AZ-900 Practice Question 69: Composite SLA Chained Services

To reason about a chain of dependent services, you multiply the decimal SLA of each service in the request path: here, roughly 0.9995 × 0.9999 ≈ 99.94%, below either single figure, because the request now has two independent ways to fail instead of one. Microsoft is explicit that this composite figure is only an approximate maximum limit, not a guarantee, since it assumes the services fail independently, but the direction it illustrates, that chaining required dependencies can only pull availability down, is the fact being tested.

Option A is the "weakest link" fallacy in reverse: availability is bounded by the lower of the two SLAs, and adding a second, less-than-100%-available dependency can only pull the combined figure below that lower number, never up toward the higher one. Option B is wrong because percentages don't add in a dependency chain; summing two figures near 100% would produce a nonsensical result above 100%, and requiring more dependencies always reduces combined reliability, never increases it. Option D is wrong because Microsoft publishes an SLA per service; combining SLAs for a specific architecture is analysis the customer does for their own solution, not a figure Azure calculates or publishes automatically.

Reference: How to read a service-level agreement (SLA)