Hot Spare - Explanation

Explanation

In designing a reliable system, it is recognized that there will be failures. At the extreme, a complete system can be duplicated and kept up to date—so in the event of the primary system failing, the secondary system can be switched in with little or no interruption. More often, a hot spare is a single vital component without which the entire system would fail. The spare component is integrated into the system in such a way that in the event of a problem, the system can be altered to use the spare component. This may be done automatically or manually, but in either case it is normal to have some means of error detection. A hot spare does not necessarily give 100% availability or protect against temporary loss of the system during the switching process; it is designed to significantly reduce the time that the system is unavailable.

Hot standby may have a slightly different connotation of being active but not productive to hot spare, that is it is a state rather than object. For example, in a national power grid, the supply of power needs to be balanced to demand over a short term. It can take many hours to bring a coal-fired power station up to productive temperatures. To allow for load balancing, generator turbines may be kept running with the generators switched off so as peaks of demand occur, the generators can rapidly be switched on to balance the load. Being in the state of being ready to run is known as hot standby. Though it is not a modern phenomenon, steam train operators might hold a spare steam engine at a terminus fired up, as starting an engine cold would take a significant amount of time.

The spare may be similar component or system, or it may be a system of reduced performance, designed to cope for the duration of the time to repair and recover the original component. In high availability systems, it is common to design so that not only is there a spare that can quickly be switched in, but also that the failed component can be repaired or replaced without stopping the system - this is known as hot swapping. It may be considered that the probability of a second failure is low, and therefore the system is designed simply to allow operation to continue until a suitable maintenance period. The appropriate solution is normally determined by balancing the costs of implementing the availability against the likelihood of a problem and the severity of that problem.