In general, consensus-based systems operate using majority quorums, i.e., a group of replicas may tolerate failures (if Byzantine fault tolerance, in which the system is resistant to replicas returning incorrect results, is required, then replicas may tolerate failures [Cas99]). For non-Byzantine failures, the minimum number of replicas that can be deployed is three — if two are deployed, then there is no tolerance for failure of any process. Three replicas may tolerate one failure.
