SOA-C02 Reliability and Business Continuity Practice Question
A company runs a web application on Amazon EC2 instances in an Auto Scaling group that spans two Availability Zones. The application uses an Application Load Balancer (ALB) that is deployed across the same Availability Zones. The SysOps administrator wants to ensure the application remains available if an entire Availability Zone fails. Which configuration is essential for this high availability?
⚠ Common exam trap
A common mix-up: candidates confuse cross-zone load balancing (which balances traffic) with instance distribution across AZs (which ensures survival), leading them to select Option B instead of recognizing that without instances in each AZ, no load balancing can save the application.
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
✓
Configure the Auto Scaling group with at least one instance in each Availability Zone.
For high availability across an Availability Zone (AZ) failure, the Auto Scaling group must have at least one healthy instance in each AZ. This ensures that if one AZ becomes unavailable, the ALB can route traffic to instances in the remaining AZ. Without this minimum distribution, a single AZ failure could leave the application with zero healthy targets if all instances were in the failed AZ.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure the Auto Scaling group with at least one instance in each Availability Zone.
Why this is correct
Configuring the Auto Scaling group to maintain at least one instance in each Availability Zone (AZ) ensures that if an entire AZ becomes unavailable, the remaining AZs still have healthy instances to serve traffic. Auto Scaling also performs AZ rebalancing, which automatically detects when one AZ has fewer instances and launches replacements in that AZ to maintain a balanced distribution. This is the fundamental mechanism for achieving fault tolerance at the AZ level within a single region, which is exactly what the requirement demands.
- ✗
Enable cross-zone load balancing on the Application Load Balancer.
Why it's wrong here
Cross-zone load balancing on an Application Load Balancer (ALB) distributes incoming traffic evenly across all registered instances in all enabled AZs, rather than confining traffic to instances in the same AZ as the client. However, this feature only affects traffic distribution among existing instances; it does not influence where Auto Scaling places instances. Furthermore, cross-zone load balancing is already enabled by default for ALBs, and if all instances reside in a single AZ, the feature cannot provide any protection against an AZ-wide outage.
- ✗
Use an Amazon Route 53 health check to route traffic away from a failed AZ.
Why it's wrong here
Amazon Route 53 health checks with failover routing can redirect DNS traffic from an unhealthy endpoint to a healthy endpoint, typically across AWS regions, but this is not the mechanism for handling an AZ failure within a region. An Application Load Balancer already monitors the health of its registered targets at the instance level and automatically stops routing requests to unhealthy instances, including instances in an impaired AZ. Using Route 53 for intra-region AZ failover would be redundant and much slower than the ALB's real-time health checking, which operates at the request level.
- ✗
Attach an Elastic IP address to each instance in the Auto Scaling group to ensure IP persistence.
Why it's wrong here
Attaching an Elastic IP (EIP) address to each instance only guarantees a fixed public IPv4 address for that individual instance; it does not provide any form of high availability or automatic failover if the instance or its AZ fails. An EIP remains allocated to the stopped or failed instance until you manually reassign it, so traffic cannot be redirected automatically to a healthy replacement. The Application Load Balancer already offers a stable DNS name and routes traffic to healthy instances, making EIP persistence unnecessary and potentially harmful by coupling clients to a single instance's IP address.
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Same concept, more angles
1 more way this is tested on SOA-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
easy- A.1
- ✓ B.2
- C.3
- D.4
Why B: 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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SOA-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SOA-C02 exam.