The Logical Timestamp Skew Anomaly in Event-Replicated Transaction Schedulers

To sidestep reasoning about the complex effects of concurrent execution, many system designers have conveniently embraced strict serializability on the strength of its claims, support from commercial and open-source database communities and ubiquitous levels of industry adoption. Crucially, distributed components are built on this model; multiple schedulers are composed in an event-driven architecture to form larger, ostensibly correct systems. This paper examines the oft-misconstrued position of strict serializability as a composable correctness criterion in the design of such systems. An anomaly is presented wherein a strict serializable scheduler in one system produces a history that cannot be serially applied to even a weak prefix-consistent replica in logical timestamp order. Several solutions are presented under varying isolation properties, including novel isolation properties contributed by this paper. We also distinguish between concurrent schedulers based on their propensity to produce deterministic histories. It is further shown that every nondeterministic scheduler is anomaly-prone, every nonconcurrent scheduler is anomaly-free, and that at least one deterministic concurrent scheduler is anomaly-free.