Question 220 of 724
ALB 503 Errors: Diagnose with Access Logs
An application running on EC2 instances behind an Application Load Balancer (ALB) occasionally returns HTTP 503 errors. The instances are in an Auto Scaling group. Which action should be taken to resolve this issue?
Quick Answer
The correct action is to review the ALB access logs to identify the target response codes. HTTP 503 errors from an Application Load Balancer mean the targets—your EC2 instances—are failing to respond successfully, often due to overload, application crashes, or health check failures. Access logs capture the exact response code returned by each target (e.g., a 503 from the instance itself versus a connection timeout), allowing you to distinguish between a saturated Auto Scaling group and a misconfigured application. On the AWS Certified Developer Associate DVA-C02 exam, this question tests your ability to diagnose load balancer issues systematically rather than jumping to scaling or health check tweaks. A common trap is assuming you should immediately increase instance count or modify health check thresholds, but the logs must be your first step to confirm the root cause. Memory tip: “503 from ALB? Log the target’s reply.”
⚠ Common exam trap
The trap here is that candidates often jump to scaling or instance size changes (Option D) without first using access logs to diagnose whether the 503s originate from the ALB or the targets, leading to ineffective fixes.
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
✓
Review the ALB access logs to identify the target response codes.
HTTP 503 errors from an ALB indicate that the targets (EC2 instances) are not responding successfully. Reviewing ALB access logs reveals the specific target response codes (e.g., 503 from the target itself or connection timeouts), which helps pinpoint whether the issue is due to overloaded instances, application errors, or health check failures. This diagnostic step is essential before making any configuration changes.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enable cross-zone load balancing on the ALB.
Why it's wrong here
Cross-zone balancing distributes traffic evenly but doesn't directly address 503 errors.
- ✓
Review the ALB access logs to identify the target response codes.
Why this is correct
Access logs show whether the 503 is from targets or the ALB, guiding further action.
- ✗
Increase the ALB idle timeout setting.
Why it's wrong here
Idle timeout affects connection persistence, not 503 errors.
- ✗
Increase the size of the EC2 instances.
Why it's wrong here
While helpful if overloaded, it's a guess without data; access logs should be checked first.
Visual reference
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Same concept, more angles
2 more ways this is tested on DVA-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 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?
medium- A.Increase the number of EC2 instances in the target group.
- ✓ B.Examine the ALB access logs for 503 responses.
- C.Check the Route 53 record for the ALB.
- D.Verify that the EC2 instances are in a running state.
Why B: 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.
Variation 2. An application uses an Application Load Balancer (ALB) with a target group of EC2 instances. Users report intermittent HTTP 503 errors. The ALB access logs show that the error occurs when the request rate exceeds 10,000 requests per second. What is the most likely cause?
hard- A.The EC2 instances are failing health checks.
- B.The SSL certificate is expiring.
- ✓ C.The ALB is exceeding its connection limit.
- D.The target group's connection draining is too short.
Why C: Application Load Balancers have a default limit of 10,000 new connections per second (this can be increased via a limit increase request). When the request rate exceeds this limit, the ALB returns HTTP 503 errors. Option A is incorrect because if EC2 instances were failing health checks, the ALB would route traffic to healthy instances, and the error would not be tied to a specific request rate. Option B is incorrect because an expiring SSL certificate would cause certificate validation errors, not 503 errors. Option D is incorrect because connection draining affects the graceful shutdown of instances and would not cause 503 errors when the request rate exceeds the ALB's limits.
Last reviewed: Jun 24, 2026
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