The use of more elements than necessary to maintain the performance of a system in the event of failure of one or more of the elements.

System failure is the failure of a system to achieve a goal—e.g., communicate a message, maintain a structural load, or maintain operation. It is inevitable that elements within a system will fail. It is not inevitable, however, that the system as a whole fails. Redundancy is the surest method of preventing system failure. There are four kinds of redundancy: diverse, homogenous, active, and passive.

Diverse redundancy

The use of multiple elements of different types (e.g., use of text, audio, and video to present the same information). Diverse redundancy is resistant to a single cause of failure but is complex to implement and maintain. For example, high-speed trains often have diverse redundancy in their braking systems—one electric brake, one hydraulic brake, and one pneumatic brake. A single cause is unlikely to result in a cascade failure in all three braking systems.

Homogenous redundancy

The use of multiple elements of a single type (e.g., use of multiple independent strands to compose a rope). Homogenous redundancy is relatively simple to implement and maintain but is susceptible to single causes of failure—i.e., the type of cause that results in failure in one element can result in failure of other redundant elements. For example, a sharp edge that severs one strand of a rope can sever others.

Active redundancy

The application of redundant elements at all times (e.g., using multiple independent pillars to support a roof). Active redundancy guards against both system and element failure—i.e., it distributes loads across all elements such that the load on each element and the overall system is reduced. Active redundancy also allows for element failure, repair, and substitution with minimal disruption of system performance.

Passive redundancy

The application of redundant elements only when an active element fails (e.g., using a spare tire on a vehicle in the event of a flat tire). Passive redundancy is ideal for noncritical elements, but it will result in system failure when used for elements critical to system operation. Passive redundancy is the simplest and most common kind of redundancy.

Use diverse redundancy for critical systems when the probable causes of failure cannot be anticipated. Use homogenous redundancy when the probable causes of failure can be anticipated. Use active redundancy for critical systems that must maintain stable performance in the event of element failure or extreme changes in system load. Use passive redundancy for noncritical elements within systems, or systems in which performance interruptions are tolerable. The four kinds of redundancy should be used in combination to achieve highly reliable systems.

The Super Cow entry in the Houston Cow Parade 2001 had a unique design specification—it was to sit atop a thirty-story tower crane for the duration of hurricane season. Since the consequences of Super Cow taking flight in high winds could be grave, various forms of redundancy were applied to keep him attached. Despite many severe thunderstorms (wind gusts in excess of 60 MPH), Super Cow experienced no failure, damage, or unintended flights during his four-month stay on the crane.

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