Steelhead: Interleaving Partially Synchronous and Asynchronous Commit Rules on a Shared DAG

2026-09-24

AI Summary

This study addresses the inherent tension in consensus protocols, which are efficient under partial synchrony yet prone to stalling in asynchronous environments. To resolve this, we propose a hybrid consensus algorithm built upon a shared directed acyclic graph (DAG). The approach alternates between partially synchronous and asynchronous commit rules, dynamically adjusting execution periods to balance performance with liveness. It enables adaptive mode switching without additional communication overhead and incorporates a hidden leader mechanism to guarantee asynchronous liveness. The protocol is formally verified using Lean 4. Experimental results demonstrate that the proposed method achieves throughput comparable to synchronous protocols under favorable network conditions while preserving liveness approaching that of asynchronous protocols in adverse environments, thereby effectively reconciling efficiency with robustness.

Abstract

Dual-mode consensus protocols are fast when the network is partially synchronous and remain live under asynchrony. We introduce Steelhead, a dual-mode mechanism that composes a partially synchronous and an asynchronous commit rule over one DAG: every k-th round is decided by the asynchronous rule, whose leader a common coin reveals after the votes, and all other rounds by the partially synchronous rule. Every interval, validators replay the committed DAG under each candidate period, adopt the one with the fewest expected message delays, and fall back to k = 1 when the output stalls; the asynchronous rule applied to the coin rounds alone keeps the protocol live. Steelhead sends no message beyond the DAG's blocks, not even to agree on the period, and opens a coin only on the rounds that need a hidden leader. It is generic over pairs of DAG commit rules that share a committee; we instantiate it with Mysticeti and Mahi-Mahi at n >= 3f+1 and with the two variants of BlueBottle at n >= 5f+1. We prove it safe and live, and provide mechanized proofs in Lean 4. In simulation, Steelhead matches the partially synchronous protocol in a healthy network, stays close to the asynchronous one when network conditions stall the partially synchronous one, and adapts quickly in both directions.

Problem

• dual-mode consensus
• partially synchronous
• asynchronous
• DAG
• liveness

Innovation

• Dual-mode consensus
• Shared DAG
• Adaptive commit rules
• Common coin
• Mechanized proofs