docs: def start(self, io_loop):
          """
          Run the ``before_run`` callbacks and queue to ``on_start`` callbacks.
  
          :param tornado.ioloop.IOLoop io_loop: loop to start the app on.
  
          """
          for callback in self.before_run_callbacks:
              try:
                  callback(self.tornado_application, io_loop)
              except Exception:
                  self.logger.error('before_run callback %r cancelled start',
                                    callback, exc_info=1)
                  self.stop(io_loop)
                  raise
  
          for callback in self.on_start_callbacks:
              io_loop.spawn_callback(callback, self.tornado_application, io_loop)