  Blockchain technology has sparked renewed interest in planetary-scale
Byzantine fault-tolerant (BFT) state machine replication (SMR). While recent
works have mainly focused on improving the scalability and throughput of these
protocols, few have addressed latency. We present FlashConsensus, a novel
transformation for optimizing the latency of quorum-based BFT consensus
protocols. FLASHCONSENSUS uses an adaptive resilience threshold that enables
faster transaction ordering when the system contains few faulty replicas. Our
construction exploits adaptive weighted replication to automatically assign
high voting power to the fastest replicas, forming small quorums that
significantly speed up consensus. Even when using such quorums with a smaller
resilience threshold, FlashConsensus still satisfies the standard SMR safety
and liveness guarantees with optimal resilience, thanks to the judicious
integration of abortable SMR and BFT forensics techniques. Our experiments with
tens of replicas spread in all continents show that FLASHCONSENSUS can order
transactions with finality in less than 0.4s, half the time of a PBFT-like
protocol (with optimal consensus latency) in the same network, and matching the
latency of this protocol running on the theoretically best possible internet
links (transmitting at 67% of the speed of light).
