An application on EC2 instances is behind an Application Load Balancer. A CodeDeploy release fails during the AllowTraffic lifecycle event, but deployment logs show no explicit cause. What would explain this failure?

During the CodeDeploy AllowTraffic lifecycle event, the Application Load Balancer (ALB) shifts traffic to the newly deployed instances. For this to succeed, the instances must pass the health checks configured for their target group. If these health checks are misconfigured (e.g., wrong port, path, or unhealthy thresholds), the ALB will not register the new instances as healthy, causing the AllowTraffic event to fail and the deployment to roll back. Incorrect options: A broken script in appspec.yml would likely cause a failure during an earlier lifecycle event (e.g., BeforeInstall, ApplicationStart) when the script is executed, not specifically during AllowTraffic. Lack of permissions for the user would typically result in an immediate access denied error, not a silent failure during AllowTraffic after instances are provisioned. The CodeDeploy agent is essential for the deployment itself. If it were missing, the deployment would fail much earlier, likely during the Install or BeforeInstall phases, as CodeDeploy wouldn't be able to interact with the instance.