Zeno: Eventually Consistent Byzantine-Fault Tolerance

Many distributed services are hosted at large, shared, geographically diverse data centers, and they use replication to achieve high availability despite the failure of an entire data center. Recent events show that non-crash faults occur in these services and may lead to long outages. While Byzantine Fault Tolerance (BFT) could be used to withstand these faults, current BFT protocols can become unavailable if a small fraction of their replicas are unreachable. This is because existing BFT protocols favor strong safety guarantees (consistency) over liveness (availability). This paper presents a novel BFT state machine replication protocol called Zeno, that trades consistency for higher availability. In particular, Zeno replaces linearizability with eventual consistency, where clients can temporarily miss each other’s updates but when the network is stable the states from the individual partitions are merged by having the replicas agree on a total order for the requests. We have built a prototype of Zeno and our evaluation using micro-benchmarks shows that Zeno provides better availability than traditional BFT protocols, and that its impact on performance is low, even when partitions occur or heal.