Implementing newly-launched synchronous workflows for step functions

Event Gateway
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5

Hi,

I just had similar use case. Like you said, it’s a new feature, serverless-step-functions plugin does not support it yet (https://github.com/serverless-operations/serverless-step-functions/issues/378).

I managed to make it work with CloudFormation so I though I will share it with you.
You need to use step functions of type express and Http API gateway.
Step functions need to be express type, because they offer synchronous execution.

You need to create:

  1. Step function, e.g. by using serverless-step-functions plugin

     stepFunctions:
   validate: true
  stateMachines:
    myStateMachine:
      type: EXPRESS
      tracingConfig:
        enabled: true
      id: MyStateMachine
     name: 'My step function
     definition: ${file(state-machines/state-machine.json)}

  2. Execution role for HttpApi integration

     InvokeStepFunctionRole:
    Type: AWS::IAM::Role
    Properties:
  AssumeRolePolicyDocument:
    Statement:
      - Effect: Allow
        Principal:
        Service:
          - apigateway.amazonaws.com
        Action:
         - sts:AssumeRole
  Policies:
   - PolicyName: InvokeStepFunctionPolicy
     PolicyDocument:
       Statement:
         - Effect: Allow
           Action:
             - states:DescribeExecution
             - states:StartSyncExecution
           Resource:
             - !Ref MyStateMachine

  3. Given you have a gateway already created (HttpApi) resource, you need to create integration

    HttpApiStepFunctionIntegration:
  Type: AWS::ApiGatewayV2::Integration
  Properties:
   ApiId: !Ref HttpApi
   CredentialsArn: !GetAtt InvokeStepFunctionRole.Arn
   Description: 'Start my step machine step function'
   IntegrationType: AWS_PROXY
   IntegrationSubtype: StepFunctions-StartSyncExecution
   PayloadFormatVersion: "1.0"
   RequestParameters: {"Input": "$request.body", "StateMachineArn": "#{MyStateMachine}"}
  • note: #{MyStateMachine} is serverless pseudo parameters syntax for !Ref MyStateMachine

  1. Http Api route with integration from point 3.

     HttpApiRoute:
   DependsOn:
     - HttpApiStepFunctionIntegration
     - HttpApi
   Type: AWS::ApiGatewayV2::Route
   Properties:
     ApiId: !Ref HttpApi
     RouteKey: POST /start-stepfunction-sync
     Target: !Join ['/', ['integrations', !Ref HttpApiStepFunctionIntegration]]

The only drawback I noticed is the format of the response which cannot be mapped at the moment according to documentation, only headers and status codes can be mapped: [Modifying the response of the StartSyncExecution · Issue #6 · aws-samples/contact-form-processing-with-synchronous-express-workflows · GitHub]

Cheers,
jimmy