def get_ring(self, tree_map, parent_map):
        """
        get a ring connection used to recover local data
        """
        assert parent_map[0] == -1
        rlst = self.find_share_ring(tree_map, parent_map, 0)
        assert len(rlst) == len(tree_map)
        ring_map = {}
        nslave = len(tree_map)
        for r in range(nslave):
            rprev = (r + nslave - 1) % nslave
            rnext = (r + 1) % nslave
            ring_map[rlst[r]] = (rlst[rprev], rlst[rnext])
        return ring_map
get a ring connection used to recover local data