Shoal: Improving DAG-BFT latency and robustness

Alexander (Sasha) Spiegelman

The Narwhal system is a state-of-the-art Byzantine fault-tolerant scalable architecture that involves constructing a directed acyclic graph (DAG) of messages among a set of validators in a Blockchain network. Bullshark is a zero-overhead consensus protocol on top of the Narwhal’s DAG that can order over 100,000 transactions per second. Unfortunately, the high throughput of Bullshark comes with a latency price because of the DAG construction, increasing the latency compared to the state-of-the-art leader-based BFT consensus protocols. Sasha Spiegelman (Aptos Labs) introduces Shoal, a protocol-agnostic framework for enhancing Narwhal-based consensus. By incorporating leader reputation and pipelining support, Shoal significantly reduces latency. Moreover, the combination of properties of the DAG construction and the leader reputation mechanism enables the elimination of timeouts in all but extremely uncommon scenarios in practice, a property we call “Prevalent Responsiveness.” Sasha explains how he has integrated Shoal instantiated with Bullshark, the fastest existing Narwhal-based consensus protocol, in an open-source Blockchain project, and provides experimental evaluations demonstrating up to 40% latency reduction in the failure-free executions, and up-to 80% reduction in executions with failures against the vanilla Bullshark implementation.

About the speaker

Sasha is a Blockchain researcher at Aptos. He obtained his PhD from the Technion, where he worked with Idit Keidar on Distributed Systems. Prior to joining Aptos, Alexander worked as a research scientist at Novi (Meta) Research and VMware Research.

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