Leader Election for Replicated Services Using Application Scores

Replicated services often rely on a leader to order client requests and broadcast state updates. In this work, we present POLE, a leader election algorithm that select leaders using application-specific scores. This flexibility given to the application enables the algorithm to tailor leader election according to metrics that are relevant in practical settings and that have been overlooked by existing approaches. Recovery time and request latency are examples of such metrics. To evaluate POLE, we use ZooKeeper, an open-source replicated service used for coordinating Web-scale applications. Our evaluation over realistic widearea settings shows that application scores can have a significant impact on performance, and that just optimizing the latency of consensus does not translate into lower latency for clients. An important conclusion from our results is that obtaining a general strategy that satisfies a wide range of requirements is difficult, which implies that configurability is indispensable for practical leader election.