Callback Pattern Example (Amazon SQS, Amazon SNS, Lambda) - Amazon Step Functions
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Callback Pattern Example (Amazon SQS, Amazon SNS, Lambda)

This sample project demonstrates how to have Amazon Step Functions pause during a task, and wait for an external process to return a task token that was generated when the task started.

When this sample project is deployed and an execution is started, the following steps occur:

  1. Step Functions passes a message that includes a task token to an Amazon Simple Queue Service (Amazon SQS) queue.

  2. Step Functions then pauses, waiting for that token to be returned.

  3. The Amazon SQS queue triggers an Amazon Lambda function that calls SendTaskSuccess with that same task token.

  4. When the task token is received, the workflow continues.

  5. The "Notify Success" task publishes an Amazon Simple Notification Service (Amazon SNS) message that the callback was received.

To learn how to implement the callback pattern in Step Functions, see Wait for a Callback with the Task Token.

For more information about how Amazon Step Functions can control other Amazon services, see Using Amazon Step Functions with other services.

Step 1: Create the state machine and provision resources

  1. Open the Step Functions console and choose Create state machine.

  2. Type Callback pattern example in the search box, and then choose Callback pattern example from the search results that are returned.

  3. Choose Next to continue.

  4. Step Functions lists the Amazon Web Services used in the sample project you selected. It also shows a workflow graph for the sample project. Deploy this project to your Amazon Web Services account or use it as a starting point for building your own projects. Based on how you want to proceed, choose Run a demo or Build on it.

    This sample project deploys the following resources:

    • An Amazon SQS message queue.

    • A Lambda function that calls the Step Functions API action SendTaskSuccess.

    • An Amazon SNS topic to notify about the success or failure of a task indicating whether or not the workflow can continue.

    • An Amazon Step Functions state machine

    • Related Amazon Identity and Access Management (IAM) roles

    The following image shows the workflow graph for the Callback pattern example sample project:

            Workflow graph of the Manage a batch job sample project.
  5. Choose Use template to continue with your selection.

  6. Do one of the following:

    • If you selected Build on it, Step Functions creates the workflow prototype for the sample project you selected. Step Functions doesn't deploy the resources listed in the workflow definition.

      In Workflow Studio's Design mode, drag and drop states from the States browser to continue building your workflow protoype. Or switch to the Code mode that provides an integrated code editor similar to VS Code for updating the Amazon States Language (ASL) definition of your state machine within the Step Functions console. For more information about using Workflow Studio to build your state machines, see Using Workflow Studio.


      Remember to update the placeholder Amazon Resource Name (ARN) for the resources used in the sample project before you run your workflow.

    • If you selected Run a demo, Step Functions creates a read-only sample project which uses an Amazon CloudFormation template to deploy the Amazon resources listed in that template to your Amazon Web Services account.


      To view the state machine definition of the sample project, choose Code.

      When you're ready, choose Deploy and run to deploy the sample project and create the resources.

      It can take up to 10 minutes for these resources and related IAM permissions to be created. While your resources are being deployed, you can open the CloudFormation Stack ID link to see which resources are being provisioned.

      After all the resources in the sample project are created, you can see the new sample project listed on the State machines page.


      Standard charges may apply for each service used in the CloudFormation template.

Step 2: Run the state machine

  1. On the State machines page, choose your sample project.

  2. On the sample project page, choose Start execution.

  3. In the Start execution dialog box, do the following:

    1. (Optional) To identify your execution, you can specify a name for it in the Name box. By default, Step Functions generates a unique execution name automatically.


      Step Functions allows you to create names for state machines, executions, and activities, rate controls, and labels that contain non-ASCII characters. These non-ASCII names don't work with Amazon CloudWatch. To ensure that you can track CloudWatch metrics, choose a name that uses only ASCII characters.

    2. (Optional) In the Input box, enter input values in JSON format to run your workflow.

      If you chose to Run a demo, you need not provide any execution input.


      If the demo project you deployed contains prepopulated execution input data, use that input to run the state machine.

    3. Choose Start execution.

    4. The Step Functions console directs you to a page that's titled with your execution ID. This page is known as the Execution Details page. On this page, you can review the execution results as the execution progresses or after it's complete.

      To review the execution results, choose individual states on the Graph view, and then choose the individual tabs on the Step details pane to view each state's details including input, output, and definition respectively. For details about the execution information you can view on the Execution Details page, see Execution Details page – Interface overview.

    For example, to review how Step Functions progressed through the workflow and received a callback from Amazon SQS, review the entries in the Events table. The following image shows the execution output for the Notify Success step. It also shows the first five events from the execution event history. Expand each event to view more details about that event.

            Excerpt of the execution output for the Notify Success step selected in Graph view. The image also shows an excerpt from the execution event history for the Callback pattern example sample project.

Lambda Callback Example

To see how the components of this sample project work together, see the resources that were deployed in your Amazon account. For example, here is the Lambda function that calls Step Functions with the task token.

console.log('Loading function'); const aws = require('aws-sdk'); exports.lambda_handler = (event, context, callback) => { const stepfunctions = new aws.StepFunctions(); for (const record of event.Records) { const messageBody = JSON.parse(record.body); const taskToken = messageBody.TaskToken; const params = { output: "\"Callback task completed successfully.\"", taskToken: taskToken }; console.log(`Calling Step Functions to complete callback task with params ${JSON.stringify(params)}`); stepfunctions.sendTaskSuccess(params, (err, data) => { if (err) { console.error(err.message); callback(err.message); return; } console.log(data); callback(null); }); } };