DOP-C02 Resilient Cloud Solutions Practice Question
A company is deploying a critical application on Amazon EC2 instances behind an Application Load Balancer (ALB) across multiple Availability Zones. The application must be resilient to the failure of an entire Availability Zone. Which design should the company implement?
⚠ Common exam trap
Many exam-takers assume that simply placing instances in multiple AZs behind a load balancer is sufficient, but they overlook the critical requirement that the load balancer itself must be deployed across multiple AZs to avoid being a single point of failure.
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 Availability Zones and place them behind an Application Load Balancer with cross-zone load balancing enabled.
Deploying EC2 instances across at least two Availability Zones (AZs) behind an Application Load Balancer (ALB) with cross-zone load balancing enabled ensures that if an entire AZ fails, the ALB can route traffic to healthy instances in the remaining AZs. Cross-zone load balancing allows the ALB to distribute incoming requests evenly across all registered instances in all enabled AZs, which improves fault tolerance and resource utilization. This design meets the requirement for resilience to an AZ failure by eliminating a single point of failure at the AZ level.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Launch EC2 instances in at least two Availability Zones and place them behind an Application Load Balancer with cross-zone load balancing enabled.
Why this is correct
This is the correct approach. An Application Load Balancer is a regional service; by enabling subnets in at least two Availability Zones, you create redundant ALB nodes. The ALB performs health checks and automatically routes traffic to healthy EC2 instances across both AZs. Cross-zone load balancing ensures each instance receives an equal share of requests, so the architecture tolerates an AZ failure and even an instance failure without manual intervention.
- ✗
Use one EC2 instance in a single Availability Zone behind a Network Load Balancer.
Why it's wrong here
A Network Load Balancer can distribute traffic, but with only one EC2 instance in a single Availability Zone, the entire workload is a single point of failure. If that instance or its AZ becomes unavailable, the NLB has no healthy target to forward traffic to. The NLB itself is not the problem; the critical flaw is the complete lack of redundancy in compute capacity. This design provides no automatic failover or resilience against either instance or AZ failure.
- ✗
Launch EC2 instances in one Availability Zone and use an Application Load Balancer to distribute traffic.
Why it's wrong here
While an Application Load Balancer is regional, it requires healthy targets to route traffic. If all EC2 instances are in one Availability Zone, that AZ is a single point of failure; if it goes down, the ALB has no healthy instances to send requests to. This setup does not achieve high availability because the failure domain is not isolated. It only provides load distribution within a single AZ, not resilience against AZ loss.
- ✗
Deploy EC2 instances in two Availability Zones but use a single Application Load Balancer in one AZ.
Why it's wrong here
An Application Load Balancer only becomes highly available if you explicitly enable subnets in multiple Availability Zones. If you deploy the ALB with a single subnet in one AZ, the ALB node is a single point of failure even though your instances are in two AZs. Traffic must first pass through that ALB node, so if it fails, all access is lost. This misconfiguration negates the resilience provided by the multi-AZ instances.
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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.