Learn about availability, how to measure it, and its importance.

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) = (Total Time Amount Of Time Service Was Down)Total Time100\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.

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