This is one of the series of posts that I have planned to write about high availability. This post exclusively tries to explain High Availability the way I understood.
The term Availability refers to the time required for system to respond to user request, i.e. the time period over which service is available often documented in Service Level Agreement (SLA). Since there could be different kinds of Outages that may cause services to become unavailable, there is need to manage those Downtimes (measured in time units, of course) caused by those outages. Minimizing these downtimes is collectively managed by designing Fault Tolerant systems and SLA.
So Highly Available systems are combination of SLA and fault tolerant systems essentially providing characteristics that totally/partially avoids downtimes or loss of service.