availabilitydiffer

Availabilitydiffer is a metric used in reliability engineering and service management to quantify the difference in system availability between two configurations, time periods, or service instances. It expresses how much one system’s availability deviates from another, facilitating comparisons across releases, regions, or design choices.

Formally, if A1 and A2 denote availabilities on a common basis (values between 0 and 1), availabilitydiffer = |A1 − A2|. Availability itself is commonly defined as the fraction of time a system is operational, which can be computed as MTBF/(MTBF+MTTR) or as uptime divided by the measurement window. When both methods are applicable, they should use the same definition to ensure meaningful results.

Applications include benchmarking between configurations, monitoring improvements after maintenance, and SLA evaluation. It supports quick, scalar assessment of reliability changes and can be used for pairwise comparisons across components, services, or regions. For time-series data, availabilitydiffer can be tracked as a function of window length to observe trends.

Limitations: the metric summarizes only a single scalar and ignores outage duration distribution, latency of downtime, and variance in failure rates. It is sensitive to the chosen measurement window and definition of availability. A small availabilitydiffer does not necessarily imply small risk if downtimes are rare but long.

See also: availability, MTBF, MTTR, reliability, uptime, Service Level Agreement.