Why does GetAttr not work on cloudformation template parameters?

Question

Have a collection of cloudformation templates in a parent-child relationship and want to pass an AWS::IAM::Role into the parameters of a child stack and use GetAttr to get the Arn.

This fails validation because can only call GetAttr on resources, not on parameters.

Anyone know/guess why this is designed in this way?

It's not a problem as it can be worked around by just passing the Arn into the stack, I'm just curious really

Answer 1

What Fn::GetAttr and Parameters are trying to do in AWS CloudFormation is fundamentally different. As per AWS docs:

The intrinsic function Fn::GetAtt returns the value of an attribute from a resource in the template.

[1] http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-getatt.html

You can use the optional Parameters section to pass values into your template when you create a stack. With parameters, you can create templates that are customized each time you create a stack.

[2] http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/parameters-section-structure.html

I believe your confusion is stemming from the fact that you're trying to think of this in terms of the object-oriented/some other programming paradigm, where Resources and Parameters are some kind of objects and Fn::GetAttr is a generic function which retrieves the value of a reference that's passed in as an argument.

Answer 2

In my case, I wanted to access the resource attributes like ARN and Name in the nested stack. If I pass resource string and use GetAtt to get these, I dont need to pass them as two parameters. With this limitation, I had to pass them as two parameters.