Availability modeling of modular software

Dependability evaluation is a basic component in assessing the quality of repairable systems. A general model (Op) is presented and is specifically designed for software systems; it allows the evaluation of various dependability metrics, in particular, of availability measures. Op is of the structural type, based on Markov process theory. In particular, Op is an attempt to overcome some limitations of the well-known Littlewood reliability model for modular software. This paper gives the: mathematical results necessary to the transient analysis of this general model; and algorithms that can efficiently evaluate it. More specifically, from the parameters describing the: evolution of the execution process when there is no failure; failure processes together with the way they affect the execution; and recovery process, the results are obtained for the: distribution function of the number of failures in a fixed mission; and dependability metrics which are much more informative than the usual ones in a white-box approach. The estimation procedures of the Op parameters are briefly discussed. Some simple examples illustrate the interest in such a structural view and explain how to consider reliability growth of part of the software with the transformation approach developed by Laprie et al. The complete transient analysis of Op allows discussion of the Poisson approximation by Littlewood for his model.