AZ-900 Describe cloud concepts Practice Question
A company runs business-critical ERP software on Azure. If the ERP application has a 99.9% SLA and the Azure SQL Database backing it has a 99.99% SLA, what is the overall composite SLA?
⚠ Common exam trap
The trap here is that candidates mistakenly pick the lowest SLA (Option C) or highest SLA (Option A) instead of multiplying the probabilities, failing to recognize that composite SLA for dependent services is a product, not a min or max.
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 99.89%
The composite SLA for dependent services is calculated by multiplying their individual SLAs. Here, 99.9% (0.999) multiplied by 99.99% (0.9999) equals approximately 0.9989, or 99.89%. This reflects the overall availability when both the ERP application and the Azure SQL Database must be operational for the system to function.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
99.99% (take the highest SLA)
Why it's wrong here
Selecting the highest individual SLA, 99.99%, ignores the fundamental rule of composite availability for dependent services. In a chain where the ERP app or the Azure SQL Database can each cause failure, the combined probability of uptime is the product of their individual probabilities, never the maximum. Even if SQL is extremely reliable, the app's 99.9% SLA caps the overall availability far below 99.99%.
- ✓
Approximately 99.89%
Why this is correct
The composite SLA for the ERP system is determined by multiplying the availability percentages of its dependent components. Since the ERP application (99.9% SLA) relies directly on the Azure SQL Database (99.99% SLA), their individual availabilities are multiplied: 0.999 * 0.9999 = 0.9989001. This calculation reflects the serial dependency where the failure of either component impacts the entire system's uptime, resulting in an overall availability of approximately 99.89%.
- ✗
99.9% (take the lowest SLA)
Why it's wrong here
Choosing the lowest SLA, 99.9%, is closer but still incorrect because the composite availability is not the minimum but the joint probability of both components being available. Since the app and database must both be up, the correct calculation is 0.999 × 0.9999 = 0.9989001, which is slightly lower than the app's 99.9%. The database's 99.99% SLA adds an additional, albeit small, chance of failure, so you cannot simply take the lower number.
- ✗
200% because two services are running
Why it's wrong here
The idea that two services yield 200% availability reflects a basic misunderstanding of probability and SLA math. Availability percentages are bounded between 0% and 100%; a probability cannot exceed 1.0. Moreover, adding SLAs would only apply to parallel redundant components, not to serial dependencies like an app and its database, where both must work sequentially.
Go deeper
Related to this question
Learn chapter
High Availability and Scalability
Key term
Azure SQL Database
Azure SQL Database is a fully managed relational database-as-a-service (DBaaS) in Microsoft Azure, based on the SQL Server engine, that handles scaling, backups, patching, and high availability automatically.
Key term
SLA
A Service Level Agreement (SLA) is a contract between a service provider and a customer that defines the level of service expected, including metrics like uptime, response time, and penalties for non-compliance.
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