Causal Order Protocols for Group Communication

Group communication (or multicast) is a powerful abstraction that can be used whenever groups of distributed processes cooperate for the execution of a given task. Due to the uncertainties inherent to distributed systems (emerging from communication or process failures), multicast messages may arrive in an inconsistent order at different destination processes. Further complications will arise when groups overlap (i.e. a process is allowed to belong to distinct groups). In this paper, we address the problem of causal order violation in message delivery. We present three causal order protocols for overlapping groups with different trade-offs between message space overhead2 and message delivery delays. The first protocol favours message space overhead (small one) with a longer message delivery delay. The second protocol favours message delivery delay but with a larger message space overhead. The third protocol is a compromise solution between message delivery delay and message space overhead, comparing favourably with respect to existing solutions. Key works: group communication, multicast protocols, causal order, logical clocks, distributed algorithms, distributed systems, network protocols.