nested cloudformation get vpcid on vpc creation - amazon-web-services

Question

Following several threads on SO and aws forums, I am trying to get a basic nested cloudformation example working.
The ChildStack01 creates a VPC, then ChildStack02 adds a subnet. but after trying several combinations I get the same type of error, Output 'VpcID' not found in stack
{
"AWSTemplateFormatVersion": "2010-09-09",
"Resources": {
"ChildStack01": {
"Type": "AWS::CloudFormation::Stack",
"Properties": {
"TemplateURL": "https://s3-eu-west-1.amazonaws.com/cf-templates-1u1ziwem31f87-eu-west-1/xxx",
"TimeoutInMinutes": "60"
}
},
"ChildStack02": {
"Type": "AWS::CloudFormation::Stack",
"Properties": {
"TemplateURL": "https://s3-eu-west-1.amazonaws.com/cf-templates-1u1ziwem31f87-eu-west-1/yyy",
"Parameters": {
"VpcId" : { "Fn::GetAtt" : [ "ChildStack01", "Outputs.VpcID" ] }
},
"TimeoutInMinutes": "60"
}
}
}
I have tried adding a parameter with
"VPC" : {
"Description" : "VPC ID",
"Type": "AWS::EC2::VPC::Id"
}
but then generates an error as there is no reference value for VPC listed in http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-ref.html
Is there a basic way to get the VPC-id after it has been created?
thanks
Art

Answer 1

You should look at Exports.
In your VPC stack, create an Export section:
Outputs:
VpcId:
Value: !Ref VPC
Export:
Name: Unique-VpcId
You can then import that value in another stack:
VpcId:
Fn::ImportValue: Unique-VpcId
You should of course include some way of generating unique export names (they have to be unique within a region) rather than hard-coding as in my example.

Answer 2

In CF Template create VPC, use as below:
"Resources": {
"VPC": {
"Type": "AWS::EC2::VPC",
"Properties": {
....
}
}
},
"Outputs": {
"VPC": {
"Description": "VPC",
"Value": {
"Ref": "VPC"
},
"Export": {
"Name": {
"Fn::Sub": "${AWS::StackName}-VPC"
}
}
}
}
In CF Template that you want to use output of 1st CF Template, as:
"Parameters": {
"VPCStackName": {
"Description": "Name of VPC CF Stack",
"Type": "String",
"Default": "SOME_NAME"
}
},
"Resources": {
"Subnet1": {
"Type": "AWS::EC2::Subnet",
"Properties": {
"VpcId": {
"Fn::ImportValue": {
"Fn::Sub": "${VPCStackName}-VPC"
}
},
"CidrBlock": {
"Ref": "CidrBlockSubnet1"
},
"AvailabilityZone": {
"Ref": "AZ_NAME"
}
}
}
}