SOA-C02 Networking and Content Delivery Practice Question
A company is using an Application Load Balancer (ALB) to distribute traffic to a fleet of EC2 instances. The SysOps administrator receives reports that some users are experiencing intermittent HTTP 503 errors. What is the most likely cause?
⚠ Common exam trap
Watch out — candidates often confuse HTTP 503 errors with connectivity or timeout issues, but the ALB specifically returns 503 only when no healthy targets exist, not for security group or timeout misconfigurations.
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
✓
The health checks are failing for the target group, causing the ALB to stop sending traffic to all instances.
HTTP 503 errors from an Application Load Balancer typically indicate that the target group has no healthy registered targets. When health checks fail for all instances in the target group, the ALB cannot route traffic to any backend, resulting in a 503 response. This is the most common cause of intermittent 503 errors in ALB architectures.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The security group attached to the ALB does not allow inbound traffic on port 443.
Why it's wrong here
This would block traffic entirely, causing client timeout or refusal, not 503.
- ✓
The health checks are failing for the target group, causing the ALB to stop sending traffic to all instances.
Why this is correct
If all targets are unhealthy, ALB returns 503.
- ✗
The EC2 instances do not have the correct IAM role to register with the ALB.
Why it's wrong here
IAM role is not required for target registration.
- ✗
The ALB idle timeout is set too low.
Why it's wrong here
Low idle timeout causes 504 errors, not 503.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SOA-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 SOA-C02 exam.