How to Calculate and Design IT Service Availability

Architecture Techniques

Image by the author from Canva Pro

A common problem we do not deal with the attention it deserves is the proper specification, design and estimation of IT services and systems availability. The result is the shockingly high number of outages of web portals, mobile applications, and many business support systems not exposed to the ordinary person.

This article lays down a systematic method to think about the availability of a business service and what underlies it.

Three foundations of service availability

A. The inherent availability of the physical IT system

1. The formula below expresses the availability of a physical system or software application:
   A = MTBF / (MTBF + MTTR)
   MTBF = Mean Time Between Failures, MTTR = Mean Time To Repair. E.g., A = 0.999 when MTBF = 3 years and MTTR = 1 day.
2. Availability values come out as fractions under 1, e.g., 0.999; we commonly refer to them by multiplying by 100, e.g., as 99.9% availability.
3. We use the number of 9s in the value as a reference, and we have three 9s (99.9%), four 9s (99.99%), five 9s (99.999), etc., although other values such as 99.5 or 99.95 are also common.