SOA-C02 Reliability and Business Continuity Practice Question

A company is designing a highly available web application on AWS. The application runs on EC2 instances behind an Application Load Balancer. Which configuration ensures that the application remains available if an entire AWS Availability Zone fails?

⚠ Common exam trap

Test-takers frequently confuse high availability with scalability or performance, mistakenly thinking that larger instances (Option C) or multi-Region deployment (Option D) are required, when the core requirement is simply eliminating a single AZ as a point of failure by using multiple AZs within the same region.

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

✓

Launch EC2 instances in at least two different Availability Zones.

Deploying EC2 instances in at least two different Availability Zones (AZs) ensures that if one AZ fails, the Application Load Balancer (ALB) can route traffic to healthy instances in the remaining AZ(s). ALBs are regional constructs that automatically distribute traffic across registered targets in multiple AZs, and they perform health checks to detect and route away from failed AZs. This design meets the high availability requirement by eliminating the AZ as a single point of failure.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Deploy EC2 instances in multiple subnets of the same Availability Zone.
Why it's wrong here
Subnets in the same Availability Zone do not provide fault isolation because an Availability Zone is a single failure domain comprising one or more data centers with shared power, cooling, and network connectivity. Even spreading instances across multiple subnets within the same AZ leaves them vulnerable to a complete AZ outage, such as a regional power failure or cooling system malfunction that takes down the entire zone.
✓
Launch EC2 instances in at least two different Availability Zones.
Why this is correct
Launching EC2 instances in at least two different Availability Zones is the correct approach because each AZ is an isolated, independent failure domain with separate power, cooling, and physical infrastructure. A load balancer can then distribute traffic across these AZs, so if one AZ becomes unavailable, the remaining AZs continue to serve traffic, maintaining high availability within the same AWS Region without cross-region latency.
✗
Use a larger EC2 instance type to handle the load.
Why it's wrong here
Using a larger EC2 instance type addresses capacity and performance needs by increasing the instance's vCPUs, memory, and network bandwidth, but it does nothing to eliminate the single point of failure inherent in being located within a single Availability Zone. Vertical scaling merely increases the maximum load one instance can handle; if the underlying AZ experiences an outage, the larger instance still becomes unavailable, so it does not improve availability.
✗
Use EC2 instances in multiple AWS Regions.
Why it's wrong here
Deploying EC2 instances in multiple AWS Regions provides protection against region-wide disasters, but it is an over-engineered solution for the need to survive an Availability Zone failure. Multi-Region deployment introduces significant cross-region data transfer costs, higher latency for users not near the secondary region, and complex replication or failover logic, whereas distributing instances across multiple AZs within a single Region already achieves high availability at lower cost and latency.

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SOA-C02 Reliability and Business Continuity Practice Question

Why each option matters

Option-by-option breakdown

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