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Availability is a measure of the readiness of a dependable system at a point in time, i.e., when a client needs to use a service provided by the system, the probability that the system is there to provide the service to the client. The availability of a system is determined by two factors:

 ◾ Mean time to failure (MTTF). It characterizes how long the system can run without a failure.

 ◾ Mean time to repair (MTTR). It characterizes how long the system can be repaired and recovered to be fully functional again.

Availability is defined to be MTTF/(MTTF + MTTR). Hence, the larger the MTTF, and higher the availability of a system. Similarly, the smaller the MTTR, the higher the availability of the system.

The availability of a system is typically represented in terms of how many 9s. For example, if a system is claimed to offer five 9s availability, it means that the system will be available with a probability of 99.999%, i.e., the system has 10⁻⁵ probability to be not available when a client wants to access the service offered by the system at any time, which means that the system may have at most 5.256 minutes of down time a year.