docs: def notify_observers(self, event=None, msg=None):
          """Notify observers."""
          for observer, events in list(self._observers.items()):
              # LOG.debug("trying to notify the observer")
              if events is None or event is None or event in events:
                  try:
                      observer(self, event, msg)
                  except (Exception,) as ex:  # pylint: disable=broad-except
                      self.unregister_observer(observer)
                      errmsg = "Exception in message dispatch: Handler '{0}' unregistered for event '{1}'  ".format(
                          observer.__class__.__name__, event)
                      LOG.error(errmsg, exc_info=ex)