code: def instance():
          
          if not NotificationDB.__instance:
              if settings.NOTIFICATION_DB_PATH:
                  NotificationDB.__instance = NotificationDB(settings.notification_leveldb_path)
              else:
                  logger.info("Notification DB Path not configured in settings")
          return NotificationDB.__instance
docs: Singleton accessor for NotificationDB
          Returns:
              NotificationDB: The current instance of the NotificationDB