What would cause this?
The appspec.yml file contains an invalid script to execute 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 does not involve executing scripts from the appspec.yml file. It is used to make the newly deployed version available by shifting traffic to the instances. If there was an issue with a script, the failure would likely occur in the BeforeAllowTraffic or AfterAllowTraffic hooks.
The permissions to interact with the ALB are not required for the CodeDeploy agent to perform the AllowTraffic operation. CodeDeploy interacts with the instances directly, not through the ALB, during this lifecycle event.
Misconfigured health checks on the ALB target group can prevent instances from passing the health check, thus causing the AllowTraffic event to fail. The deployment cannot proceed if the instances are not considered healthy, as traffic cannot be routed to them.
The CodeDeploy agent is responsible for handling deployments on EC2 instances. However, the failure occurs during the AllowTraffic event, which is related to traffic routing, not the agent installation. The agent should be installed on the EC2 instances, but it is not the cause of failure during this lifecycle event.
I believe the answer is:
The health checks specified for the ALB target group are misconfigured.