The Hashgraph Protocol: Efficient Asynchronous BFT for High-Throughput Distributed Ledgers

Atomic broadcast protocols are increasingly used to build distributed ledgers. The most robust protocols achieve byzantine fault tolerance (BFT) and operate in asynchronous networks. Recent proposals such as HoneyBadgerBFT (ACM CCS `16) and BEAT (ACM CCS `18) achieve optimal communication complexity, growing linearly as a function of the number of nodes present. Although asymptotically optimal, their practical performance precludes their use in demanding applications. Further performance improvements to HoneyBadgerBFT and BEAT are not obvious as they run two separate sub-protocols for broadcast and voting, each of which has already been optimized. We describe how hashgraph - an asynchronous BFT atomic broadcast protocol (ABFT) - departs in structure from prior work by not using communication to vote, only to broadcast transactions. We perform an extensive empirical study to understand how hashgraph's structure affects performance. We observe that hashgraph can improve latency by an order of magnitude over HoneyBadgerBFT and BEAT, while keeping throughput constant with the same number of nodes; similarly, throughput can increase by up to an order of magnitude while maintaining latency. Furthermore, we test hashgraph's capability for high performance, and conclude that it can achieve sufficiently high throughput and low latency to support demanding practical applications.