Causal multicasts in overlapping groups: towards a low cost approach

Concepts of groups (to structure processes) and causality (to structure, send, and deliver events of messages) are of major importance in the design of distributed systems. By combining both concepts, the ISIS system defines causal multicasts in overlapping groups. The authors present a simple and efficient protocol that implements such causal multicasts. It compares favorably to the ISIS protocol, as it uses only one vector of integers (the size of this vector being the total number of groups) to timestamp messages. This low cost in the size of timestamps is obtained by using (sometimes) additional resynchronization messages. It is shown that there is a tradeoff between the delay in the delivery of messages and the size of timestamps carried by messages for timestamp-based protocols implementing causal order