How to reference a dynamic lambda ARN in AWS Step Functions written in the Serverless Framework?

Question

EDIT: Just found this https://github.com/serverless-operations/serverless-step-functions/issues/209 Basically that example, but instead of hardcoding the ARN, I want to use a input variable, if that makes sense.

Here's the context:

Using Serverless' error destinations, an error payload is sent to SQS, which triggers a lambda that starts a state machine to perform a retry.

For example, if Lambda A fails, the failure is sent to SQS -> Lambda B which triggers a state machine to retry Lambda A.

I'm defining my state machine in Serverless.yml like so (This is what I've been trying so far):

stepFunctions:
  stateMachines:
    MyStateMachine:
      name: RetryLambdaMachine
      definition:
        Comment: Example to test retries
        StartAt: StepOne
        States:
          StepOne:
            Type: Task
            Resource: arn:aws:states:::lambda:invoke
            Parameters:
            - FunctionName.$: $$.lambdaArn 
            #### ^^ This is where I need Lambda A to be referenced ####
            Retry:
            - ErrorEquals: 
              - States.ALL
              MaxAttempts: 2
            Catch:
            - ErrorEquals: ["States.ALL"]
              Next: CatchAllFallback
            End: true
          CatchAllFallback:
            Type: Task
            Resource: 
              Fn::GetAtt: [lambda_c, Arn]
            End: true

Here is the error I get when I run serverless deploy:

Error: The CloudFormation template is invalid: [/Resources/RetryLambdasMachineRole/Type/Policies/0/PolicyDocument/Statement/0/Resource/0] 'null' values are not allowed in templates

How do I reference Lambda A as a variable? I know that the error attributes will contain Lambda A's ARN, but how do I pass that to the state machine for the retry step?

Note: Lambda A is NOT defined within this Serverless.yml, it could come from anywhere. The ARN will be passed in as part of the error event coming from SQS, as stated above.

Answer 1

I found out that my original code was actually pretty close. The only issue was that there was a hyphen in front of the FunctionName key and and extra '$' on lambdaArn.

    States:
      StepOne:
        Type: Task
        Resource: arn:aws:states:::lambda:invoke
        Parameters:
          FunctionName.$: $.lambdaArn

This allows the lambda to be passed in as a dynamic resource.