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QScale: Probabilistic Chained Consensus for Moderate-Scale Systems

Hasan Heydari, Alysson Bessani, Kartik Nayak

TL;DR

QScale addresses the need for scalable distributed ledgers in moderate-size settings (hundreds to thousands of processes) by achieving sub-linear per-process communication and sub-quadratic total communication while keeping latency low. It combines a HotStuff-like leader-based structure with probabilistic sampling and a lightweight propagation sub-protocol, using random samples of size $O(\sqrt{n})$ to drive efficiency. The protocol certifies blocks via quorum-like voting to form a chain of $\kappa$ consecutive certified blocks, which provides probabilistic safety and guaranteed liveness, with tunable parameters to balance risk and speed. Practically, QScale offers a viable path for moderate-scale deployments (e.g., Redbelly, SUI) by delivering strong fault tolerance with significantly reduced communication overhead compared to traditional all-to-all schemes or large fixed committees.

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 $\widetilde{O}(κ\sqrt{n})$ 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.

QScale: Probabilistic Chained Consensus for Moderate-Scale Systems

TL;DR

QScale addresses the need for scalable distributed ledgers in moderate-size settings (hundreds to thousands of processes) by achieving sub-linear per-process communication and sub-quadratic total communication while keeping latency low. It combines a HotStuff-like leader-based structure with probabilistic sampling and a lightweight propagation sub-protocol, using random samples of size to drive efficiency. The protocol certifies blocks via quorum-like voting to form a chain of consecutive certified blocks, which provides probabilistic safety and guaranteed liveness, with tunable parameters to balance risk and speed. Practically, QScale offers a viable path for moderate-scale deployments (e.g., Redbelly, SUI) by delivering strong fault tolerance with significantly reduced communication overhead compared to traditional all-to-all schemes or large fixed committees.

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 communication complexity per-block in expectation, total communication complexity per-block in expectation, and a best-case latency of rounds while ensuring safety and liveness with overwhelming probability, with being a small security parameter.

Paper Structure

This paper contains 18 sections, 16 theorems, 46 equations, 3 figures, 3 tables, 2 algorithms.

Table of Contents

  1. Introduction
  2. Preliminaries
  3. System Model
  4. Distributed Ledger
  5. QScale
  6. Overview
  7. Protocol
  8. Complexity Analysis
  9. Correctness Proofs
  10. Evaluation
  11. Conclusion
  12. Evaluation under Synchrony
  13. Static Committee
  14. Probability Bounds
  15. Propagation Sub-Protocol
  16. ...and 3 more sections

Key Result

theorem thmcountertheorem

In a message-passing system with $n$ processes, where up to $f = \epsilon\,n$ processes may be Byzantine under a static corruption adversary, QScale solves the probabilistic distributed ledger by providing the following per-block guarantees:

Figures (3)

  • Figure 1: Execution of the QScale over two epochs. Blue arrows illustrate messages sent by the propagation sub-protocol, while black arrows indicate messages sent during the propose, disseminate, and vote rounds. The value shown at the top of each of the first three rounds indicates the communication complexity incurred in that round.
  • Figure 2: The probability of safety violation (top) and of committing in $\kappa$ epochs (bottom) under partial synchrony with $n=500$.
  • Figure 3: Evaluation under synchrony with $n=500$.

Theorems & Definitions (31)

  • theorem thmcountertheorem: Informal main result
  • theorem thmcountertheorem
  • proof
  • theorem thmcountertheorem
  • proof
  • lemma thmcounterlemma
  • proof
  • lemma thmcounterlemma
  • proof
  • theorem thmcountertheorem
  • ...and 21 more