Combine SLAs to compute the composite SLA

January 17, 2021

1 min read

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After you’ve identified the SLA for the individual workloads in the Special Orders application, you might notice that those SLAs are not all the same. How does this affect our overall application SLA requirement of 99.9 percent? To work that out, you’ll need to do some math.

The process of combining SLAs helps you compute the composite SLA for a set of services. Computing the composite SLA requires that you multiply the SLA of each individual service.

From Service Level Agreements , you discover the SLA for each Azure service that you need. They are:

Service	SLA
Azure Virtual Machines	99.9 percent
Azure SQL Database	99.99 percent
Azure Load Balancer	99.99 percent

Composite SLA

Recall that you need two virtual machines. Therefore, you include the Virtual Machines SLA of 99.9 percent two times in the formula.

Note that even though all of the individual services have SLAs equal to or better than the application SLA, combining them results in an overall number that’s lower than the 99.9 percent you need. Why? Because using multiple services adds an extra level of complexity and slightly increases the risk of failure.

You see here that the composite SLA of 99.78 percent doesn’t meet the required SLA of 99.9 percent. You might go back to team and ask whether this is acceptable. Or you might implement some other strategies into your design to improve this SLA.