SOA-C02 Reliability and Business Continuity Practice Question
A web application runs on Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer. To achieve high availability, what is the minimum number of Availability Zones (AZs) that must be configured for the Auto Scaling group?
⚠ Common exam trap
Many exam-takers think a single AZ is sufficient if the Auto Scaling group can replace failed instances, but they overlook that the AZ itself is a failure domain, and high availability requires redundancy across at least two AZs.
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
✓
2
For high availability, an Auto Scaling group must span at least two Availability Zones (AZs) to ensure that if one AZ fails, the application remains available from the other AZ. A single AZ would create a single point of failure, violating the high-availability requirement. The Application Load Balancer distributes traffic across healthy instances in all configured AZs, so two AZs are the minimum to achieve fault tolerance.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
1
Why it's wrong here
A single Availability Zone represents a single failure domain because all instances in that zone are dependent on the same underlying power, networking, and hardware. If that AZ experiences an outage, the Auto Scaling group cannot maintain availability because all instances are in the same location, leading to downtime. Therefore, one AZ fails to meet the basic requirement of high availability, which requires isolation from a single point of failure.
- ✓
2
Why this is correct
Placing instances across two Availability Zones in an Auto Scaling group ensures that if one AZ fails, the remaining AZ's instances continue serving traffic, and the group can automatically launch new instances in that healthy AZ to maintain the desired capacity. This configuration meets the classic high-availability requirement because AWS guarantees independent failure of AZs, so two AZs provide sufficient redundancy. For most production workloads, two AZs are the minimum architectural baseline for high availability.
- ✗
3
Why it's wrong here
Using three Availability Zones adds even greater resilience by spreading instances across three isolated locations, allowing the group to continue operating even during a rare event that impacts one or even two AZs simultaneously. However, high availability is already achieved with two AZs, so three AZs are an enhancement to improve fault tolerance and capacity burst availability, not the minimum required for basic HA. The phrase 'minimum required' makes this option incorrect.
- ✗
4
Why it's wrong here
Four Availability Zones provide the highest level of fault tolerance among the options, but they far exceed the minimum requirement for high availability, and not all AWS regions have four AZs available for use. This configuration is typically reserved for workloads that need extreme resilience or massive scaling capacity, and it introduces higher inter-AZ data transfer costs and management complexity. Since the question asks for the minimum to maintain availability, four is incorrect.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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