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Reliability

In a large, complex system, the concept of reliability can get rather confusing—in large part because the reliability of the overall system is only loosely related to the reliability of the components used to build it. It is possible to build highly reliable systems out of mostly reliable components, and it is possible to build fragile systems out of very reliable components. For example, the telephone system in the United States is very reliable because of the way the system is engineered, not just because it is built from highly reliable components.

A second source of confusion is the difference between the concepts of hardware reliability and software reliability. Hardware reliability is much easier to understand. In most cases, the hardware stops working, and the failure is more or less apparent (unless it is masked by redundancy). The failure may be signaled by software that isn't getting the right result from the hardware, but (unlike many software bugs) it is uncommon for hardware to silently give the wrong answer. In contrast, software reliability is mostly a question of bugs. A program might crash, or it might report an error, or it might give the wrong answer.


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