Software Availability is defined as expected fraction of time for which a component or a system is functioning acceptably. If a program has a constant failure intensity ie. it is being operational without any changes or modifications or repairs, then Availability is calculated as the ratio –
Availability = Uptime / Uptime + Downtime
Here, availability is derived as the time interval over which a measurement is made approaches infinity.
Downtime = Failure intensity * Mean Time to repair (MTTR)
MTTR is the average time required to restore the database for the program or reload the program and resume execution.