On Barriers and the Gap between Active and Passive Replication (Full Version)

TL;DR

This paper analyzes the differences between active and passive replication, introduces a barrier function tau to unify existing algorithms, and proposes a modular implementation that matches the efficiency of tau-based methods.

Contribution

It introduces a barrier function tau to unify and explain differences between primary-order atomic broadcast algorithms and presents a modular implementation matching existing complexities.

Findings

Barrier function tau explains differences between algorithms.

Existing implementations have higher time complexity.

Proposed modular algorithm matches the efficiency of tau-based methods.

Abstract

Active replication is commonly built on top of the atomic broadcast primitive. Passive replication, which has been recently used in the popular ZooKeeper coordination system, can be naturally built on top of the primary-order atomic broadcast primitive. Passive replication differs from active replication in that it requires processes to cross a barrier before they become primaries and start broadcasting messages. In this paper, we propose a barrier function tau that explains and encapsulates the differences between existing primary-order atomic broadcast algorithms, namely semi-passive replication and Zookeeper atomic broadcast (Zab), as well as the differences between Paxos and Zab. We also show that implementing primary-order atomic broadcast on top of a generic consensus primitive and tau inherently results in higher time complexity than atomic broadcast, as witnessed by existing algorithms. We overcome this problem by presenting an alternative, primary-order atomic broadcast implementation that builds on top of a generic consensus primitive and uses consensus itself to form a barrier. This algorithm is modular and matches the time complexity of existing tau-based algorithms.