CONSENSUS IN THE PRESENCE OF PARTIAL SYNCHRONY

The concept of partial synchrony in a distributed system is introduced. Partial synchrony lies between the cases of a synchronous system and an asynchronous system. In a synchronous system, there is a known fixed upper bound DELTA on the time required for a message to be sent from one processor to another and a known fixed upper bound PHI on the relative speeds of different processors. In an asynchronous system no fixed upper bounds DELTA and PHI exist. In one version of partial synchrony, fixed bounds DELTA and PHI exist, but they are not known a priori. The problem is to design protocols that work correctly in the partially synchronous system regardless of the actual values of the bounds DELTA and PHI . In another version of partial synchrony, the bounds are known, but are only guaranteed to hold starting at some unknown time T, and protocols must be designed to work correctly regardless of when time T occurs. Fault-tolerant consensus protocols are given for various cases of partial synchrony and various fault models.