11.1 Availability and reliability
In Chapter 10, I introduced the concepts of system reliability and system availability. If we think of systems as delivering some kind of service (to deliver cash, control brakes, or connect phone calls, for example), then the availability of that service is whether or not that service is up and running and its reliability is whether or not that service delivers correct results. Availability and reliability can both be expressed as probabilities. If the availability is 0.999, this means that, over some time period, the system is available for 99.9% of that time. If, on average, 2 inputs in every 1000 result in failures, then the reliability, expressed as a rate of occurrence of failure, is 0.002.
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