Fault-tolerant technique is an effective approach to achieve high system reliability. This paper proposes six hierarchical fault-tolerant software reliability models with multi-level of Recovery Block (RB) modules, multi-level of N-Version Programming (NVP) modules and combinations of RB and NVP modules called hybrid fault-tolerant architectures. These system reliabilities and costs are evaluated at various degrees of failure dependencies, and then compare with those of the classical RB and NVP models. Reliability results with s-independent failure assumption are compared with those considering failure dependency assumption. System cost for each model is evaluated as well.
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