Efficient Voting Protocols with Witnesses

Witnesses are small entities recording the state of a replicated object. They are especially useful in voting protocols as these protocols require 2n + 1 replicas to guarantee continuous access to the data in the presence of n site failures but allow some of these replicas to be replaced by a witness. We propose in this paper two orthogonal enhancements to the original voting with witnesses protocol. Our first proposal introduces a new kind of witness that can be stored in volatile memory and is put in a state of temporary amnesia when it recovers from a hardware failure. Our second proposal allows the set of all current replicas to act as a majority even when it only has a minority of the votes; it is then referred to as a leading minority. We show that under standard Markovian assumptions two full replicas and one volatile witness managed by a majority consensus voting protocol recognizing leading minorities provide a data availability comparable to that achieved by three full replicas managed by majority consensus voting.