Since each of the levels in a hierarchical system could have various characteristics, different fault-tolerant schemes could be appropriate at different levels. A stochastic Petri net (SPN) is used to investigate various fault-tolerant schemes in this context. The basic SPN is augmented by parameterized subnet primitives to model the fault-tolerant schemes. Both centralized and distributed fault-tolerant schemes are considered. The two schemes are investigated by considering the individual levels in a hierarchical system independently. In the case of distributed fault tolerance, two different checkpointing strategies are considered. The first scheme is called the arbitrary checkpointing strategy. Each process in this scheme does its checkpointing independently; thus, the domino effect may occur. The second scheme is called the planned strategy. Here, process checkpointing is constrained to ensure no domino effect. The results show that, under certain conditions, an arbitrary checkpointing strategy can perform better than a planned strategy. The effect of integration on the fault-tolerant strategies of the various levels of a hierarchy are studied.
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