Service availability design requires a perspective that can account for
and coordinate resources and technologies throughout the system. It requires real-time
checkpointing of current data to hot standby resources and rapid, seamless switchover
in case of active resource failure. Because the continuity of service ultimately depends
on the resources and the functioning of the system as a whole, service availability
management requires an integrated, total-system approach.
High.Availability.Environment
To understand availability, we first need to understand reliability. The reliability of an element
is the conditional probability that the element will operate during a specified period
of time. A system may be considered highly reliable (that is, it may fail very infrequently),
but, if it is out of service for a significant period of time as a result of a failure, it will not be
considered highly available (Singh, 2001). As another definition, high availability is more
than excellent reliability. It is achieved through high service availability, the avoidance of
downtime (including all maintenance downtime), and extreme data integrity, with no loss
of stored data under any circumstances (High Availability White Paper, 2001).
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