ZPaxos: An Asynchronous BFT Paxos with a Leaderless Synchronous Group

Abstract

Increased resource overhead and low throughput are major issues associated with current BFT consensus systems. EPaxos [48], a leaderless crash fault-tolerant protocol can perform commits quickly within a single phase using a fast quorum provided that the commands are independent. If the commands have dependencies, EPaxos needs to run an additional phase on a majority quorum. As a result, it would lose its throughput due to the number of command instances it needs to keep in memory and the high volume of messages it needs to exchange with other replicas to reach consensus. Alternatively, XPaxos [43] which is a leader-based system solves practical BFT with minimal resources while assuring high throughput. This paper describes an improved consensus mechanism, ZPaxos, which is an enhanced EPaxos embedding two key features of XPaxos i.e., a synchronous group with fault detection, and a recovery protocol under weak asynchrony assumptions. The new protocol achieves consensus in a single round with a leaderless synchronous group which allows the removal and addition of a node at a time whenever it sees a crashed or a byzantine replica while the system continues servicing requests without any significant interruptions throughout view synchrony. Transaction throughput and latency of ZPaxos which can handle both byzantine and crash failures exhibit superior performance to that of EPaxos which handles crash failures only, which are two leaderless core group architectures.