11.2. High Availability and Fault Tolerance
High availability (HA) is a measure of the percentage of the time that a system is running. Saying a system has HA implies that a system is up and running nearly all the time. Fault tolerance (FT) means that a system is capable of surviving the loss of one or more critical components while still retaining functionality. The two terms often get confused, but they have entirely different meanings.
Take the Hubble Space Telescope as an example. Hubble shipped with six gyroscopes and needed three for normal observations. As gyroscopes failed, NASA found a way to maintain functionality by taking some of the operational gyros and moving them to stabilize the telescope to keep their observations running. ...
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