docs: def _setup_backend(self):
          ''' Send sockets facing services (e.g. online monitor, other forwarders)
          '''
          self.backends = []
          self.be_poller = zmq.Poller()
          for actual_backend_address in self.backend_address:
              # publisher or client socket
              actual_backend = (actual_backend_address,
                                self.context.socket(self.backend_socket_type))
              # Wait 0.5 s before termating socket
              actual_backend[1].setsockopt(zmq.LINGER, 500)
              # Buffer only 100 meassages, then throw data away
              actual_backend[1].set_hwm(10)
              actual_backend[1].bind(actual_backend_address)
              self.backends.append(actual_backend)
              if self.backend_socket_type != zmq.DEALER:
                  self.be_poller.register(actual_backend[1], zmq.POLLIN)
          self.be_stop = threading.Event()