Replicated data types that unify eventual consistency and observable atomic consistency

Abstract

Strong consistency is widely used in systems such as relational databases. In a distributed system, strong consistency ensures that all clients observe consistent data updates atomically on all servers. However, such systems need to sacrifice availability when synchronization occurs.We propose a new consistency protocol, the observable atomic consistency protocol (OACP) to make write-dominant applications as fast as possible and as consistent as needed. OACP combines the advantages of (1) mergeable data types, specifically, convergent replicated data types, to reduce synchronization and (2) reliable total order broadcast to provide on-demand strong consistency. We also provide a high-level programming interface to improve the efficiency and correctness of distributed programming.We present a formal, mechanized model of OACP in rewriting logic and verify key correctness properties using the model checking tool Maude. Furthermore, we provide a prototype implementation of OACP based on Akka, a widely-used actor-based middleware. Our experimental evaluation shows that OACP can reduce coordination overhead compared to the state-of-the-art Raft consensus protocol. Our results also suggest that OACP increases availability through mergeable data types and provides acceptable latency for achieving strong consistency, enabling a principled relaxation of strong consistency to improve performance.