Lambda@Edge with serverless

Lambda@Edge allows you to run lambda functions in response to CloudFront events. In order to use a lambda function with CloudFront, you need to make sure that your function can assume edgelambda identity. I want to show you an easy way to do it with serverless.

Missing identity

As of right now, serverless framework has no native support for lambda@edge. There is a plugin though that allows you to associate your lambda functions with a CloudFront distribution’s behavior.

The plugin works great if you deploy and control both your lambda functions and its associations with the CloudFront distributions. You might, however, be deploying a global function that is to be used by different teams on different distributions. Here’s a good example - a function that supports redirecting / to /index.html deeper in the URL hierarchy than the site root.

Serverless allows you to define additional IAM role statements in iamRoleStatements block but doesn’t seem to have a shortcut for the iamRoleLambdaExecution. You can certainly configure your own custom IAM::Role but that’s a pretty involved excercise if all you want to achieve is this:

Easy way out

If you don’t define your own IAM::Role, serverless will create one for you. The easiest way to see how it looks is to run sls package, look inside your .serverless folder, and inspect the CloudFormation JSON that will orchestrate your deployment. Look for IamRoleLambdaExecution in the Resources group.

Serverless carries a template that it uses as a starting point to build the role definition. The good news is that serverless merges it into the list of other resources that you might have defined in your serverless.yml. Take a look at the code if you want to see how it does it.

The name of the roles seems to always default to IamRoleLambdaExecution (here and here). Knowing how lodash’s merge works, all we need to do now is to give our resources definition a little boost.

In my serverless.yml:

Resources:
  IamRoleLambdaExecution:
    Type: "AWS::IAM::Role"
    Properties:
      AssumeRolePolicyDocument:
        Statement: 
          - Effect: Allow
            Principal: 
              Service:
                - lambda.amazonaws.com
                - edgelambda.amazonaws.com

And that’s it. Serverless will merge its template over this structure and will keep the edgelambda principal in there. Enjoy!