In hardware systems, high reliability or availability (Specify Reliability Specifically) is often achieved through redundancy. Thus, if a system component is expected to exhibit a mean-time-between-failures of x, we can manufacture two or three such components and run them in either:
- Parallel. For example, they all do all the work and, when their responses differ, one is turned off with no impact on overall system functionality.
- Or cold standby. A backup computer might be powered on only when a hardware failure is detected in the operational computer.
Manufacturing cost is slightly more than doubled. Design cost increases slightly. Reliability increases exponentially.
In software systems, we cannot use the same approach. If we make two copies of the same software, no increase in reliability will be achieved. In one fails, the other will as well. What can be done, however, is to design (using two different design teams) two versions of the software from the same requirements specification, and deploy them in parallel. Development cost doubles. Reliability increases exponentially. Notice that, in the case of hardware, design increases in cost only slightly, whereas software design cost (the primary cost of software) doubles. Ultrahigh reliability in software is very expensive (High Quality Software Is Possible).
Reference:
Musa, J., et al., Software Reliability, New York: McGraw Hill, 1987.
