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With the emergence of cross-organization attack-prone byzantine fault-tolerant (BFT) systems, so-called Blockchains, providing asynchronous state machine replication (SMR) solutions is no longer a theoretical concern. This paper introduces ACE: a general framework for the software design of fault-tolerant SMR systems. We first propose a new leader-based-view (LBV) abstraction that encapsulates the core properties provided by each view in a partially synchronous consensus algorithm, designed according to the leader-based view-by-view paradigm (e.g., PBFT and Paxos). Then, we compose several LBV instances in a non-trivial way in order to boost asynchronous liveness of existing SMR solutions. ACE is model agnostic - it abstracts away any model assumptions that consensus protocols may have, e.g., the ratio and types of faulty parties. For example, when the LBV abstraction is instantiated with a partially synchronous consensus algorithm designed to tolerate crash failures, e.g., Paxos or Raft, ACE yields an asynchronous SMR for $n = 2f+1$ parties. However, if the LBV abstraction is instantiated with a byzantine protocol like PBFT or HotStuff, then ACE yields an asynchronous byzantine SMR for $n = 3f+1$ parties. To demonstrate the power of ACE, we implement it in C++, instantiate the LBV abstraction with a view implementation of HotStuff -- a state of the art partially synchronous byzantine agreement protocol -- and compare it with the base HotStuff implementation under different adversarial scenarios. Our evaluation shows that while ACE is outperformed by HotStuff in the optimistic, synchronous, failure-free case, ACE has absolute superiority during network asynchrony and attacks. |
