Harmonia: Enhancing Liveness in Two-Phase HotStuff without Sacrificing Core Properties

HotStuff stands out as the first Byzantine Fault Tolerant (BFT) state machine replication protocol to achieve linear communication complexity while maintaining optimistic responsiveness and fairness. However, it is required at least three phases to commit a block, which leads to high latency. Recent studies have focused on developing two-phase variants of HotStuff to reduce its block commitment latency. Nonetheless, two-phase HotStuff suffers from the liveness issue, forcing these protocols to sacrifice one of the three fundamental properties of HotStuff when addressing this problem, thereby facing a trade-off among these properties. In this paper, we introduce a novel protocol, Harmonia, which, to the best of our knowledge, is the first BFT protocol to simultaneously achieve linear communication complexity, optimistic responsiveness, fairness, and the minimum commit latency of two phases. Harmonia overcomes the liveness issue of two-phase HotStuff through its specialized block unlocking mechanism while preserving all the original properties of HotStuff. Experimental results show that Harmonia consistently outperforms HotStuff across various metrics.