DVA-C02 Troubleshooting and Optimization Practice Question
A web application running on EC2 instances behind an Application Load Balancer (ALB) is experiencing intermittent 503 errors. The ALB target group health checks are succeeding. Which step should the developer take FIRST to diagnose the issue?
⚠ Common exam trap
Test-takers frequently assume 503 errors always mean unhealthy instances, so they jump to checking instance state or scaling, ignoring that health checks are passing and that ALB-level issues (like connection limits or timeouts) are the actual cause.
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
✓
Examine the ALB access logs for 503 responses.
The correct first step is to examine the ALB access logs for 503 responses. Since health checks are succeeding, the EC2 instances are considered healthy by the target group, but the ALB itself may be returning 503 errors due to issues like request rate limits, connection limits, or backend response timeouts. Access logs provide detailed HTTP response codes and timestamps, allowing you to identify the pattern and cause of the 503 errors without making assumptions about instance count or state.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the number of EC2 instances in the target group.
Why it's wrong here
Increasing EC2 instances is a scaling action primarily used to handle increased traffic load or improve availability, not a diagnostic step for intermittent 503 errors when existing instances are passing health checks. If instances are healthy but still returning 503s, the issue likely lies with application-level problems, misconfigurations, or specific request patterns overloading individual instances, which adding more instances might only temporarily mask or exacerbate without identifying the root cause. This approach doesn't help pinpoint *why* the 503s are occurring.
- ✓
Examine the ALB access logs for 503 responses.
Why this is correct
Examining ALB access logs is the most effective diagnostic step because these logs capture detailed information about every request processed by the load balancer, including the HTTP status code returned to the client and the target status code from the EC2 instance. Filtering for 503 responses ("HTTP 503" or "target_status_code:503") allows identification of specific request patterns, source IPs, or target groups that are experiencing issues. This data helps pinpoint whether the 503s are due to application errors, target connection issues, or other load balancer-related problems.
- ✗
Check the Route 53 record for the ALB.
Why it's wrong here
Checking the Route 53 record for the ALB is irrelevant for diagnosing intermittent 503 errors. Route 53 is responsible for DNS resolution, translating the domain name into the ALB's IP address. If the application is intermittently accessible and only some requests are failing with 503s, it indicates that DNS resolution is working correctly and clients are successfully reaching the ALB. A Route 53 issue would typically manifest as complete unavailability or incorrect routing, not intermittent application-level errors.
- ✗
Verify that the EC2 instances are in a running state.
Why it's wrong here
While verifying EC2 instances are in a running state is a fundamental check, it's insufficient for diagnosing intermittent 503 errors when health checks are already passing. An instance can be running and respond successfully to a basic health check (e.g., /health endpoint) but still experience application-level issues, resource exhaustion, or specific code errors that cause it to return 503s for certain application requests. The health check might not cover the specific problematic application logic, making this a superficial check for the described problem.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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