What should the SysOps administrator do to troubleshoot this issue?
Verify that the Auto Scaling group is configured to use all AWS Regions.
Verify that the application is running on the protocol and the port that the listener is expecting.
Verify the listener priority in the ALB. Change the priority if necessary.
Verify the maximum number of instances in the Auto Scaling group. Change the number if necessary.
Explanations:
Verifying that the Auto Scaling group is configured to use all AWS Regions is unnecessary. The issue likely lies with the ALB health check or the EC2 instance itself, not the region configuration.
The most common cause of EC2 instance health check failures is a mismatch between the protocol/port the application is running on and what the ALB expects. Verifying that the application matches the expected protocol (HTTP/HTTPS) and port (e.g., 80, 443) is key to resolving health check failures.
Listener priority affects routing rules for the ALB but does not typically cause EC2 health check failures. The issue is likely related to the health check configuration or application status.
The maximum number of instances in the Auto Scaling group is unlikely to be the issue, as the health check failures are more related to the health of the instances themselves rather than the scaling limit.
I think the answer is:
Verify that the application is running on the protocol and the port that the listener is expecting.