Example of invoking a Step Function Express Workflow from Amazon API Gateway

order-service-stack.ts

// create the state machine defintion for cancelling an order
const cancelOrderStateMachineDefinition: sfn.TaskStateBase =
  new tasks.LambdaInvoke(this, "CancelOrder", {
    lambdaFunction: cancelOrderHandler,
    resultPath: "$",
    timeout: Duration.seconds(30),
    comment: "Cancel order task",
    retryOnServiceExceptions: true,
  }).addCatch(
    new tasks.SqsSendMessage(this, "SendSQSFailure", {
      queue: new sqs.Queue(this, "CancelOrderLambdaFailureDLQ", {
        queueName: "cancel-order-lambda-failure-dlq",
      }),
      messageBody: sfn.TaskInput.fromJsonPathAt(
        "States.StringToJson($.Cause)"
      ),
    })
  );

// create the state machine for cancelling orders
const cancelOrderStateMachine: sfn.StateMachine = new sfn.StateMachine(
  this,
  "CancelOrderStateMachine",
  {
    definition: cancelOrderStateMachineDefinition,
    logs: {
      level: sfn.LogLevel.ALL,
      destination: new logs.LogGroup(
        this,
        "cancelOrderStateMachineLogGroup",
        {
          retention: logs.RetentionDays.ONE_DAY,
        }
      ),
      includeExecutionData: true,
    },
    tracingEnabled: true,
    stateMachineName: "CancelOrderStateMachine",
    stateMachineType: sfn.StateMachineType.EXPRESS,
    timeout: Duration.seconds(30),
  }
);

// create the orders resource
const orders: apigw.Resource = onlineOrdersApi.root.addResource("orders");

// create the role for invoking the step function
const apigwRole: iam.Role = new iam.Role(this, "OnlineOrdersRole", {
  assumedBy: new iam.ServicePrincipal("apigateway"),
  inlinePolicies: {
    putEvents: new iam.PolicyDocument({
      statements: [
        new iam.PolicyStatement({
          actions: ["events:PutEvents"],
          resources: [eventBus.eventBusArn],
        }),
        new iam.PolicyStatement({
          actions: ["states:StartSyncExecution"],
          effect: iam.Effect.ALLOW,
          resources: [cancelOrderStateMachine.stateMachineArn],
        }),
      ],
    }),
  },
});

// step function options for the api gateway integration
const stepFunctionOptions: apigw.IntegrationOptions = {
  credentialsRole: apigwRole,
  integrationResponses: [
    {
      statusCode: "200",
      responseTemplates: {
        "application/json": "Cancelled",
      },
    },
  ],
  passthroughBehavior: apigw.PassthroughBehavior.NEVER,
  requestTemplates: {
    "application/json": `{
          "input": "{\\"actionType\\": \\"cancel\\", \\"body\\": $util.escapeJavaScript($input.json('$'))}",
          "stateMachineArn": "${cancelOrderStateMachine.stateMachineArn}"
        }`,
  },
};

// the cancel order endpoint persists a message directly with state machine
orders.addMethod(
  "PUT",
  new apigw.Integration({
    type: apigw.IntegrationType.AWS,
    uri: `arn:aws:apigateway:${cdk.Aws.REGION}:states:action/StartSyncExecution`,
    integrationHttpMethod: "POST",
    options: stepFunctionOptions,
  }),
  { methodResponses: [{ statusCode: "200" }] }
);