Kronos: the design and implementation of an event ordering service

This paper proposes a new approach to determining the order of interdependent operations in a distributed system. The key idea behind our approach is to factor the task of tracking happens-before relationships out of components that comprise the system, and to centralize them in a separate event ordering service. This not only simplifies implementation of individual components by freeing them from having to propagate dependence information, but also enables dependence relationships to be maintained across multiple independent systems. A novel API enables the system to detect and take advantage of concurrency whenever possible by maintaining fine-grained information and binding events to a time order as late as possible. We demonstrate the benefits of this approach through several example applications, including a transactional key-value store, and an online graph store. Experiments show that our event ordering service scales well and has low overhead in practice.