Detection of Weak Unstable Predicates in Distributed Programs

|This paper discusses detection of global predicates in a distributed program. Earlier algorithms for detection of global predicates proposed by Chandy and Lamport work only for stable predicates. A predicate is stable if it does not turn false once it becomes true. Our algorithms detect even unstable predicates without excessive overhead. In the past, such predicates have been regarded as too dii-cult to detect. The predicates are speciied using a logic described formally in this paper. We discuss detection of weak conjunctive predicates which are formed by conjunction of predicates local to processes in the system. Our detection methods will detect if such a predicate is true for any in-terleaving of events in the system, whether the predicate is stable or not. Also, any predicate which can be reduced to a set of weak conjunctive predicates is detectable. This class of predicates captures many global predicates that are of interest to a programmer. The message complexity of our algorithm is bounded by the number of messages used by the program. The main application of our results are in debugging and testing of distributed programs. Our algorithms have been incorporated in a distributed debugger which runs on a network of Sun Workstations under SunOS.