Serverless Step Functions: Avoid Leaky Abstractions

I have some exciting news to share with you about the  plugin.

One of the main pain points of using the plugin was that you needed to use fully-formed ARNs. We addressed this in  by supporting CloudFormation intrinsic functions 

Fn:GetAtt

Ref

functions:
  hello-world:
    handler: hello-world.handler

stepFunctions:
  stateMachines:
    myStateMachine:
      definition:
        StartAt: HelloWorld
        States:
          HelloWorld:
            Type: Task
            Resource:
              Fn::GetAtt: [HelloDashworldLambdaFunction, Arn]
            End: true

But this is still a leaky abstraction - you have to know how the  converts local function names into CloudFormation logical IDs.

Newcomers would often get confused here.

“How did you get the logical ID HelloDashworldLambdaFunction from hello-world?”

I can hardly blame them for not knowing. This is an implementation detail in the Serverless framework, one that you shouldn’t have to care about!

Which is why I’m really happy to tell you that, as of  you can reference local functions using their local names in the state machine definition.

functions:
  hello-world:
    handler: hello-world.handler

stepFunctions:
  stateMachines:
    myStateMachine:
      definition:
        StartAt: firstTask
        States:
          firstTask:
            Type: Task
            Resource:
              Fn::GetAtt: [hello-world, Arn]
            Next: secondTask
          secondTask:
            Type: Task
            Resource: arn:aws:states:::lambda:invoke
            Parameters:
              FunctionName:
                # you can use Ref to get the function name
                Ref: hello-world
              Payload:
                answer: 42
            Next: thirdTask
          thirdTask:
            Type: Task
            Resource: arn:aws:states:::lambda:invoke.waitForTaskToken
            Parameters:
              FunctionName:
                # you can also use Fn::GetAtt to get the ARN
                Fn::GetAtt: [hello-world, Arn]
              Payload:
                token.$: $$.Task.Token
            End: true

In the above example, when we reference the local function 

hello-world

HelloDashworldLambdaFunction

As you can see from the example, it applies when you use either 

Ref

Fn::GetAtt

Task

Task

Hi, my name is Yan Cui. I’m an  and the author of . I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

You can contact me via ,  and .

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Originally published at  on July 31, 2019.