Detection of response time failures of real-time software

Classical software reliability research has tended to focus on behavioral type failures which typically manifest themselves as incorrect or missing outputs. In real time software, a correct output which is not produced within a specified response time interval may also constitute a failure. As a result, response time failures must also be taken into account when real time software reliability is assessed. The paper considers the case where the detection of response times failures is done by a separate unit which observes the inputs and outputs of the target software. Automatic detection of such failures is complicated by state dependencies which require the unit to track a target's state as well as the elapsed times between specified stimulus and response pairs. A novel black box approach is described for detecting response time failures and quality of service degradations of session oriented, real time software. The behavior of the target software is assumed to be specified in a formalism based on the notion of communicating extended finite state machines. The response time failure detection unit implemented independently of the target software, interprets a formal model derived directly from the target's requirement specifications. The model is used both to track the state of the target and to determine when to start and stop time interval timing measurements. Measurements of multiple response time intervals may occur simultaneously. The approach was evaluated on the call processing program of a small telephone exchange. Some results of the evaluation are presented and discussed.