High Availability Definition

The high availability definition is referred to as the ability of a computer system to function or operate for a long period without any failures. A highly available system ensures to operate continuously for a longer time than a normal period.

One of the main characteristics of a highly available system is, if one component goes down it makes sure the entire system does not go down with it. A highly available system can instantly redirect requests to a backup system if a component fails. Usually, this process is accomplished by following a failover mechanism.

In such a failover mechanism, the components that are more likely to fail have a cloned backup component. The cloned component is called the hot spare. If it is a server, server A has a cloned server B. The cloned server B is offline but constantly monitors server A. As soon as server A goes down, server B switches on and all the traffic is redirected to server B which is the hot spare.

How to calculate availability?

If a system has an availability of 99.99% or more, it is considered a highly available system. System availability can be calculated by dividing the system uptime by the total time passed since the system started. The total time is the summation of system uptime and downtime.

Let’s assume you have been running a web server for a month and it has only gone down for 4.38 minutes during that month.

The total duration the server was running = 43800 minutes

The uptime of the server = (43800 – 4.38 ) minutes = 43795.62 minutes

The availability of the server = (43795.62/43800) * 100 % = 99.99 %