      socket: socket,
      transport: transport
      }}
  end

  def handle_info(:accept_ack, state = %{ref: ref, transport: transport, socket: socket}) do
    Logger.debug "Accepting ack and setting state to :once"
    :ok = :ranch.accept_ack(ref)
    :ok = transport.setopts(socket, active: :once)
    {:noreply, state}
  end

  def handle_info({:tcp, socket, msg}, state = %{transport: transport, socket: socket}) do
    Logger.debug "Got a tcp message"
    :ok = transport.setopts(socket, active: :once)
    parse_line(msg)
    {:noreply, state}
  end

  defp parse_line(line) do
    Logger.debug "Got a tcp package!"
    Logger.debug "#{inspect line}"
  end

end

supervisor.ex

#started with supervisor(Core.SyslogListener, []) from the main Application file

defmodule Core.SyslogListener do

  require Logger
  
  @port Application.get_env(:core, :syslog_port, 514)
  def start_link do
    opts = [port: @port]
    Logger.debug "Setting up syslog listener at port #{@port}"
    {:ok, _} = :ranch.start_listener(:syslog_listener, 100, :ranch_tcp, opts, Core.SyslogService, [])
  end
end

output:

[debug] module=Core.SyslogListener function=start_link/0 Setting up syslog listener at port 514
Interactive Elixir (1.3.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> [debug] module=Core.SyslogService function=start_link/4 Trying to start_link
iex(1)> [debug] module=Core.SyslogService function=init/4 Got start link request
[debug] module=Core.SyslogService function=init/4 Start link request acked
[debug] module=Core.SyslogService function=init/4 Sent message to self to start ranch acking
iex(1)> [debug] module=Core.SyslogService function=start_link/4 start_link result: {:ok, #PID<0.1092.0>}
iex(1)>