How to include Serverless Repository apps in serverless.yml

Over the past year, the Serverless Application Repository (SAR) service has improved a lot. I have grown to enjoy it more and more, and have contributed a few applications myself:

• lambda-janitor: cron job to delete old, unused versions of all Lambda functions in the region to free up storage space.
• auto-subscribe-log-group-to-arn: subscribes new and existing CloudWatch log groups to a Lambda function, Kinesis stream, or Firehose delivery stream by ARN.
• auto-set-log-group-retention: updates the retention policy for new and existing CloudWatch log groups to a specified number of days to reduce CloudWatch Logs cost.

However, CloudFormation does not natively support the AWS::Serverless::Application resource type. Until recently, I had to use the SAM deployment framework in order to include a SAR app in my stack. That is, until the 1.41.0 release of the Serverless framework.

In this release, the Serverless framework has added support for CloudFormation’s Transform directive. This means we can use the same magic source (CloudFormation macros) that powers SAM!

To add a Serverless Repository app to your serverless.yml you will need to:

1. Add Transform: AWS::Serverless-2016–10–31 to the resources section of your serverless.yml. This enables a global macro that will run over the whole CloudFormation stack and transform special resources as necessary. In this case, it’ll transform AWS::Serverless::Application resources into nested CloudFormation stacks.
2. Add Serverless Repository apps as additional CloudFormation resources. These should have the resource type AWS::Serverless::Application.

For example, to include the auto-subscribe-log-group-to-arn app as part of a serverless.yml, I will need the following:

service: sar-app-in-sls-demo

provider:
  name: aws
  runtime: nodejs8.10

functions:
  hello:
    handler: handler.hello

resources:
  Transform: AWS::Serverless-2016-10-31
  Resources:
    SubscribeToApiGatewayLogs:
      Type: AWS::Serverless::Application
      Properties:
        Location:
          ApplicationId: arn:aws:serverlessrepo:us-east-1:374852340823:applications/auto-subscribe-log-group-to-arn
          SemanticVersion: 1.1.0
        Parameters:
          DestinationArn: !GetAtt HelloLambdaFunction.Arn
          Prefix: "API-Gateway-Execution-Logs"
          FilterPattern: ""

And that’s it, that’s how you can include Serverless Repository apps in your serverless.yml.