anchor.diff:  -record(state, {
     max_backends,             % maximum number of backends on this host
     available_ports,          % available ports on this node
  +  app_pids,                 % pids that are running the apps
     current_backends  = []    % backends hosted on this app_handler
   }).
   -define(SERVER, ?MODULE).
  
positive.code:   
    {ok, #state{
      max_backends = MaxBackends,
      available_ports = AvailablePorts
    }}.
  
  
negative.code: %%                                      {stop, Reason, State}
  %% Description: Handling call messages
  %%-------------------------------------------------------------------- 
  handle_call({start_new_instance, App, From}, _From, #state{
                                        current_backends = CurrBackends,
                                        app_pids = AppPids, 
                                        available_ports = AvailablePorts} = State) ->
    Port = hd(AvailablePorts),
    ?LOG(info, "internal_start_new_instance(~p, ~p, ~p)", [App, Port, From]),
    Backend = internal_start_new_instance(App, Port, From),
    NewAvailablePorts = lists:delete(Port, AvailablePorts),
    {reply, ok, State#state{
      current_backends = [Backend|CurrBackends], 
      available_ports = NewAvailablePorts,
      app_pids = [{Backend#backend.pid, Backend}|AppPids]}
    };
    
  handle_call({stop_instance, Backend, App, From}, _From, #state{current_backends = CurrBackends, available_ports = AvailablePorts} = State) ->
    Port = Backend#backend.port,
  