Availability

Availability describes the degree to which the software system will work as required when it is needed. It is the probability that the software system is operating properly when needed by a user and is not experiencing unplanned downtime due to failure and repair.

It is typically measured in terms of nines, as in how many nines represent the availability level (99.9%, 99.99%, or 99.999%).

Calculating availability based on time

Availability can be calculated with the following formula:

MTBF is the mean time between failures, which is the average time between two failures of the system. MTTR is the mean time to repair, which is the average time to troubleshoot and repair the system back to its operational state. Only unplanned outages are considered when calculating system downtime.

To achieve high availability, which is considered five nines, or 99.999% availability, your software system can only be down for 5 minutes and 15 seconds or less during the course of a whole year! The following...