Building and Using Quorums Despite any Number of Process of Crashes

Failure detectors of the class denoted Pt eventually suspect all crashed processes in a permanent way (completeness) and ensure that, at any time, no more than n− t−1 alive processes are falsely suspected (accuracy), n being the total number of processes. This paper first shows that a simple combination of such a failure detector with a two-step communication pattern can provide the processes with an interesting intersection property on sets of values. As an example illustrating the benefit and the property that such a combination can provide when designing protocols, a leader-based consensus protocol whose design relies on its systematic use is presented. Then the paper presents a Pt-based protocol that builds quorums in systems where up to t processes can crash with t < n. Key-words: Asynchronous distributed system, Consensus, Distributed algorithm, Fault tolerance, Leader oracle, Quorum, Process crash, Unreliable failure detector.