Fault tolerant software uses redundancy to improve reliability; but such redundancy requires additional resources and tends to be costly, therefore the redundancy level needs to be optimized. Our optimization models determine the optimal level of redundancy within a software system under the assumption that functionally equivalent software components fail independently. A framework illustrates the tradeoff between the cost of using N-version programming and the improved reliability for a software system. The 2 models deal with: a single task, and multitask software. These software systems consist of several modules where each module performs a subtask and, by sequential execution of modules, a major task is performed. Major assumptions are: 1) several versions of each module, each with an estimated cost and reliability, are available, 2) these module versions fail independently. Optimization models are used to select the optimal set of versions for each module such that the system reliability is maximized and total cost remains within budget.
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