Hierarchical supervisors for automatic detection of software failures

Software supervision is an approach to the automatic detection of software failures. A supervisor observes the inputs and outputs of a target system. It uses a model of correct behavior, derived from the target system´s requirement specification. Discrepancies between specified and observed behaviors are reported as failures. Applications of the supervisor include online failure detection in real time reactive systems, fault localization and automatic collection of failure data. The paper describes a hierarchical approach to supervision. The approach differs from previous approaches in that supervision is split into two sub problems: tracking the behavior of the target system and detailed behavior checking. The architecture of the hierarchical supervisor has two layers: the path detection layer and the base supervisor layer. The hierarchical approach results in a significant reduction in computational cost arising from specification nondeterminism. The approach was evaluated by supervising the call processing software of a small telephone exchange, executed under random telephone traffic at different loads. Several thousand failures were individually seeded into the output generated by the exchange. The supervisor was able to detect the presence of all seeded failures. Reductions in computational cost of several orders of magnitude were measured in comparison with the direct, single layer supervisor