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Cloud Computing · Intermediate

Composite SLA

Overall availability of a solution calculated by multiplying the SLAs of the dependent services it uses.

Definition

When an application depends on several services in series, its expected availability is the product of their individual SLAs, which is lower than any single SLA. Adding redundant independent paths raises availability, calculated as 1 minus the product of their failure probabilities.

Example

A web app at 99.95 percent depending on a database at 99.99 percent has a composite SLA of about 99.94 percent.

Exam tip

Multiply serial dependencies; for redundant paths compute 1 - (1 - A)(1 - B). Expect a calculation on Azure architecture exams.

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