Serverless Framework and AWS Step Functions (AWS State Machine) triggered from a file uploaded to S3

Question

I'm trying to start an AWS State Machine (step function) once a file is uploaded to an S3 bucket. The bucket already exists and the project was created using Serverless Framework.

For that, I created this function in my serverless.yml

    functions:
    
      imagewasuploadedevent:
        handler: src/stepfunctions/imageWasUploadedEvent.handler
        events:
         - s3:
            bucket: !Ref AttachmentsBucket
            existing: true
        iamRoleStatements:
          - Effect: "Allow"
            Action:
              - "states:StartExecution"
            Resource:
              - "*"

This function should be triggered when a file is uploaded to the S3 bucket "AttachmentsBucket" that already exists and I hope it starts the State Machine processing

Now the Step Function definition is below

stepFunctions:
  stateMachines:
    ValidateImageStateMachine:
      definition:
        Comment: "This state function validates the images after users upload them to S3"
        StartAt: imagewasuploadedevent
        States:
          imageWasUploadedEvent:
            Type: Task
            Resource:
              Fn::GetAtt: [imagewasuploadedevent, Arn]
            End: true

The plug-ins section is making use of "serverless-step-functions" plugins

plugins:
  - serverless-python-requirements
  - serverless-iam-roles-per-function
  - serverless-step-functions

However, CloudFormation of the stack is failing with the following error

 An error occurred: 

ValidateImageStateMachineStepFunctionsStateMachine - Invalid State Machine 
Definition: 'MISSING_TRANSITION_TARGET: Missing 'Next' target: imagewasuploadedevent at /StartAt, MISSING_TRANSITION_TARGET: State "imageWasUploadedEvent" is not reachable. at /States/imageWasUploadedEvent' (Service: AWSStepFunctions; Status Code: 400; Error Code: InvalidDefinition; Request ID: 39217fe8-9dcd-4386-8549-b995619d2db6; Proxy: null).

I'm suspecting it has nothing to do with I'm doing but the fact that we can only use this plug-in to initiate State Machines through API Gateway functions and schedules and cloud watch events.

Could someone point me here in the right direction?

Thanks

Btw. is there any way to trigger it without Lambda function? Of course using Serverless and step functions plugin — PatrykMilewski
From S3 I guess the way of triggering StepFunction is through CloudWtach event rules. It is explained here. I was unable at the end to do it this way using the Serverless Framework and step functions plugin — MasterOfTheHouse
Thanks! Yes it's very complicated, I have no idea how to make it work, I will use Lambda then — PatrykMilewski

maafk maafk · Accepted Answer · 2020-10-27T01:22:41

I think this has to do with the case sensitivity of the Amazon States Language.

Try updating the StartAt with imageWasUploadedEvent

stepFunctions:
  stateMachines:
    ValidateImageStateMachine:
      definition:
        Comment: "This state function validates the images after users upload them to S3"
        StartAt: imageWasUploadedEvent
        States:
          imageWasUploadedEvent:
            Type: Task
            Resource:
              Fn::GetAtt: [imagewasuploadedevent, Arn]
            End: true

Serverless Framework and AWS Step Functions (AWS State Machine) triggered from a file uploaded to S3

1 Answers