We can contrast the main difference in concept between fault tolerance and high availability.
Fault tolerance provides additional resources that allow an application to continue
functioning after a component failure without interruption (A Framework for System High
Availability, 2000; Brown, 2000; Providing Open Architecture, 2001). Many of the highavailability
solutions on the market today actually provide fault tolerance for a particular
application component. Disk mirroring, where there are two disk drives with identical copies
of the data, is an example of a fault-tolerant component. If one of the disk drives fails,
there is another copy of the data that is instantly available for the application to continue
execution. A fault-tolerant system would be used in a situation where no downtime can be
tolerated at all, such as air-traffic-control systems, emergency-response systems, or financial-
trading systems.
Metrics.of.High.Availability
Availability is a function of system reliability and reparability, augmented and supported by
redundancy. To quantify and identify the availability of any system, academic and industry
sectors have defined two metrics that formalize the definition of a system??™s availability:
??? Reparability??”Mean.
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