Concurrency and Replica Control for Constraint-Based Database Caching

Efficient and dynamic reallocation of data is a major challenge of distributed data management, because current solutions require constant monitoring and manual adjustment. In contrast, future solutions should provide autonomic mechanisms to achieve self-tuning and exhibit high degrees of flexibility and availability. Constraint-based database caching (CbDBC) is one of the most ambitious approaches to reach these goals, because it is able to dynamically respond to workload changes and keep subsets of data near by the application. In turn, caching of data always generates replicas whose consistency needs to be controlled—for reasons of data independence, transparent for both application and underlying DBMS. Hence, such a task can best be approached by a middleware-based solution. This paper discusses challenges arising when distributed replicas are synchronized within CbDBC. We compare proposals using eager and lazy update propagation and review their feasibility within middleware-based realizations. Because constraints have to be maintained by the cache, they restrict the implementation of concurrency control mechanisms. Therefore, we explore, as a novel contribution, the far-reaching influence of these constraints.