Cloudformation - Passing Parameters to second level stack

Question

I'm trying to pass some parameters to nested stacks.

My current configuration is the following:

root template:

Parameters:
  SubnetIds:
    Description: The array of Subnet IDs assigned to the lambdas
    Type: List<AWS::EC2::Subnet::Id>
  SecurityGroupIds:
    Description: The array of Security Groups Assigned to the lambda functions
    Type: List<AWS::EC2::SecurityGroup::Id>

Resources:
 Myresource1:
    Type: 'AWS::Serverless::Application'
    Properties:
      Location: 'resource1/template.yaml'
      Parameters:
        SubnetIds: !Join [',', !Ref SubnetIds]
        SecurityGroupIds: !Join [',', !Ref SecurityGroupIds]

first nested stack:

Parameters:
  SubnetIds:
    Description: The array of Subnet IDs assigned to the lambdas
    Type: List<AWS::EC2::Subnet::Id>
  SecurityGroupIds:
    Description: The array of Security Groups Assigned to the lambda functions
    Type: List<AWS::EC2::SecurityGroup::Id>

Resources:
  MySecondLevelResource:
    Type: 'AWS::Serverless::Application'
    Properties:
      Location: 'app/template.yaml'
      Parameters:
        SubnetIds: !Ref SubnetIds
        SecurityGroupIds: !Ref SecurityGroupIds

second level nested stack:

Parameters:
  SubnetIds:
    Description: The array of Subnet IDs assigned to the lambdas
    Type: CommaDelimitedList
  SecurityGroupIds:
    Description: The array of Security Groups Assigned to the lambda functions
    Type: CommaDelimitedList

With this configuration I get an error when AWS tries to deploy the first nested stack because it is expecting a String or object of Strings. I tried also to use the CommaDelimitedList type in the first level stack but then I still get an error in the second level. So far no luck on that.

Has anyone experienced this situation or any ideas on how it can be solved?

Answer 1

First, you have a major mistake in your templates:

SubnetIds: !Join [',', !Ref SecurityGroupIds]

The use of SecurityGroupIds will lead to failure as SecurityGroupIds are not SubnetIds, regardless of any other issues.

Also nested stacks are created using AWS::CloudFormation::Stack which has different syntax that what you are using. So the way you pass arguments is correct if you are actually using nested stacks by means of AWS::CloudFormation::Stack.