fc abstract

Group communication services have been successfully used to construct applications with high availability, dependability, and real-time responsiveness requirements. Flow-control techniques enable group members to manage their local buffers, which they use to temporarily store multicast updates. Despite buffer overflow being one of the main causes of process failures, flow control has not been studied much in literature. We present two general-purpose, flow-control techniques, termed as conservative and optimistic, that can be incorporated in a large number of atomic multicast services. We present discrete event simulation results that compare these two flow-control techniques when incorporated in two different atomic multicast protocols: a positive acknowledgement protocol and a negative acknowledgement protocol. We investigate three performance indices: average delivery time, average stability time, and percentage of time an application multicast request is denied by group members due to flow control. These performance indices have been measured by varying four operating parameters: group size, interarrival time between update arrivals, update arrival pattern, and communication failure frequency. Our measurements show that an optimistic flow-control technique is almost always preferable to a conservative one.