Using conditions in AWS CloudFormation templates allows you to define resources and configurations that are selectively included or excluded based on certain conditions. This feature is particularly useful when you want to create stacks that behave differently in different environments (e.g., development, testing, production) or when you want to include optional resources.

Here’s how to use conditions in your CloudFormation templates:

Define Parameters (Optional): You often start by defining parameters that will be used in your conditions. Parameters allow you to pass values into your template at runtime.

Parameters:

  EnvironmentType:

    Description: Type of environment to deploy

    Type: String

    AllowedValues:

      - dev

      - prod

Define Conditions: Conditions are defined in the Conditions section of the template. Each condition uses intrinsic functions to evaluate expressions based on parameters or pseudo parameters.

 IsProduction: !Equals [!Ref EnvironmentType, "prod"]

Use Conditions in Resources or Resource Properties: You can then reference these conditions in the Resources section to conditionally create resources or set properties.

Resources:

  MyBucket:

    Type: "AWS::S3::Bucket"

    Condition: IsProduction

    Properties:

      BucketName: !Sub "${AWS::StackName}-prod-bucket"

  MyDevBucket:

    Type: "AWS::S3::Bucket"

    Condition: !Not [IsProduction]

    Properties:

      BucketName: !Sub "${AWS::StackName}-dev-bucket"

Use Conditions with Resource Properties: You can also use conditions within resource properties to set values conditionally.

Resources:

  MyInstance:

    Type: "AWS::EC2::Instance"

    Properties:

      InstanceType: !If

        - IsProduction

        - "t2.large"

        - "t2.micro"

      ImageId: "ami-0ff8a43207f55f723"

Example Template

Here’s a complete example demonstrating the use of conditions in an AWS CloudFormation template:

Parameters:

  EnvironmentType:

    Description: Type of environment

    Type: String

    AllowedValues:

      - dev

      - prod

    Default: dev



Conditions:

  IsProduction:

    Fn::Equals: 

      - !Ref EnvironmentType

      - prod



Resources:

  MyBucket:

    Type: "AWS::S3::Bucket"

    Condition: IsProduction

    Properties:

      BucketName: !Sub "${AWS::StackName}-prod-bucket"



  MyBucketPolicy:

    Type: "AWS::S3::BucketPolicy"

    Condition: IsProduction

    Properties:

      Bucket: !Ref MyBucket

      PolicyDocument:

        Statement:

          - Action: "s3:*"

            Effect: Allow

            Resource: !Sub "arn:aws:s3:::${MyBucket}/*"

            Principal: "*"



Outputs:

  BucketName:

    Description: "The name of the S3 bucket"

    Value: !If

      - IsProduction

      - !Ref MyBucket

      - "No bucket created in dev environment"

 

Key Points

Parameters: Used to pass dynamic values into the template.

Conditions: Defined using intrinsic functions like !Equals, !And, !Or, !Not, etc.

Resources: Use the Condition key to conditionally create resources.

Properties: Use the !If intrinsic function to conditionally set property values.