Ridge: High-Throughput, Low-Latency Atomic Multicast

It has been shown that the highest throughput for broadcasting messages in a point-to-point network is achieved with a ring topology. Although several ring-based group communication protocols have benefited from this observation, broadcasting messages along a ring overlay may lead to high latencies: In a system with n processes, at least n-1 communication steps are necessary for all processes to deliver a message. In this work, we argue that it is possible to reach optimal throughput without resorting to a ring topology (or to ip-multicast, typically unavailable in wide-area networks). This can be done by routing messages through different paths, while carefully using the available bandwidth at each process, resulting in a significantly lower latency for every message (potentially a single communication step). Based on this idea, we propose Ridge, a Paxos-based atomic multicast protocol where each message is initially forwarded to a single destination, the distributor, whose responsibility is to propagate the message to all other destinations. To utilize all bandwidth available in the system, processes alternate in the role of distributor. By doing this, the maximum system throughput matches that of ring-based protocols, with a latency that is not significantly dependent on the size of the system. Finally, we show that Ridge can also deliver messages optimistically, with even lower latency.