def describe_app_blb_rs(self, blb_id, sg_id,
                            marker=None, max_keys=None,
                            config=None):
        """
        Query the list of backend servers under the specified LoadBalancer
        and server group

        :param blb_id:
            Id of LoadBalancer
        :type blb_id:string

        :param sg_id:
            id of the server group
        :type sg_id:string

        :param marker:
            The optional parameter marker specified in the original
            request to specify where in the results to begin listing.
            Together with the marker, specifies the list result which
            listing should begin. If the marker is not specified,
            the list result will listing from the first one.
        :type marker: string

        :param max_keys:
            The optional parameter to specifies the max number of
            list result to return.
            The default value is 1000.
        :type max_keys: int

        :param config:
        :type config: baidubce.BceClientConfiguration

        :return:
        :rtype baidubce.bce_response.BceResponse
        """
        path = utils.append_uri(self.version, 'appblb', blb_id, 'blbrs')
        params = {}
        params[b'sgId'] = compat.convert_to_string(sg_id)

        if marker is not None:
            params[b'marker'] = marker
        if max_keys is not None:
            params[b'maxKeys'] = max_keys

        return self._send_request(http_methods.GET, path, params=params,
                                  config=config)