Non-monotonic snapshot isolation: Scalable and strong consistency for geo-replicated transactional systems

Modern cloud systems are geo-replicated to improve application latency and availability. Transactional consistency is essential for application developers, however, the corresponding concurrency control and commitment protocols are costly in a geo-replicated setting. To minimize this cost, we identify the following essential scalability properties: (i) only replicas updated by a transaction $T$ make steps to execute $T$, (ii) a read-only transaction never waits for concurrent transactions and always commits, (iii) a transaction may read object versions committed after it started, and (iv) two transactions synchronize with each other only if their writes conflict. We present Non-Monotonic Snapshot Isolation (NMSI), the first strong consistency criterion to allow implementations with all four properties. We also present a practical implementation of NMSI called Jessy, which we compare experimentally against a number of well-known criteria. Our measurements show that the latency and throughput of NMSI are comparable to the weakest criterion, read-committed, and between two to fourteen times faster than well-known strong consistencies.