SAP-C02 Continuous Improvement for Existing Solutions Practice Question
A company has a web application behind an Application Load Balancer that uses sticky sessions. The application is deployed on EC2 instances in an Auto Scaling group. During a deployment, the team notices that users are experiencing errors after new instances are launched. What is the MOST likely 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
✓
The Auto Scaling group's scale-in policy is terminating instances with active sessions.
The correct answer. The issue occurs during deployment when new instances are launched. With sticky sessions enabled, user sessions are tied to specific instances. If the Auto Scaling group's scale-in policy terminates instances that have active sessions (e.g., during a rolling update or instance replacement), users lose their session data and experience errors. Option A is incorrect because the deregistration delay is about waiting for in-flight requests to complete, not about session stickiness. Option B is incorrect because if stickiness duration were too long, requests would still be routed to terminated instances, but the issue here is after new instances are launched (scale-out), not scale-in. Option D is incorrect because health checks are configured and would pass for new healthy instances; the errors stem from missing session data on the new instances.
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 target group's deregistration delay is too short.
Why it's wrong here
Deregistration delay handles connection draining, not session creation.
- ✗
The stickiness duration is set too long, causing requests to be routed to terminated instances.
Why it's wrong here
Stickiness duration is not the issue; the problem is that new instances lack session state.
- ✓
The Auto Scaling group's scale-in policy is terminating instances with active sessions.
Why this is correct
Stickiness ties sessions to instances; if an instance is terminated, users lose session data. The correct fix is to externalize session state.
- ✗
The ALB health check is not configured for the new instances.
Why it's wrong here
Health checks would mark instances unhealthy, not cause errors.
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