Conflict-free Replicated Priority Queue: Design, Verification and Evaluation.

Internet-scale distributed systems often rely on replication to achieve fault-tolerance and load distribution. To provide low latency and high availability, the systems are often required to accept updates on one replica immediately and then propagate the updates among replicas asynchronously. Conflict-free Replicated Data Type (CRDT) is a principled approach to addressing the challenge for these systems to resolve conflicts among concurrent updates. Although many CRDTs have been studied, little research has been done on Conflict-free Replicated Priority Queue (CRPQ), which is a collection of elements that focuses on maintaining element orderings based on their priority values, and can be used in many applications scenarios such as task scheduling and network routing. In this work, we discuss the design rationales of CRPQs and introduce two CRPQ designs: Add-Win CRPQ and Remove-Win CRPQ. The correctness of the designs is formally verified using TLA+. We also demonstrate the effectiveness of our designs by implementing them over Redis. Our evaluation shows that both CRPQs perform well in terms of data consistency and memory overhead.