PhysiCS-NMSI: efficient consistent snapshots for scalable snapshot isolation

Non-Monotonic Snapshot Isolation (NMSI), a variant of the widely deployed Snapshot Isolation (SI), aims at improving scalability by relaxing snapshots. In contrast to SI, NMSI snapshots are causally consistent, which allows for more parallelism and a reduced abort rate. This work documents the design of PhysiCS-NMSI, a transactional protocol implementing NMSI in a partitioned data store. It is the first protocol to rely on a single scalar taken from a physical clock for tracking causal dependencies and building causally consistent snapshots. Its commit protocol ensures atomicity and the absence of write-write conflicts. We argue that PhysiCS-NMSI approach increases concurrency and reduces abort rate and metadata overhead as compared to state-of-art systems.