Before we go further into the details, we still need to clarify two terms that are important to our explanation:

• Uptime
• Downtime

The term, uptime, is easily explained, it is just another name for what I mentioned in the previous section of the chapter, operation time. In other words, the period of time when an application runs without errors and without interruptions.

The term, downtime, then designates as a counterpart, the period where it comes to an interruption. However, it does not matter to us whether the disruption has been deliberately caused (for example, in the case of maintenance tasks), or is a consequence of errors, damage or even a total failure.

Why are these terms important to us? Both values are part of the calculation of the degree of availability, and thus also a part of the definition of a SLA.