def in_flight_request_count(self, node_id=None):
        """Get the number of in-flight requests for a node or all nodes.

        Arguments:
            node_id (int, optional): a specific node to check. If unspecified,
                return the total for all nodes

        Returns:
            int: pending in-flight requests for the node, or all nodes if None
        """
        if node_id is not None:
            conn = self._conns.get(node_id)
            if conn is None:
                return 0
            return len(conn.in_flight_requests)
        else:
            return sum([len(conn.in_flight_requests)
                        for conn in list(self._conns.values())])