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D. Tang, H. Hecht, An Approach to Measuring and Assessing Dependability for
Critical Software Systems, 8th IEEE International Symposium on Software Reliability
Engineering, Albuquerque, New Mexico, November, 1997
Traditional software testing methods combined with probabilistic models cannot measure
and assess dependability for software that requires very high reliability (failure
rate < 10 6/hour) and availability (>0.999999). This paper proposes a novel
approach, drawing on findings and methods that have been described individually
but have never been combined, applied in the late testing phase or early operational
phase, to quantify dependability for a category of critical software with such high
requirements. The concepts that are integrated are: operational profile, rare conditions,
importance sampling, stress testing, and measurement-based dependability evaluation.
In the approach, importance sampling is applied on the operational profile to guide
the testing of critical operations of the software, thereby accelerating the occurrence
of rare conditions which have been shown to be a leading cause of failure in critical
systems. The failure rates measured in the testing are then transformed to those
that would occur in the normal operation by the likelihood ratio function of the
importance sampling theory, and finally dependability for the tested software system
is evaluated by using measurement-based dependability modeling techniques. When
the acceleration factor is large (over 100), which is typical for a category of
software, it is possible to quantify a very high reliability or availability in
a reasonable test duration. Some feasible methods to implement the approach are
discussed based on real data.
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