Always trigger stack rerun on update

Question

I have a CFN root stack with nested stacks in it, one of which is essentially a lambda function. I want that nested stack to run on any update triggered on the root stack. It may be possible that there is no parameter/template changes to this nested stack, but I still want to rerun it.

detailed use-case: The nested stack retrieves details about a VPC and produces outputs based on that. While the inputs to this stack may not have changed, the outputs may change based on changes in the VPC. Therefore I need a way to tell AWS to `ALWAYS` run this stack in case of updates.

Answer

I can think of two ways to achieve this.

First 
Set the lambda up as an cloudwatch rule which is triggered by this specific stack update - I believe this would have to be a "AWS API Call via CloudTrail" event. Your nested stack would create the cloudwatch rule and potentially run the lambda on the initial creation. But I don't think this will work in your case as you need the output within the stack on the update

Second - one I have used multiple times
Create a parameter to the parent stack, pass to nested stack and add as a environment var within your lambda. When I have used this method I usually triggered via a pipeline so I make the parameter the pipeline-execution-id so it changes on every execution.

Always trigger stack rerun on update

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