Chasing the Tail of Atomic Broadcast Protocols

Many applications today rely on multiple services, whose results are combined to form the application's response. In such contexts, the most unreliable service and the slowest service determine the application's reliability and response time, respectively. State-machine replication and atomic broadcast are fundamental abstractions to build highly available services. In this paper, we consider the latency variability of atomic broadcast protocols. This is important because atomic broadcast has a direct impact on the response time of services. We study four high performance atomic broadcast protocols representative of different classes of protocol design and characterize their latency tail distribution under different workloads. Next, we assess how key design features of each protocol can possibly be related to the observed latency tail distributions. Our observations hint at request batching as a simple yet effective way to shorten the latency tails of some of the studied protocols, an improvement within the reach of application implementers. Indeed, our observation is not only verified experimentally, it allows us to assess which of the protocol's key design principles favor the construction of latency predictable protocols.