Availability is the proportion of time a service is up and usable. High availability means designing a system so it stays available even when some part of it fails, usually by removing single points of failure: running more than one instance so that another can take over, storing more than one copy of data, and spreading those copies across separate hardware or locations. In Azure you build high availability from features such as multiple virtual machines behind a load balancer, availability zones and redundant storage. High availability is related to, but not the same as, disaster recovery: high availability keeps a service running through everyday failures, while disaster recovery restores it after a large event such as the loss of a region.
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