The Timewheel Asynchronous Group Communication Protocol

We propose a membership and a reliable broadcast protocol that support several diierent semantics simultaneously and provide good overall performance. The broadcast protocol supports three atomicity semantics|weak, strong, and strict atomicity|and three ordering semantics| unordered, total order, and time order. These result in nine group communicationsemantics and an application can dynamically choose the desired atomicity and order semantics when broadcasting an update. These protocols provide fast delivery and stability times, use a small number of messages to accomplish a broadcast or a membership change, distribute evenly the processing load among group members, and use eecient ow control techniques. The good overall performance is maintained in the absence of any failures and normal update arrival rates, in the presence of communication or process failures, and when updates arrive at a very fast or very slow rate.