Parameterizing AWS CloudFormation Templates

AWS Cloudformation templates enable users to define and provision AWS infrastructure and resources in a declarative manner. Parameterizing your CloudFormation template is considered a best practice because it enhances the flexibility, maintainability, and reusability of your infrastructure code. These templates serve as a blueprint during the deployment process.

In the example template, we have declared two parameters: InstanceTypeParameter and KeyNameParameter. These parameters must be provided by the user during the deployment process. Lets break them down:

1. InstanceTypeParameter: This enables users to choose the instance type when the stack is created. A valid type must be used (e.g., t2.micro, t2.small, t2.medium). If the user specifies a type that in not an allowed AllowedValues, the deployment will fail. If no instance type is specified, the deployment will use t2.micro by default.

InstanceTypeParameter:
  Type: String
  Description: EC2 instance type
  Default: t2.micro
  AllowedValues:
    - t2.micro
    - t2.small
    - t2.medium
  ConstraintDescription: Must be a valid EC2 instance type.

2. KeyNameParameter: This lets the users select an existing EC2 KeyPair, which enables SSH access to the instance. Since there is no Default value, the user is required to specify one during deployment. Otherwise, the deployment will fail.

KeyNameParameter:
Type: AWS::EC2::KeyPair::KeyName
Description: Name of an existing EC2 KeyPair to enable SSH access to the instance.

Finally, these parameters are specified during the deployment process. For example, if using the AWS CLI to create the stack, here's what the command would look like:

aws cloudformation create-stack --stack-name MyStackName \
--template-body file://my-template.yaml \
--parameters ParameterKey=InstanceTypeParameter,ParameterValue=t2.micro \ ParameterKey=KeyNameParameter,ParameterValue=my-key-pair-name

Parameterizing templates is particularly useful because they're reusable, eliminating the need to create a new unique template for every deployment.

For example, if we wanted to use create another EC2 instance, but using a t2.medium type instead, we can still use same template. We would just need to specify a different value for the InstanceTypeParameter:

    aws cloudformation create-stack --stack-name NewStackName \
    --template-body file://my-template.yaml \
    --parameters ParameterKey=InstanceTypeParameter,ParameterValue=t2.medium \
    ParameterKey=KeyNameParameter,ParameterValue=my-key-pair-name

For more details, you can refer to the AWS CloudFormation documentation.