Paper 2026/2024

Optimistic History-Independent Sleepy Atomic Broadcast

Abstract

The latency of sleepy consensus protocols has been improved significantly, with recent designs achieving constant-round confirmation. However, existing protocols either rely on unrealistic additional assumptions about access to historical messages, where messages sent in round $t$ may depend on messages received in rounds $[t-k,t-1]$ for some $k>1$, or do not consider optimizing performance under optimistic conditions. In this work, we study history-independent sleepy atomic broadcast protocols while simultaneously considering optimistic conditions, where messages sent in round $t$ depend only on messages received in round $t-1$. We first design a sleepy atomic broadcast protocol that achieves a confirmation latency of $2\Delta$ ($\Delta$ is the upper bound on network delay) and an optimal latency of $\Delta$ under up to $1/5$ corruption. Second, we present a super-optimistic atomic broadcast protocol, achieving a best-case latency of $2\Delta$ under fault-free executions. Building on this insight, we propose an optimistic sleepy atomic broadcast protocol that relaxes the super-optimistic condition to tolerate sub-optimal faults. As a building block, we introduce an optimistic graded agreement protocol, $OptimisticGA$, which enables a dynamic optimistic quorum with four decision grades. Built on $OptimisticGA$, our optimistic atomic broadcast protocol achieves a good-case latency of $2\Delta$ when the leader is honest, while tolerating up to $1/6$ corruptions rather than requiring a fault-free setting. When corruption is more than $1/6$, it seamlessly adapts to $1/3$ resilience with a good-case latency of $3\Delta$. All protocols require no historical messages and tolerate an adversary that grows proportionally with honest nodes, with a minimal delay of $\Delta$.