DVA-C02 Troubleshooting and Optimization Practice Question
A web application running on Amazon EC2 instances behind an Application Load Balancer (ALB) is experiencing intermittent 503 errors. Which TWO steps should be taken to diagnose the issue?
⚠ Common exam trap
Watch out — candidates often confuse Route 53 health checks (DNS-level) with ALB target group health checks (application-level), or assume that security groups or EBS volumes are the root cause of HTTP 503 errors when they are not directly related to load balancer routing failures.
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
✓
Check the CPU utilization of the EC2 instances.
High CPU utilization on EC2 instances can cause them to become unresponsive or fail to respond to health checks within the ALB's configured timeout, leading to 503 errors. The ALB routes traffic only to healthy targets; if instances are overwhelmed, they may fail health checks or drop requests, resulting in a 503 response to clients.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Check the Route 53 health checks for the domain.
Why it's wrong here
Route 53 health checks primarily influence DNS routing decisions, such as failing over to a secondary resource record set if the primary becomes unhealthy. They do not directly cause 503 Service Unavailable errors originating from an Application Load Balancer (ALB), which typically indicate issues with the ALB's registered targets. An ALB would still attempt to forward requests even if Route 53 marked its associated DNS record as unhealthy, leading to different error types or routing behavior, not an ALB-generated 503.
- ✓
Check the CPU utilization of the EC2 instances.
Why this is correct
High CPU utilization on EC2 instances can severely impact their ability to process requests and respond to health checks in a timely manner. If instances are consistently overloaded, they may fail the Application Load Balancer's (ALB) health checks, causing the ALB to mark them as unhealthy. Consequently, the ALB will stop routing traffic to these instances and return 503 Service Unavailable errors to clients, as it has no healthy targets to forward requests to.
- ✓
Check the target group health check settings and instance health status.
Why this is correct
The Application Load Balancer (ALB) relies on target group health checks to determine the operational status of registered EC2 instances. If instances within a target group fail these configured health checks (e.g., due to application errors, network issues, or resource exhaustion), the ALB marks them as unhealthy. When an ALB has no healthy targets available in its target groups to process incoming requests, it responds to clients with a 503 Service Unavailable error.
- ✗
Check the security group rules for the ALB.
Why it's wrong here
Incorrect security group rules for the Application Load Balancer (ALB) itself, or between the ALB and its target instances, typically manifest as connection timeouts or "504 Gateway Timeout" errors. If the ALB cannot receive traffic from clients or cannot establish connections to its backend EC2 instances due to security group restrictions, it would not generate a "503 Service Unavailable" error, which specifically indicates that the server (ALB) is unable to handle the request because no healthy targets are available.
- ✗
Check the EBS volume type of the EC2 instances.
Why it's wrong here
The EBS volume type (e.g., gp2, gp3, io1) primarily affects the I/O performance characteristics of the attached storage for an EC2 instance. While extremely poor EBS performance could indirectly contribute to application slowness or unresponsiveness, leading to failed health checks, the volume type itself is not a direct cause of an Application Load Balancer (ALB) returning a 503 Service Unavailable error. A 503 error directly signifies an issue with the ALB's ability to find or communicate with healthy targets, not typically a storage-specific bottleneck.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DVA-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 DVA-C02 exam.