def instance(cls, interval=5):
        '''
        Returns existing instance of messenger. If one does not exist it will
        be created and returned.

        :param int interval:
            Number of miliseconds that represents interval when messages will
            be processed.
            Note that this parameter will be used only the first time when
            instance is requested, every other time it will be ignored
            because existing instance of :class:`._Messenger` is returned.
        '''
        if not cls._instance:
            cls._instance = _Messenger(interval)
        return cls._instance