HP: Hybrid Paxos for WANs

Implementing a fault-tolerant state machine boils down to reaching consensus on a sequence of commands. In wide area networks (WANs), where network delays are typically large and unpredictable, choosing the best consensus protocol is difficult. During normal operation, Classic Paxos (CP) requires three message delays, whereas Fast Paxos (FP) requires only two. However, when collisions occur, due to interfering commands issued concurrently, FP requires four extra message delays. In addition, FP uses larger quorums than CP. Therefore, CP can outperform FP in many situations. We present Hybrid Paxos (HP), a consensus protocol that combines the features of FP and CP. HP implements generalized consensus, where collisions are caused only by interfering commands. In the absence of collisions HP requires two message delays, and only one extra message delay otherwise. Our evaluation shows that when collisions are rare, the latency of HP reaches the theoretical minimum. When collisions are frequent, HP behaves like CP.