Consistency Oracles: Towards an Interactive and Flexible Consistency Model Specification

Many modern distributed storage systems emphasize availability and partition tolerance over consistency, leading to many systems that provide weak data consistency. However, weak data consistency is difficult for both system designers and users to reason about. Formal specifications offer precise descriptions of consistency behavior, but they require expertise and specialized tools to apply to real software systems. In this paper, we propose and describe consistency oracles, an alternative way of specifying the consistency model of a system that provides interactive answers, making them easier and more flexible to use in a variety of ways. A consistency oracle mimics the interface of a distributed storage system, but returns all possible values that may be returned under a given consistency model. This allows consistency oracles to be directly applied in the testing and verification of both distributed storage systems and the client software that uses those systems.