Safe Combination of Data-Centric and Operation-Centric Consistency

Programming distributed systems requires maintaining consistency among data replicas. In recent years, various frameworks have proposed language-level abstractions for this, falling into two fundamental approaches: data-centric and operation-centric solutions. The former allow developers to explicitly assign consistency levels to data, the latter enable attaching consistency constraints to operations. In practice, developers may benefit from both in the same application: data-centric consistency harmonizes well with object-oriented programming, yet one may need the flexibility to access the same data with a different consistency level depending on the operation. Currently, there is no solution that integrates both: it is a conceptual challenge to unify these two models and design a type system capable of ensuring their correct interaction. We present ConOpY, a programming language that integrates both data-centric and operation-centric consistency into the same design. The ConOpY type system guarantees the proper usage of consistency levels, preventing consistency violations resulting from an improper mix of consistency models. ConOpY is implemented as a Java extension based on annotations.