Accessing name of parent Cloudformation stack in nested stack - amazon-web-services

Question

I'm using a nested Cloudformation template structured like this:
masterTemplate -> childA -> childB
Each of the JSON template files is stored in S3 with a bucket name of "${masterStackName}-env-upload". This works fine from the parent template, as I can simply do:
"TemplateURL": {
"Fn::Join": [
"",
[
"https://s3.amazonaws.com/",
{
"Ref": "AWS::StackName"
},
"-env-upload/device-specific-template-HALO-BUILD-VERSION.json"
]
]
},
However, when childA attempts to do the same thing to launch the childB template, "AWS::StackName" becomes the generated name for childA - meaning that it is trying to access a non-existent bucket.
My question is: how can I pass down the name of the master/parent stack to the child stacks? I attempted to do this as a Parameter, but "Ref" is not allowed for parameter values (so I couldn't do "Ref" : "AWS::StackName" for the value).
Any help is appreciated!

Answer 1

It is in fact possible to pass the parent stack name to the child stacks using parameters:
Here's an exemple:
parent.yml
---
AWSTemplateFormatVersion: '2010-09-09'
Resources:
ChildA:
Type: AWS::CloudFormation::Stack
Properties:
TemplateURL: ...test/test-child-a.yml
Parameters:
ParentStackName: !Ref AWS::StackName
test-child-a.yml
---
AWSTemplateFormatVersion: '2010-09-09'
Parameters:
ParentStackName:
Type: String
Resources:
TestBucket:
Type: AWS::S3::Bucket
Properties:
BucketName: !Ref ParentStackName

Answer 2

One option is to decouple the stack name from the S3 bucket name, and specify the S3 bucket as a parameter in the masterTemplate stack, and then reference it in the Outputs section. The
In the master:
"Outputs": {
"EnvUploadBucketName" : {
"Value" : { "Ref" : "paramEnvUploadBucketName" }
}
}
In the child:
"TemplateURL": {
"Fn::Join": [
"",
[
"https://s3.amazonaws.com/",
{ "Fn::GetAtt" : [ "masterTemplate", "Outputs.EnvUploadBucketName" ] }
"/device-specific-template-HALO-BUILD-VERSION.json"
]
]
}
In this case, EnvUploadBucketName would be the name of the upload bucket, passed as an output from the masterTemplate stack.