Abstract
Multiple peer processes are cooperating by exchanging messages in group communication. In this paper, we discuss a hierarchical group of processes to reduce communication and computation overheads for large number of processes, which is composed of disjointed subgroups. Each subgroup has gateway processes which communicate with other subgroups. Even if messages are causally ordered in a subgroup, the messages may not be causally ordered in another subgroup. We discuss how to globally causally order messages by using local mechanisms to causally order messages in subgroups. In addition, we present a protocol for a hierarchical group.