Availability

What is availability?

Availability is the percentage of time that some service or infrastructure is accessible to clients and is operated upon under normal conditions. For example, if a service has 100% availability, it means that the said service functions and responds as intended (operates normally) all the time.

Measuring availability

Mathematically, availability, A, is a ratio. The higher the A value, the better. We can also write this up as a formula:

A (in percent) = $\frac{(Total\ Time\ - Amount\ Of\ Time\ Service\ Was\ Down)}{Total\ Time} * 100$

We measure availability as a number of nines. The following table shows how much downtime is permitted when we’re using a given number of nines.