Yan Cui

AWS Serverless Hero. Independent Consultant. Developer Advocate at Lumigo.

Serverless Step Functions: Avoid Leaky Abstractions

I have some exciting news to share with you about the Serverless Step Functions plugin.
One of the main pain points of using the plugin was that you needed to use fully-formed ARNs. We addressed this in v1.18.0 by supporting CloudFormation intrinsic functions 
Fn:GetAtt
 and 
Ref
. This makes it possible for you to reference a local function instead.
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 Serverless framework 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 v2.2.0 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
 we no longer need to use its CloudFormation logical ID 
HelloDashworldLambdaFunction
.
As you can see from the example, it applies when you use either 
Ref
 or 
Fn::GetAtt
 functions in a 
Task
 state. And it applies to all 3 ways of invoking a Lambda function from a 
Task
 state.
Hi, my name is Yan Cui. I’m an AWS Serverless Hero and the author of Production-Ready Serverless. 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 EmailTwitter and LinkedIn.
Check out my new course, Complete Guide to AWS Step Functions.
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Originally published at https://theburningmonk.com on July 31, 2019.

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