AWS CloudFormation for update (rerun?) of downstream nested Stacks

Question

I'm trying to do the following: the parent Stacks launches the first child Stack which creates a fully configured EC2 instance. Once that is completed, the parent Stack kicks off a second Stack that uses a Lambda function to create an AMI, which is then used for an AutoScaling setup even further downstream. This is working perfectly.

Now the challenge: when I update the metadata for the EC2 instance from the first child Stack I would really like the second Stack to be triggered. In other words: I want to be able to change the seed instance and have the CloudFormation Stack update, creating a new AMI.

I'm able to get the seed instance to update, but the second child Stack isn't triggered :-(

I've Google everything I could think of, but Update Policy doesn't apply, manually kicking off the second child is defying the point of having nested Stacks and I'm pretty sure I'm missing some obvious feature or clever trick, so I'm asking you guys to help me out. Please.

Answer 1

Have you tried using Lambda-backed custom resource? You can have the service token of resource as the Lambda and use DependsOn with the first nested stack. It will kick off whenever the CF script runs or updates.

You can also lookup the stack itself from the Lambda function to determine any changes if you want.

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/template-custom-resources.html