Diagnosing 503 Errors from ALB
A company uses an Application Load Balancer (ALB) in front of an Auto Scaling group of EC2 instances. The application is experiencing intermittent HTTP 503 errors. The DevOps team needs to diagnose the cause. Which THREE of the following should the team investigate? (Choose THREE.)
Quick Answer
The answer is to investigate health check configuration and target group health status, insufficient instances in the Auto Scaling group, and the ALB’s idle timeout settings. When the ALB’s health checks fail or target instances are marked unhealthy, the load balancer has no healthy targets to route traffic to, resulting in a 503 error. Similarly, if the Auto Scaling group lacks enough running instances to handle the current request volume, the ALB cannot distribute the load and returns 503. The idle timeout setting can also cause 503 errors if it is set too low, forcing premature closure of connections before the application finishes processing. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this question tests your understanding of ALB error codes and their root causes, often using 503 as a trap to distinguish from SSL-related 502 errors or security group issues. Remember the mnemonic “HIT” for Health, Instances, and Timeout to quickly recall the three common 503 triggers.
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
✓
Auto Scaling group minimum capacity and scaling policy
The correct options are C, D, and E. Option C is correct because if the Auto Scaling group's minimum capacity is too low or scaling policies are not responsive, there may be insufficient instances to handle the load, causing HTTP 503 errors. Option D is correct because a low idle timeout setting on the ALB can cause premature closure of idle connections, leading to 503 errors for long-lived requests. Option E is correct because if health checks are misconfigured or instances are unhealthy, the ALB will route traffic to unhealthy targets or have no healthy targets, resulting in 503 errors. Option A is incorrect because security group inbound rules affect whether traffic can reach the ALB; if they block traffic, the client would receive a timeout or connection refused, not a 503 from the ALB. Option B is incorrect because SSL certificate expiration causes SSL handshake failures, resulting in 502 Bad Gateway or connection errors, not 503.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Security group inbound rules for the ALB
Why it's wrong here
Inbound rules affect client access, but 503 is from ALB.
- ✗
SSL certificate expiration on the ALB
Why it's wrong here
SSL issues cause 502 or 400, not 503.
- ✓
Auto Scaling group minimum capacity and scaling policy
Why this is correct
Not enough instances can cause 503.
- ✓
ALB idle timeout settings
Why this is correct
Timeout can cause connection drops leading to 503.
- ✓
Health check configuration and target group health status
Why this is correct
Unhealthy targets cause 503.
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Same concept, more angles
1 more way this is tested on DOP-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 company runs a multi-tier web application on EC2 instances behind an Application Load Balancer. The application experiences intermittent 503 errors during peak traffic. The Auto Scaling group is configured with a step scaling policy based on CPU utilization. CloudWatch metrics show that CPU utilization never exceeds 70%, but the ALB target group reports that some targets are unhealthy. What is the MOST likely cause?
hard- ✓ A.The application health check endpoint is returning HTTP 5xx or timing out.
- B.The ALB is misconfigured with an incorrect security group blocking traffic to the targets.
- C.The ALB connection draining settings are too short, causing in-flight requests to fail.
- D.The step scaling policy is too aggressive and is terminating instances prematurely.
Why A: Health checks are failing, causing the ALB to stop sending traffic to those instances, which results in 503 errors. The CPU utilization might be low because the unhealthy instances are not receiving traffic. Option B is wrong because if the scaling policy were too aggressive, you'd see more instances and possibly lower CPU, not 503 errors. Option C is wrong because connection draining does not cause health check failures. Option D is wrong because the application itself is failing health checks, not the ALB configuration.
JA
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.