WatCA: The Waterloo Consistency Analyzer

Today's online applications depend on fast storage and retrieval of up-to-date data at web scale. To meet this growing demand, the designers of distributed storage systems have devised a rich variety of data replication protocols, offering different trade-offs between consistency, latency, and availability. Understanding the sweet spot, and testing whether a system delivers a particular level of consistency, are challenging problems as consistency itself is difficult to reason about. This demo paper describes an interactive software tool for measuring and visualizing the consistency actually observed by client applications accessing a key-value storage system in real time. The tool can be used to evaluate performance trade-offs in a system with tunable consistency, or to verify the correctness of a storage system that guarantees certain forms of so-called "strong consistency".