What would cause this?
The appspec.yml file contains an invalid script that runs in the AllowTraffic lifecycle hook.
The user who initiated the deployment does not have the necessary permissions to interact with the ALB.
The health checks specified for the ALB target group are misconfigured.
The CodeDeploy agent was not installed in the EC2 instances that are part of the ALB target group.
Explanations:
The AllowTraffic lifecycle event is triggered after the deployment is successful. If there were issues with the appspec.yml file or scripts, the failure would typically occur before reaching the AllowTraffic phase.
The failure is not related to user permissions but rather to the deployment process or EC2 instance health. The deployment can still proceed as long as the user has sufficient permissions for CodeDeploy actions.
If the ALB health checks are misconfigured, instances may not be marked as healthy, which causes the deployment to fail during the AllowTraffic phase when CodeDeploy attempts to send traffic to them.
The CodeDeploy agent is not responsible for the AllowTraffic lifecycle event. This event interacts directly with the ALB and instances, and if the agent were missing, it would cause failures in earlier stages, not during AllowTraffic.
I organize that the answer is:
The health checks specified for the ALB target group are misconfigured.