On Barriers and the Gap between Active and Passive Replication

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 i¾? that explains and encapsulates the differences between existing primary-order atomic broadcast algorithms. We also show that implementing primary-order atomic broadcast on top of a generic consensus primitive and i¾? 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 i¾?-based algorithms.