Dependability analysis of iterative fault-tolerant software considering correlation

We consider the dependability of fault tolerant software executed iteratively as e.g. in process control applications. We recall themodels usually, adopted for evaluating the probnbility of mission survival (reliability at a certain time) and performabiliry, and show the results obtained hy applying these models to the adaptive scheme for software fault-tolerance SCOP. "Self-Configuring Optimal Programming" and to the more popular schemes, recovery blocks and multiple version programming. In the case considered. SCOP is cqui valent to N-version programming in terms of the probability of delivering correct results. but achieves better performance by delaying the execution of some of the variants until it is made necessary by an error. A discussion follows highlighting the limits in the realism of these analyses, due to the assumptions made to obtain mathematically tractable models and to the lack of experimental data.As a limitations, we present a simple black-box model of an iterative software component which takes into account dependencies among input values of successive iterations and the possibility that repeated. non fatal failures may together cause mission failure. Using this model we show the effects of different distributions of the correlations among, inputs on the dependability indicators selected.