AWS Certified CloudOps Engineer Associate SOA-C03 Practice Question

An e-commerce application runs on EC2 instances in two Availability Zones, fronted by an Application Load Balancer (ALB). Some checkout requests take 3 to 4 minutes to complete, and users intermittently receive 504 Gateway Timeout responses. CloudWatch shows the targets are healthy and no Auto Scaling scale-in events occurred. Which change will most effectively prevent these timeouts without redesigning the application?

Enable connection draining by setting the target group deregistration delay to 300 seconds.
Enable cross-zone load balancing on the ALB.
Increase the ALB idle timeout to a value higher than the longest expected request processing time.
Replace the ALB with a Network Load Balancer to remove all timeout limits.

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Answer Description

The ALB returns a 504 Gateway Timeout when it closes the connection because no response is received from the target before the load balancer's idle timeout elapses. The default idle timeout for an ALB is 60 seconds, which is shorter than the 3- to 4-minute checkout operations. Increasing the idle timeout to exceed the longest expected request duration allows the load balancer to keep the connection open until the target responds, eliminating the observed 504 errors.

Changing the deregistration delay only affects in-flight requests during scale-in, not long-running requests on healthy targets. Enabling cross-zone load balancing distributes traffic across zones and does not influence connection timeouts. A Network Load Balancer also has a connection idle timeout (with a default of 350 seconds for TCP listeners), so replacing the ALB with an NLB would not remove timeout limits. Simply adjusting the ALB's idle timeout is the most direct and cost-effective fix.

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