A contribution to the evaluation of the reliability of iterative-execution software

This paper deals with the reliability of software executed iteratively, as for example in process control applications. The probability of mission survival is evaluated taking account of two characteristics of iterative software: (a) system failure, defined in terms of the behaviour of the software over successive iterations, because the controlled system can usually tolerate short bursts of errors; (b) the probabilistic correlation between successive executions of the software, which is to be expected for various reasons. The paper presents models accounting for these characteristics and evaluates their effects. The interesting case of fault-tolerant software is considered as well. Using the example of a `pair-and-spare' type fault-tolerant scheme, the relationships between different aspects of failure behaviour that are covered by the models developed here, and those used elsewhere for fault-tolerant software, are shown.