How dynamically change S3 path inside DefinitionBody - amazon-web-services

Question

My code currently is working like this
DefinitionBody:
'Fn::Transform':
Name: AWS::Include
Parameters:
Location:
Fn::Sub: s3://${Bucket}/${AWS::StackName}/${File}
# Note: Bucket and File comes from Parameters in this case
Now I try to get {$File} dynamically using FindInMap
'Fn::Transform':
Name: AWS::Include
Parameters:
Location: !Sub
- s3://${value-from-parameters-section}/${AWS::StackName}/${spec}
- { spec: !FindInMap [ "Config", "type", "file"]}
But is throwing me this error:
Failed to digest functions within transform parameters, intrinsic functions in transform block must only contain parameter values or stack metadata
So, How I can achieve to dynamically change the File name inside the DefinitonBody?

Answer 1

You can't do this, it is not supported. AWS docs explicitly states that no intrinsic functions are supported:
Supported functions None. CloudFormation passes any intrinsic function calls included in Fn::Transform to the specified macro as literal strings.
You can re-design your function to use custom resources instead of macros. This way you can do whatever you wish.

Answer 2

In this case, I think the message is self-explanatory
Failed to digest functions within transform parameters, intrinsic functions in transform block must only contain parameter values or stack metadata
Only metadata or parameters are allowed, that is the reason this works
DefinitionBody:
'Fn::Transform':
Name: AWS::Include
Parameters:
Location:
Fn::Sub: s3://${Bucket}/${AWS::StackName}/${StackType}
In case somebody needs a workaround I just remove the mapping section and rename the name of the files to match the type I want, something like this
Parameters:
StackType:
Type: String
AllowedValues:
- typeA
- typeB
Bucket:
Type: String
Then I just use this sub to get the DefinitionBody like this:
DefinitionBody:
'Fn::Transform':
Name: AWS::Include
Parameters:
Location:
Fn::Sub: s3://${Bucket}/${AWS::StackName}/${StackType}-API.yaml
This will resolve like this:
s3://example-bucket/name-of-stack/typeA-API.yaml
or
s3://example-bucket/name-of-stack/typeB-API.yaml
So now I just need to have these files named in that way on the S3, this is similar to what mapping should do, but for now, is not supported, this will achieve what I want, is not perfect but did the trick, still remain the confusion because I think the documentation is inaccurate