Question

A company runs a stateful web application behind an Application Load Balancer (ALB) with a target group of six EC2 instances across three Availability Zones. To keep user sessions on the same backend, the team enabled duration-based cookie stickiness on the target group. After a marketing campaign, users report slow response times, and CloudWatch shows two instances consistently at high CPU while the other four remain nearly idle. What is the MOST likely cause of the uneven load and the BEST corrective action?

Accepted Answer

Session stickiness is binding long-lived client sessions to a subset of instances; the imbalance is expected with stickiness, and the team should scale based on connection distribution or reduce the stickiness duration.

Answer

Cross-zone load balancing is disabled; enable it on the ALB so requests are distributed evenly across all registered targets.

Answer

The health check interval is too long; shorten it so the ALB detects and reroutes traffic away from busy instances faster.

Answer

The target group deregistration delay is too high; lower it so busy instances drain connections and receive fewer new requests.