code: def _controller(self):
          
          def server_controller(cmd_id, cmd_body, _):
              """Server controler."""
              if not self.init_logginig:
                  # the reason put the codes here is because we cannot get
                  # kvstore.rank earlier
                  head = '%(asctime)-15s Server[' + str(
                      self.kvstore.rank) + '] %(message)s'
                  logging.basicConfig(level=logging.DEBUG, format=head)
                  self.init_logginig = True
  
              if cmd_id == 0:
                  try:
                      optimizer = pickle.loads(cmd_body)
                  except:
                      raise
                  self.kvstore.set_optimizer(optimizer)
              else:
                  print("server %d, unknown command (%d, %s)" % (
                      self.kvstore.rank, cmd_id, cmd_body))
          return server_controller
docs: Return the server controller.