Synchronous solutions for Byzantine Fault Tolerance (BFT) can tolerate up to minority faults. In this work, we present Sync HotStuff, a surprisingly simple and intuitive synchronous BFT solution that achieves consensus with a latency of 2Δ in the steady state (where Δ is a synchronous message delay upper bound). In addition, Sync HotStuff ensures safety in a weaker synchronous model in which the synchrony assumption does not have to hold for all replicas all the time. Moreover, Sync HotStuff has responsiveness, i.e., it advances at network speed when less than one-quarter of the replicas are not responding, a small sacrifice in availability compared with optimal partially synchronous solutions. Borrowing from practical partially synchronous BFT solutions, Sync HotStuff has a two-phase leader-based structure, and has been fully prototyped under the standard synchronous message delay assumption. When tolerating a single fault, Sync HotStuff achieves a throughput of over 150 Kops/sec under typical network assumptions, which is comparable to the best known partially synchronous solutions.





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