Hydrozoan improves consensus speed with adaptive commit delays
Hydrozoan: Latency-Adaptive DAG Consensus under Mixed Byzantine and Crash Faults
Distributed, Parallel, and Cluster ComputingCryptography and Security
Summary
Reaching agreement quickly and reliably in distributed computer systems is a challenging problem, especially when some parts fail or behave badly. The authors present Hydrozoan, a new method that speeds up agreement when faults are limited, but can still handle more problems by slowing down without getting stuck. Their approach flexibly adapts to different fault conditions and geographic delays, ensuring consensus while keeping high throughput. They prove their method works rigorously and show it performs well in real-world-like network conditions.
What this means in practice
- •For blockchain infrastructure teams: Enable faster transaction commits in geo-distributed blockchains with adaptive fault tolerance and no extra messaging overhead.
- •For cloud platform engineers: Improve distributed database commit latency across multiple regions by tuning the fault tolerance parameters to geographic network conditions.
Authors
Qianyu Yu, Lefteris Kokoris-Kogias, Alberto Sonnino
Abstract
DAG-based consensus protocols can achieve great throughput and the optimal three-message-delay limit for n = 3f+1 consensus. While two-delay protocols exist, they pay with reduced resilience (requiring 5f+1-style committees) or rely on fallbacks that sacrifice the DAG's high throughput. This paper introduces Hydrozoan, the first DAG protocol with a dual commit path under a hybrid fault model of f Byzantine and c crashed validators, on n = 3f+c+2p+1 validators. Leaders commit in two message delays whenever at most p validators are faulty, and in three otherwise, with no extra messages, no view changes, and multiple leaders per round. Both paths are evaluated on the same DAG, using a novel graded indirect rule to reconcile them so that every honest validator reaches the same decision. We show that under geo-distributed conditions, which path is faster is a property of geography rather than the protocol, as rounds reaching a remote region cost far more than those that do not. The (f, c, p) knobs place the fast quorum where the deployment requires it, allowing a commit in two message delays. If misconfigured, Hydrozoan can still commit in three message delays: Hydrozoan commits on whichever path fires first. We also present Optimal-Hydrozoan, a variant that tolerates one more fault on the fast path, the first construction to match the known lower bound. The safety and liveness of both protocols are machine-checked in Lean 4. Our geo-distributed evaluation shows that Hydrozoan matches Mysticeti's throughput, commits ~25% faster when the fast quorum fits fast regions, and falls back to three message delays when it does not or past p faults, where existing two-delay protocols stall.