CloudFormation protip: use !Sub instead of !Join

Yan Cui

I help clients go faster for less using serverless technologies.

This article is brought to you by

Hookdeck: The Serverless Event Gateway

Hookdeck is a reliable and scalable serverless event gateway for sending, receiving, authenticating, transforming, filtering, and routing events between services in your event-driven architecture.

Learn more

CloudFormation supports a number of intrinsic functions and Fn::Join (or !Join) is often used to construct parameterised names and paths.

The Serverless framework, for instance, uses it extensively. A quick look in a CloudFormation it generates I can see Fn::Join used for:

  • IAM policy names
  • IAM role names
  • IAM principals
  • API Gateway URIs
  • Resource ARNs

and many more.

But it’s not just the frameworks that are using Fn::Join heavily. They also show up in our own code all the time as well. For example, to construct the ARN for a resource, or the URI for an API Gateway endpoint.

I find these very hard to comprehend, and my protip for you today is to use Fn::Sub (or the !Sub shorthand) instead.

Many folks would use Fn::Sub when they need to reference pseudo parameters such as AWS::Region and AWS::AccountId, for example:

!Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:vpc/${vpc}'

But Fn::Sub also lets you provide your own parameters. For example:

  - 'arn:aws:s3:::${Bucket}/*'
  - { Bucket: Ref MyBucket }

The advantage over Fn::Join is that you can see the pattern of the interpolated string. Whereas with Fn::Join you’ll have to construct the pattern in your mind, which requires far more cognitive energy.

  - ''
  - - 'arn:aws:s3:::'
    - !Ref MyBucket
    - '/*'

Here are a few side-by-side comparisons to drive home the message.

Example 1: IAM role name

RoleName:  # hello-world-dev-{region}-lambdaRole
    - '-'
    - - 'hello-world'
      - 'dev'
      - !Ref 'AWS::Region'
      - 'lambdaRole'

with Fn::Sub:

  !Sub 'hello-world-dev-${AWS::Region}-lambdaRole

Example 2: API Gateway integration URI

Uri: # arn:{partition}:apigateway:{region}:.../{lambda}/invocations
    - ''
    - - 'arn:'
      - Ref: AWS::Partition
      - ':apigateway:'
      - Ref: AWS::Region
      - ':lambda:path/2015-03-31/functions/'
      - !GetAtt 'HelloLambdaFunction.Arn'
      - '/invocations'

with Fn::Sub:

    - 'arn:${AWS::Partition}:apigateway:${AWS::Region}:lambda:path/2015/03/31/functions/${Function}/invocations'
    - { Function: !GetAtt 'HelloLambdaFunction.Arn' }

Example 3: Lambda permission for API Gateway

SourceArn: # arn:{partition}:execute-api:{region}:.../*/*
    - ''
    - - 'arn:'
      - Ref: AWS::Partition
      - ':execute-api:'
      - Ref: AWS::Region
      - ':'
      - Ref: AWS::AccountId
      - ':'
      - Ref: ApiGatewayRestApi
      - '/*/*'

with Fn::Sub:

    - 'arn:${AWS::Partition}:execute-api:${AWS::Region}:${AWS::AccountId}:${RestApi}/*/*'
    - { RestApi: Ref: ApiGatewayRestApi }

Suffice to say that the Fn::Sub version is easier to understand in every case! Now that you have seen what Fn::Sub can do, I hope you will prefer it to Fn::Join going forward.

Finally, if you’re using the Serverless framework and need more expressive power than the intrinsic functions can offer, then check out this plugin. It lets you use a number of “extrinsic” functions such as Fn::Substring or Fn::StartsWith anywhere in your serverless.yml.

Whenever you’re ready, here are 4 ways I can help you:

  1. Production-Ready Serverless: Join 20+ AWS Heroes & Community Builders and 1000+ other students in levelling up your serverless game. This is your one-stop shop for quickly levelling up your serverless skills.
  2. Do you want to know how to test serverless architectures with a fast dev & test loop? Check out my latest course, Testing Serverless Architectures and learn the smart way to test serverless.
  3. I help clients launch product ideas, improve their development processes and upskill their teams. If you’d like to work together, then let’s get in touch.
  4. Join my community on Discord, ask questions, and join the discussion on all things AWS and Serverless.