Modeling software design diversity

Design diversity has been used for many years now as a means of achieving a degree of fault tolerance in software-based systems. While there is clear evidence that the approach can be expected to deliver some increase in reliability compared to a single version, there is no agreement about the extent of this. More importantly, it remains difficult to evaluate exactly how reliable a particular diverse fault-tolerant system is. This difficulty arises because assumptions of independence of failures between different versions have been shown to be untenable: assessment of the actual level of dependence present is therefore needed, and this is difficult. In this tutorial, we survey the modeling issues here, with an emphasis upon the impact these have upon the problem of assessing the reliability of fault-tolerant systems. The intended audience is one of designers, assessors, and project managers with only a basic knowledge of probabilities, as well as reliability experts without detailed knowledge of software, who seek an introduction to the probabilistic issues in decisions about design diversity.