In information technology, High Availability (HA) is a characteristic that aims to ensure continuous uptime of the system for a desirably long period of time. Availability of the system is usually expressed as a percentage of uptime in a given year.
High Availability differs greatly from Disaster Recovery (DR), although the two are typically discussed together in the context of ensuring the system's uptime.
For commercial systems, the system's availability is typically backed by an uptime SLA. An SLA (short for service-level agreement) is defined as an official commitment that exists between the service provider and the customer. SLAs typically refer to a percentage of uptime that is financially backed by the provider, meaning that if the system experienced a longer downtime than what is permitted by the SLA, the customer would receive a monetary refund...
