High.Availability.Overview
A system is composed of a collection of interacting components. A system provides one or
more services to its consumers. A service is the output of a system that meets the specification
for which the system was devised, or agrees with what system users have perceived as
the correct output values.
Service failures are incorrect results with respect to the specification or unexpected behavior
perceived by the users of the service. The cause of a failure is said to be a fault. Faults are
identified or detected in some manner, either by the system or by its users. Finally, an error
is a discrepancy between the displayed output (e.g., computed, measured, or observed value
or condition) and the correct or specified value or condition. Errors are often the result of
exceptional conditions or unexpected interference. If an error occurs, then a failure of some
form has occurred. Similarly, if a failure occurs, then an error of some form will occur. It is
important to detect not only active faults, but also the latent faults. Finding and removing
these faults before they become active leads to less downtime and higher availability. Availability
is defined as the readiness for usage.
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