Step Functions Express Workflows: Orchestrate Lambda at 100K Events Per Second

Resources:
  EventPipeline:
    Type: AWS::Serverless::StateMachine
    Properties:
      Type: EXPRESS
      Definition:
        Comment: High-volume event processing
        StartAt: ValidateEvent
        States:
          ValidateEvent:
            Type: Task
            Resource: !GetAtt ValidateFn.Arn
            Retry:
              - ErrorEquals: [States.TaskFailed]
                MaxAttempts: 2
                IntervalSeconds: 1
            Catch:
              - ErrorEquals: [ValidationError]
                Next: HandleInvalid
            Next: RouteEvent

          RouteEvent:
            Type: Choice
            Choices:
              - Variable: "$.type"
                StringEquals: ORDER
                Next: ProcessOrder
              - Variable: "$.type"
                StringEquals: PAYMENT
                Next: ProcessPayment
            Default: ProcessGeneric

          ProcessOrder:
            Type: Task
            Resource: !GetAtt OrderFn.Arn
            End: true

          ProcessPayment:
            Type: Task
            Resource: !GetAtt PaymentFn.Arn
            End: true

          HandleInvalid:
            Type: Task
            Resource: !GetAtt DLQFn.Arn
            End: true

      Logging:
        Level: ERROR
        IncludeExecutionData: false
        Destinations:
          - CloudWatchLogsLogGroup:
              LogGroupArn: !GetAtt Logs.Arn

// Synchronous — wait for result
import { SFNClient, StartSyncExecutionCommand } from "@aws-sdk/client-sfn";
const sfn = new SFNClient({});

export const handler = async (event) => {
  const { status, output, error } = await sfn.send(new StartSyncExecutionCommand({
    stateMachineArn: process.env.STATE_MACHINE_ARN,
    input: JSON.stringify(event),
  }));
  if (status === "FAILED") throw new Error(error);
  return JSON.parse(output);
};

// Asynchronous at high volume:
// SQS → EventBridge Pipe → Express Workflow (parallel fan-out per message)

EnrichInParallel:
  Type: Parallel
  Branches:
    - StartAt: FetchUser
      States:
        FetchUser:
          Type: Task
          Resource: !GetAtt UserFn.Arn
          End: true
    - StartAt: FetchProduct
      States:
        FetchProduct:
          Type: Task
          Resource: !GetAtt ProductFn.Arn
          End: true
    - StartAt: CheckInventory
      States:
        CheckInventory:
          Type: Task
          Resource: !GetAtt InventoryFn.Arn
          End: true
  # All 3 run simultaneously — total time = max(branch durations)
  Next: MergeResults