Availability

Let’s embark on the journey of designing a monitoring service for better availability.

What is availability?

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

Quantitatively

Mathematically, Availability, A, is a ratio; the higher the A value, the better. We can write it 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 will be permitted when we are using some number of nines.