Consensus algorithms deployed in the crash fault tolerant setting chose a leader-based architecture in order to achieve the lowest latency possible. However, when deployed in the wide area they face two key robustness challenges. First, they lose liveness when the network is unreliable because they rely on timeouts to find a leader. Second, they cannot have a high replication factor because of the high load imposed on the leader-replica making it a bottleneck. This effectively limits the replication factor allowed, for a given level of throughput, thus lowering the fault tolerance threshold. In this paper, we propose RACS and SADL, a modular state machine replication algorithm that addresses these two robustness challenges. To achieve robustness under adversarial network conditions, we propose RACS, a novel crash fault-tolerant consensus algorithm. RACS consists of two modes of operations: synchronous and asynchronous, that always ensure liveness. RACS leverages the synchronous network to minimize the communication cost to O(n) and matches the lower bound of O(n2) at adversarial-case executions. To avoid the leader bottleneck and to allow higher replication factor, without sacrificing the throughput, we then propose SADL, a novel consensus-agnostic asynchronous dissemination layer. SADL separates client command dissemination from the critical path of consensus and distributes the overhead evenly among all the replicas. The combination of RACS and SADL (SADL-RACS) provides a robust and high-performing state machine replication system. We implement and evaluate RACS and SADL-RACS in a wide-area deployment running on Amazon EC2.

翻译：在崩溃容错场景下部署的共识算法通常采用基于领导者的架构以尽可能降低延迟。然而，当部署于广域网时，它们面临两个关键的鲁棒性挑战：首先，在网络不可靠时，依赖超时机制寻找领导者会导致系统丧失活跃性；其次，领导者副本承受的高负载会形成瓶颈，使得系统无法支持高复制因子。这实质上限制了特定吞吐量水平下允许的复制因子，从而降低了容错阈值。本文提出RACS和SADL——一种应对上述两个鲁棒性挑战的模块化状态机复制算法。为在对抗性网络条件下实现鲁棒性，我们设计了新型崩溃容错共识算法RACS。RACS包含同步与异步两种运行模式，可始终确保系统活跃性。该算法利用同步网络将通信开销优化至O(n)，并在对抗性执行场景中达到O(n²)的理论下界。为消除领导者瓶颈并支持更高复制因子且不牺牲吞吐量，我们进一步提出SADL——一种与共识无关的异步传播层。SADL将客户端命令传播从共识关键路径中分离，并将负载均匀分配至所有副本。RACS与SADL的组合（SADL-RACS）构建了兼具鲁棒性与高性能的状态机复制系统。我们在Amazon EC2广域网部署环境中对RACS和SADL-RACS进行了实现与评估。