Quorum Tree Abstractions of Consensus Protocols

Abstract Distributed algorithms solving agreement problems like consensus or state machine replication are essential components of modern fault-tolerant distributed services. They also notoriously hard to understand and reason about. Their complexity stems from the different assumptions on environment they operate with, i.e., process network link failures, Byzantine failures etc. In this paper, we propose a novel abstract representation dynamics such protocols which focuses quorums responses (votes) request (proposal) that form during run protocol. We show focusing quorums, protocol can be viewed as working over tree structure where branches represent possible outcomes protocol, goal being stabilize choice fixed branch. This abstraction resembles description recent used in Blockchain infrastructures, e.g., supporting Bitcoin Hotstuff. supports reasoning about safety various algorithms, Paxos, PBFT, Raft, HotStuff, uniform way. general, it provides induction based argument for proving safe.