如何传递 CloudFormation JSON 中的参考列表？

Question

I used to have a CloudFormation stack which passed VpcId and a List<AWS::EC2::Subnet> (list of subnets). but then I decided I actually want my stack to create its own VPC. So I came up with this:

{
  "PublicSubnetOne": {
    "Type": "AWS::EC2::Subnet",
    "Properties": {
      "VpcId": {"Ref": "VPC"},
      "CidrBlock": "10.0.0.0/24",
      "AvailabilityZone": {
        "Fn::Select": ["0", {"Fn::GetAZs": ""}]
      }
    }
  },
  "PublicSubnetTwo": {
    "Type": "AWS::EC2::Subnet",
    "Properties": {
      "VpcId": {"Ref": "VPC"},
      "CidrBlock": "10.0.0.0/24",
      "AvailabilityZone": {
        "Fn::Select": ["1", {"Fn::GetAZs": ""}]
      }
    }
  },
  "ApplicationLoadBalancer" : {
    "Type" : "AWS::ElasticLoadBalancingV2::LoadBalancer",
    "Properties" : {
      "Subnets" : { "Ref": "PublicSubnetOne,PublicSubnetTwo"}
    }
  },
}

This results in the below error:

Unresolved resource dependencies [PublicSubnetOne,PublicSubnetTwo] in the Resources block of the template

How to correctly pass a List<> of Refs to a property?

Answer 1

Try this:

{"Ref": "PublicSubnetOne,PublicSubnetTwo"}
[{"Ref": "PublicSubnetOne"} , {"Ref": "PublicSubnetTwo"}]

When you say {"Ref": "PublicSubnetOne,PublicSubnetTwo"} , CloudFormation looks for something with the name verbatim "PublicSubnetOne,PublicSubnetTwo" .

You may be confused by this, because it's totally fine to pass a Parameter with --parameter-overrides (from aws cloudformation deploy ) with the value "vpc-someidhere,vpc-someidhere2" , but it doesn't work that way with refs. I assume CloudFormation formats a comma separated string into a list if a Parameter 's Type is List<>

There's also an answer that has similar keywords that actually recommends joining (Fn::Join) two IDs together, but I got an error like Subnets should be List of String with that