code: def QueueNotification(self, notification=None, timestamp=None, **kw):
      
  
      if notification is None:
        notification = rdf_flows.GrrNotification(**kw)
  
      session_id = notification.session_id
  
      if session_id:
        if timestamp is None:
          timestamp = self.frozen_timestamp
  
        notification.timestamp = timestamp
  
        # We must not store more than one notification per session id and
        # timestamp or there will be race conditions. We therefore only keep
        # the one with the highest request number (indicated by last_status).
        # Note that timestamp might be None. In that case we also only want
        # to keep the latest.
        if timestamp is None:
          ts_str = "None"
        else:
          ts_str = int(timestamp)
        key = "%s!%s" % (session_id, ts_str)
        existing = self.notifications.get(key)
        if existing is not None:
          if existing.last_status < notification.last_status:
            self.notifications[key] = notification
        else:
          self.notifications[key] = notification
docs: Queues a notification for a flow.