Understanding the use cases of the template macro

Before we dive into the internals of macros, we need to know what we can solve with them. Let's look at a few cases and examples.

Auto filling resource property values

Imagine having a launch template where you need to define an AMI ID. For Amazon Linux AMI, you could use AWS's parameter store:

Parameters:
  ImageId:
    Type: AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>
    Default: '/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2'
Resources:
  LaunchTemplate:
    Type: "AWS::EC2::LaunchTemplate"
    Properties:
      LaunchTemplateData:
        ImageId: !Ref ImageId

But what if we don't use Amazon Linux, but Ubuntu? We'd have to manually specify the AMI IDs and use mappings:

Mappings: 
...