REDUNDANCY AND DIVERSITY:
Redundancy and diversity are fundamental strategies for enhancing the dependability of any type of system. Redundancy means that spare capacity is included in a system that can be used if part of that system fails. Diversity means that redundant components of the system are of different types, thus increasing the chances that they will not fail in exactly the same way.
We use redundancy and diversity to enhance dependability in our everyday lives. As an example of redundancy, most people keep spare light bulbs in their homes so that they can quickly recover from the failure of a light bulb that is in use. Commonly, to secure our homes we use more than one lock (redundancy) and, usually, the locks used are of different types (diversity). This means that if an intruder finds a way to defeat one of the locks, they have to find a different way of defeating the other lock before they can gain entry. As a matter of routine, we should all back up our computers and so maintain redundant copies of our data. To avoid problems with disk failure, backups should be kept on a separate, diverse, external device. Software systems that are designed for dependability may include redundant components that provide the same functionality as other system components.
These are switched into the system if the primary component fails. If these redundant components are diverse (i.e., not the same as other components), a common fault in replicated components will not result in a system failure. Redundancy may also be provided by including additional checking code, which is not strictly necessary for the system to function. This code can detect some kinds of faults before they cause failures. It can invoke recovery mechanisms to ensure that the system continues to operate.
