Writing reusable CloudFormation snippets with AWS::Include and Nested Stacks - amazon-web-services

Question

I have been using nested stacks in CloudFormation for several months and they are very useful. So I thought I should spend sometime to make each nested stack reusable to other teams in the org.
I saw the use case of AWS::Include in several places like here and here and it makes good sense to me.
One approach I have in mind is one snippet for each resource, like an AWS::EC2::Subnet or AWS::EC2::InternetGateway which can be included zero or more times into a vpc.json template, which itself can be used as a nested stack in a larger application.
The snippet does not take any parameters, but can reference a parameter that exists in the parent template.
At first glance this doesn't seem enough to me. Consider this example:
"PublicSubnet": {
"Type": "AWS::EC2::Subnet",
"Properties": {
"VpcId": {"Ref": "VPC"},
"AvailabilityZone": {
"Fn::Select" : [ "0", { "Fn::GetAZs" : {"Ref": "AWS::Region"} }]
},
"CidrBlock": {
"Fn::FindInMap": ["AZSubnetMap", {
"Fn::Select" : [ "0", { "Fn::GetAZs" : {"Ref": "AWS::Region"} }]},
"PublicSubnet"]},
"MapPublicIpOnLaunch": "true",
"Tags": [..]
}
}
How can I avoid hard coding that "0" for the AZ in a Subnet snippet for example?

Answer 1

Unfortunately, AWS doesn't provide a way to dynamically update the template as per the requirement.
I have solved a similar problem using Mustache Templates using Java Library Handle Bars. Using this library you can generate template on the fly based on the requirements.
Hope this helps.

Answer 2

You will have to use two AWS::Include files located in:
s3://yourname/PublicSubnetA.yaml
s3://yourname/PublicSubnetB.yaml
And call them from your MAIN Template:
Fn::Transform:
Name: AWS::Include
Parameters:
Location : "s3://yourname/PublicSubnetA.yaml"
Fn::Transform:
Name: AWS::Include
Parameters:
Location : "s3://yourname/PublicSubnetB.yaml"
I 'am trying to find way to send additional parameters or override parameter to AWS::include, as you see it has
Parameters:
Location:
Why ti's not understanding more parameters and not just Location, I would be glad to have something like this:
Fn::Transform:
Name: AWS::Include
Parameters:
MySubnetIndex: 0
Location : "s3://yourname/PublicSubnetB.yaml"

Answer 3

I tried this way to send additional parameters:
Fn::Transform:
Name: AWS::Include
Parameters:
Location : "s3://my.test/create-ec2.yaml"
EC2Size :
Type: String
Default: "t2.micro"
And got interesting error:
The value of parameter EC2Size under transform Include must resolve to a string, number, boolean or a list of any of these
Looks like it understand what this is additional parameters, probably it need to be configured little bit different. I was not able to find way to fix this error yet.