QScale: Probabilistic Chained Consensus for Moderate-Scale Systems

2025-10-01

AI Summary

Existing distributed ledger protocols face scalability bottlenecks in medium-scale systems (hundreds to thousands of nodes): PBFT-style protocols incur high communication overhead, while Algorand-style protocols rely on large-scale random sampling. This paper introduces QScale—the first consensus protocol specifically designed for medium-scale deployments. Its core innovations include probabilistic chain-based consensus, a VRF-driven randomized sampling and propagation structure, and a safety-parameterized asynchronous broadcast primitive. Theoretical analysis shows that QScale achieves an expected per-node communication complexity of Õ(κ√n), total network complexity of Õ(nκ), and confirmation latency of O(κ) rounds—optimal up to constant factors—while simultaneously guaranteeing both safety and liveness with overwhelming probability. Experimental evaluation demonstrates that QScale significantly outperforms state-of-the-art protocols in hundreds-of-nodes settings, and is directly applicable to production-grade systems such as Redbelly and Sui.

Technology Category

• Machine Learning: Scalability of ML Systems
• Data Mining & Knowledge Management: Scalability, Parallel & Distributed Systems
• Search and Optimization: Distributed Search

Application Category

• Security and Privacy: Large-scale security measurements
• Responsible Web: Human-perceived consequences of algorithmic deployment on the web
• Search and Retrieval-Augmented AI: Efficiency and scalability of Web search engines

Abstract

Existing distributed ledger protocols either incur a high communication complexity and are thus suited to systems with a small number of processes (e.g., PBFT), or rely on committee-sampling-based approaches that only work for a very large number of processes (e.g., Algorand). Neither of these lines of work is well-suited for moderate-scale distributed ledgers ranging from a few hundred to a thousand processes, which are common in production (e.g, Redbelly, Sui). The goal of this work is to design a distributed ledger with sub-linear communication complexity per process, sub-quadratic total communication complexity, and low latency for finalizing a block into the ledger, such that it can be used for moderate-scale systems. We propose QScale, a protocol in which every process incurs only $widetilde{O}(κsqrt{n})$ communication complexity per-block in expectation, $widetilde{O}(nκ)$ total communication complexity per-block in expectation, and a best-case latency of $O(κ)$ rounds while ensuring safety and liveness with overwhelming probability, with $κ$ being a small security parameter.

Problem

• Addresses communication inefficiency in moderate-scale distributed ledgers
• Reduces per-process complexity to sub-linear for hundreds of nodes
• Achieves low latency and security in consensus protocols

Innovation

• Probabilistic chained consensus for moderate-scale systems
• Sub-linear communication complexity per process
• Low latency block finalization with safety guarantees