DOP-C02 Monitoring and Logging Practice Question
A company uses Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer. The operations team notices that some instances are failing health checks but are not being terminated by Auto Scaling. What should be investigated to resolve this issue?
⚠ Common exam trap
Test-takers frequently confuse the health check grace period with the load balancer's health check settings, assuming that if health checks are failing, the issue must be with the health check configuration (Option A) rather than the Auto Scaling group's delay in acting on those 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 health check grace period setting in the Auto Scaling group. If it is too long, instances failing health checks may not be terminated quickly.
The health check grace period in an Auto Scaling group controls how long Auto Scaling waits before checking the health of a newly launched instance. If this grace period is set too long, instances that fail health checks before the grace period expires will not be terminated promptly, leading to the observed behavior where unhealthy instances remain in service even though they are failing health checks. The default grace period is 300 seconds, but if it is excessively long, it delays the termination of instances that fail health checks during that period.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Confirm that the load balancer's health check target is pointing to the correct port and path on the instances.
Why it's wrong here
An incorrect health check target on the load balancer would cause the ELB to mark the instances as unhealthy and trigger Auto Scaling to terminate and replace them, not leave them running. The observed symptom of persistent running instances would not result from a mismatched port or path; instead you would see instance churn and replacement activity in the scaling group.
- ✓
Check the health check grace period setting in the Auto Scaling group. If it is too long, instances failing health checks may not be terminated quickly.
Why this is correct
The health check grace period tells Auto Scaling how long to wait after an instance launches before it starts evaluating the instance's health status. If this value is set too high, an instance that is already failing health checks will continue running without being terminated because the elapsed time since launch has not yet exceeded the grace period. Extending the grace period can therefore leave unhealthy instances in service, which matches the described issue.
- ✗
Ensure the instances are sending health check requests to the load balancer.
Why it's wrong here
Health checks in an Application Load Balancer or Classic Load Balancer are sent by the load balancer to the instances, not by the instances to the load balancer. Instances simply listen on the health check port and return a response; they do not initiate these probes. Therefore 'instances sending health check requests' is not a mechanism that could affect termination, making this option irrelevant to the problem.
- ✗
Verify that the security group for the instances allows inbound traffic from the load balancer on the health check port.
Why it's wrong here
Security groups control traffic flow but the health check is initiated by the ELB; if the security group blocks the ELB, the health check would fail and the instance would be marked unhealthy, leading to termination.
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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.