Thought exercise
Returning to our digital company, MilesBetter Pizza, they have encountered some service availability issues over the last few months. They have noticed that their services have different SLAs, and they need to ensure that, in total, they always meet a target total of 99.9%. However, they are unsure how to calculate and achieve this.
First, the team at MilesBetter needs to understand the concept of a composite SLA; as we learned earlier in this chapter, this means that when you combine services, the overall SLA is lower than the individual highest SLA on one of the services. This is because each service that you add increases the probability of failure and increases complexity. The following example illustrates how a composite SLA is calculated across the services:
- Service 1 = 99.9%
- Service 2 = 99.95%
- Service 3 = 99.99%
- Service 4 = 99.99%
The calculation looks as follows, which shows that the target SLA will not be met:
0.999 x 0.9995...
