AWS Certified SysOps Administrator – Associate (legacy) — Question 566

A SysOps Administrator is maintaining an application that runs on Amazon EC2 instances behind an Application Load Balancer (ALB). Users are reporting errors when attempting to launch the application. The Administrator notices an increase in the HTTPCode_ELB_5xx_Count Amazon CloudWatch metric for the load balancer.
What is a possible cause for this increase?

Answer options

• A. The ALB is associated with private subnets within the VPC.
• B. The ALB received a request from a client, but the client closed the connection.
• C. The ALB security group is not configured to allow inbound traffic from the users.
• D. The ALB target group does not contain healthy EC2 instances.

Correct answer: D

Explanation

An increase in the HTTPCode_ELB_5xx_Count metric indicates that the load balancer itself is returning 5xx server-side errors, which commonly happens when there are no healthy EC2 instances available in the target group to process requests. In this scenario, the ALB cannot route the traffic and returns an HTTP 503 Service Unavailable error. Misconfigured security groups or subnet routing issues would typically result in connection timeouts or unreachable host errors rather than ELB-generated 5xx responses.