High availability is a system design protocol and associated implementation that ensures a certain absolute degree of operational continuity during a given measurement period.
Availability refers to the ability of the user community to access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is said to be unavailable. Generally, the term downtime is used to refer to periods when a system is unavailable.
Planned and unplanned downtime
A distinction needs to be made between …