AZ-900 Describe cloud concepts Practice Question
When using Azure, a company wants to calculate the composite SLA for two services used together where both must be available. Service A has a 99.9% SLA and Service B has a 99.95% SLA. What is the composite SLA?
⚠ Common exam trap
Watch out — candidates often confuse the 'and' (multiplicative) scenario with the 'or' (redundancy) scenario, mistakenly picking the higher or lower SLA instead of performing the multiplication.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Approximately 99.85%
When two services must both be available (an 'and' scenario), the composite SLA is calculated by multiplying their individual SLAs. Service A at 99.9% (0.999) and Service B at 99.95% (0.9995) gives 0.999 × 0.9995 = 0.9985005, which rounds to approximately 99.85%. This reflects the increased probability of failure when multiple dependencies are required.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
99.95% (the higher of the two SLAs)
Why it's wrong here
Selecting the higher individual SLA (99.95%) ignores the dependency between the services: the composite solution is successful only if both services are simultaneously available. Even if the second service is up 99.95% of the time, the first service being down 0.1% of the time still causes failure during that period, and vice versa. Thus, the combined availability must be less than the higher availability, and in fact less than both individual SLAs, so 99.95% as an answer is mathematically impossible.
- ✗
99.9% (the lower of the two SLAs)
Why it's wrong here
While 99.9% is the lower of the two individual SLAs, the composite availability is even lower because the probability that both services are up is 0.999 × 0.9995 = 0.9985, which is less than 0.999. The intuitive guess that the overall SLA equals the weakest link would be accurate only if the services were independent and the system could operate with just one of them, but the scenario implies a chain of dependencies, so all components must be simultaneously available. Therefore, 99.9% overestimates the true composite SLA.
- ✓
Approximately 99.85%
Why this is correct
This is correct because when two services are part of a single end-to-end solution and both must be available for the solution to work, their SLAs are multiplied: 99.9% (0.999) × 99.95% (0.9995) = 0.9985005, which rounds to ~99.85%. The resulting composite SLA is lower than either individual SLA, reflecting the increased risk of failure when multiple dependent components are involved. This calculation is the standard method Azure documents for estimating the availability of a composite workload.
- ✗
100% because Azure guarantees maximum availability
Why it's wrong here
Azure does not provide a 100% uptime SLA for any service; SLAs are finite commitments (e.g., 99.9% or 99.95%) that account for planned maintenance and unplanned failures. Even if a service were to achieve 100% availability, a composite SLA for dependent services is always the product of individual SLAs, so it cannot exceed the lowest component and would still be below 100% if any component is less than 100%. The claim also misinterprets 'maximum availability' as a guarantee, when Azure's SLA is a contractual uptime percentage backed by service credits, not a promise of zero downtime.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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