Modeling inter-component dependencies of high-availability systems using OpenSESAME

Traditional Modeling Methods for High-Availability-Systems like Fault Trees (FT) or Reliability Block Diagrams (RBD) assume that there are no stochastic dependencies between the failure and repair behavior of the system’s components (see e.g. [3]). For instance, they imply that the unavailability of a parallel system can be calculated using the formula ∏n i=1 Ui where U1,2,...n are the unavailabilities of its subsystems. However, this assumption is over-optimistic, because of numerous dependencies between the subsystems. Using the tool OpenSESAME (Simple but Extensive Structured Availability Modeling Environment [2, 4]), these dependencies can be taken into account leading to much more realistic and trustworthy results. In contrast to modeling environments which use Markov Chains, Petri nets or stochastic process algebras as their input formalism, the learning curve of OpenSESAME is smooth: Users start with regular block diagrams and can extend them by dependencies without having to learn new formalisms.