The term redundancy refers to a parallel or alternate path in a system architecture to meet system requirements in two or more independent ways. The purpose of a redundant system is to improve reliability. There are two general approaches to redundancy implementation: system redundancy and component redundancy. System redundancy involves the connection of two identical systems in parallel between system inputs and system outputs. In the event of a failure of the active system, the inactive or backup system can be switched on to replace the failed system. A variation of system redundancy is the use of a hot backup system. In this approach, both prime and backup systems are in the “on” state. There are manual and automatic approaches to recognizing system failures and effecting a switchover from the failed system to the backup system.

Software and firmware play a dominant role in the implementation of redundant systems. Many of the built-in tests are in software or embedded in firmware. Setting and resetting the watchdog timer and updating the checkpoint restart memory are software functions. Switchover and restart are also software functions.

The concept of component redundancy suggests making redundant only those subsystems or components that are relatively unreliable. In many cases, by making a few critical components in a system redundant, one can provide enough improvement in overall reliability to meet the system reliability requirements. For example, in a redundant automobile braking system the brake pedal and linkage are not redundant (simplex) while the master brake cylinder is redundant. The same concerns in system redundancy implementation regarding failure detection and switch over are also found in component redundancy implementation. In the braking system just described, a failure of one master cylinder results in degraded braking performance. This system is better described as fault tolerant.

A. Behforooz, F. J. Hudson. Software engineering
fundamentals. F. J. Oxford University Press.

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