DOP-C02 Resilient Cloud Solutions Practice Question
A company runs a critical application on AWS that uses an Auto Scaling group of EC2 instances. The application must remain available even if an entire Availability Zone fails. Which THREE actions should the company take?
⚠ Common exam trap
The trap here is that candidates might think using multiple subnets within a single AZ (Option C) provides redundancy, but it does not protect against an AZ failure, which requires instances to be spread across at least two distinct 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
✓
Configure an ALB health check to automatically replace unhealthy instances.
Configuring an ALB health check allows the Auto Scaling group to automatically detect and replace unhealthy instances. The ALB health check pings the instances at a specified interval (e.g., every 30 seconds) and marks them as unhealthy if they fail to respond. This triggers the Auto Scaling group to terminate the unhealthy instance and launch a new one, maintaining application availability even if an instance fails within a single 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 an ALB health check to automatically replace unhealthy instances.
Why this is correct
An ALB's health check marks an instance unhealthy when it repeatedly fails the configured protocol and path check, and the ALB then stops sending traffic to that instance. When the Auto Scaling group is configured to use ELB health checks, the 'ReplaceUnhealthy' process automatically terminates the unhealthy instance and launches a new one to maintain desired capacity. Thus, the health check is the trigger that enables the self-healing replacement of failed instances, ensuring application availability.
- ✗
Use a single instance in each Availability Zone to minimize cost.
Why it's wrong here
Running a single instance per Availability Zone means each AZ has an instance that is a single point of failure; if that instance crashes or its underlying host degrades, the workload in that AZ becomes unavailable despite the AZ itself being operational. Cost optimization at the expense of redundancy directly contradicts the high-availability requirement, because the application would experience downtime lasting from failure detection to replacement. Any SLA for a critical application would be violated by this design.
- ✗
Use multiple subnets in each Availability Zone for the instances.
Why it's wrong here
Creating multiple subnets within each Availability Zone does not increase resilience, because all subnets in an AZ reside in the same physically isolated facility and share the same network, power, and cooling infrastructure; a failure at the AZ level takes down every subnet within it. AZ resilience requires placing instances in distinct Availability Zones, not merely splitting the VPC into more subnets. Over-engineering subnets also adds unnecessary routing complexity without any fault-tolerance benefit.
- ✓
Configure the Auto Scaling group to launch instances in at least two Availability Zones.
Why this is correct
Configuring the Auto Scaling group to span at least two Availability Zones means it can maintain capacity even when one AZ fails, because the launch template specifies subnets from multiple AZs and the ASG distributes instances across them. If an entire AZ becomes unavailable, the ASG's health check mechanism detects the loss and attempts to launch new instances in the remaining AZs, thereby preserving the desired instance count. This is a foundational practice for building an active-active architecture on AWS.
- ✓
Use an Elastic Load Balancer (ELB) to distribute traffic across the instances in different AZs.
Why this is correct
An Elastic Load Balancer (e.g., Application or Network Load Balancer) fronts the instances and performs health checks, routing traffic only to targets that are healthy in all enabled AZs. If a target or an entire AZ fails, the load balancer automatically shifts traffic to the remaining healthy targets, providing seamless cross-AZ failover. This distributes the incoming request load across the auto-scaled instances and eliminates the single point of failure that a direct connection to any one instance would create.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DOP-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 DOP-C02 exam.