ABSTRACT
There are as many definitions of software availability as there are authors writing about the subject. In the more traditional hardware sense of the term,
availability
is defined as the ratio of mean time to failure (MTTF) to the sum of MTTF and mean time to repair (MTTR). In the hardware world, a failure event is very easy to observe. A light bulb burned out. A disk drive went south. Consequently, it is easy to measure the precise elapsed time between failure events. Similarly, it is quite easy to measure the interval between when a light bulb burned out and when it was replaced with another. We can actually measure the length of time that it takes for a light bulb to burn out. From this we can compute the average time to failure for light bulbs. Similarly, we can measure how long it takes to change a light bulb; and from this we can compute the average time to repair a failed light bulb.
