Systematic Testing of Asynchronous Reactive Systems

We introduce the concept of a delaying explorer with the goal of performing prioritized exploration of the behaviors of an asynchronous reactive program. A delaying explorer stratifies the search space using a custom strategy, and a delay operation that allows deviation from that strategy. We show that prioritized search with a delaying explorer performs significantly better than existing prioritization techniques. We also demonstrate empirically the need for writing different delaying explorers for scalable systematic testing and hence, present a flexible delaying explorer interface. We introduce two new techniques to improve the scalability of search based on delaying explorers. First, we present an algorithm for stratified exhaustive search and use efficient state caching to avoid redundant exploration of schedules. We provide soundness and termination guarantees for our algorithm. Second, for the cases where the state of the system cannot be captured or there are resource constraints, we present an algorithm to randomly sample any execution from the stratified search space. This algorithm guarantees that any such execution that requires d delay operations is sampled with probability at least 1/L d , where L is the maximum number of program steps. We have implemented our algorithms and evaluated them on a collection of real-world fault-tolerant distributed protocols.