serverless.yml

service: serverless-stepfunctions-callback

plugins:
  - serverless-pseudo-parameters
  - serverless-step-functions

provider:
  name: aws
  runtime: python3.7
  logRetentionInDays: 7
  iamRoleStatements:
    - Effect: Allow
      Action:
      - states:SendTaskFailure
      - states:SendTaskSuccess
      Resource: 
      - "arn:aws:states:#{AWS::Region}:#{AWS::AccountId}:states:SendTaskSuccess"
      - Ref: CallbackExampleStepFunctionsStateMachine

package:
  include:
  - handler.py
  exclude:
  - "*~"
  - .venv3/**
  - README.rst
  - node_modules/**
  - package-lock.json

functions:
  SplitDoc:
    handler: handler.split_doc
  ProcessAndCheckCompletion:
    handler: handler.process_and_check_completion

stepFunctions:
  stateMachines:
    CallbackExample:
      events:                   # start the state machine with a simulated event GET /start
        - http:
            path: start
            method: GET
      definition:
        StartAt: SplitDoc
        States:
          SplitDoc:
            Type: Task          # we could use Pass and skip the Lambda, but simulate the split
            Resource: "arn:aws:lambda:#{AWS::Region}:#{AWS::AccountId}:function:${self:service}-${opt:stage}-SplitDoc"
            Next: WaitForCompletion
          WaitForCompletion:
            Type: Task
            Resource: arn:aws:states:::lambda:invoke.waitForTaskToken # this is the magick incantation
            Parameters:
              FunctionName: ${self:service}-${opt:stage}-ProcessAndCheckCompletion
              Payload:
                taskToken.$: $$.Task.Token            
            Next: ContinueProcess # the happy path
            Catch:                   # put our named failures before generic TaskFailed
            - ErrorEquals: ["ProcessingFailed"]
              Next: ProcessingFailed
            - ErrorEquals: ["States.TaskFailed"] # any other problem including Lambda Python exception
              Next: UnexpectedFailure
          ContinueProcess:
            Type: Succeed
          ProcessingFailed:
            Type: Fail
            Error: Processing failed
            Cause: We caught a problem when processing so we should clean up the artifacts
          UnexpectedFailure:
            Type: Fail
            Error: Unexpected failure
            Cause: Some unexpected error occurred, possibly a Lambda function exception, check logs