Modelling correlation among successive inputs in software dependability analysis

We consider the dependability of programs of an iterative nature. The dependability of software structures is usually analysed using models that are strongly limited in their realism by the assumptions made to obtain mathematically tractable models and by the lack of experimental data. Among the assumptions made, the independence between the outcomes of successive executions, which is often false, may lead to significant deviations of the result obtained from the real behaviour of the program under analysis. Experiments and theoretical justifications show the existence of contiguous failure regions in the program input space and that, for many applications, the inputs often follow a trajectory of contiguous points in the input space. In this work we present a model in which dependencies among input values of successive iterations are taken into account in studying the dependability of iterative software. We consider also the possibility that repeated, non fatal failures may together cause mission failure. We evaluate the effects of these different hypotheses on 1) the probability of completing a fixed-duration mission, and 2) a performability measure.