Handling Error Conditions Using a Step Functions State Machine

In this tutorial, you create an AWS Step Functions state machine with a Catch field. The Catch field uses an AWS Lambda function to respond with conditional logic based on error message type. This is a technique called function error handling.

For more information, see Function Error Handling in the AWS Lambda Developer Guide.

Note

You can also create state machines that Retry on timeouts or those that use Catch to transition to a specific state when an error or timeout occurs. For examples of these error handling techniques, see Examples Using Retry and Using Catch.

Step 1: Create a Lambda Function That Fails

Use a Lambda function to simulate an error condition.

Important

Ensure that your Lambda function is under the same AWS account and AWS Region as your state machine.

1. Open the AWS Lambda console at https://console.aws.amazon.com/lambda/.

   Choose Create function.

2. Choose Use a blueprint, enter step-functions into the filter, and then choose the step-functions-error blueprint.

3. Choose Configure.

4. In the Basic information section, configure your Lambda function:

   1. For Name, enter FailFunction.

   2. For Role, choose Create a new role with basic Lambda permissions.

5. The following code is displayed in the Lambda function code pane.

   exports.handler = async (event, context) => {
       function CustomError(message) {
           this.name = 'CustomError';
           this.message = message;
       }
       CustomError.prototype = new Error();
   
       throw new CustomError('This is a custom error!');
   };

   The context object returns the error message This is a custom error!.

6. Choose Create function.

   After your Lambda function is created, copy the function's Amazon Resource Name (ARN) displayed in the upper-right corner of the page. To copy the ARN, click . The following is an example ARN:

   arn:aws:lambda:us-east-1:123456789012:function:FailFunction

7. Choose Deploy.

Step 2: Test the Lambda Function

Test your Lambda function to see it in operation.

1. On the FailFunction page, choose Test.

2. In the Configure test event dialog box, enter FailFunction for Event name, and then choose Create.

3. On the FailFunction page, choose Test to test your Lambda function.

   The results of the test (the simulated error) are displayed in a new tab Execution results.

Step 3: Create a State Machine with a Catch Field

Use the Step Functions console to create a state machine that uses a Task state with a Catch field. Add a reference to your Lambda function in the Task state. The Lambda function is invoked and fails during execution. Step Functions retries the function twice using exponential backoff between retries.

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

2. On the Choose authoring method page, choose Write your workflow in code.

3. For Type, retain the default selection, that is, Standard.

4. In the Definition pane, paste the following code, but replace the ARN of the Lambda function that you created earlier in the Resource field.

   {
      "Comment": "A Catch example of the Amazon States Language using an AWS Lambda function",
      "StartAt": "CreateAccount",
      "States": {
         "CreateAccount": {
            "Type": "Task",
            "Resource": "arn:aws:lambda:us-east-1:123456789012:function:FailFunction",
            "Catch": [ {
               "ErrorEquals": ["CustomError"],
               "Next": "CustomErrorFallback"
            }, {
               "ErrorEquals": ["States.TaskFailed"],
               "Next": "ReservedTypeFallback"
            }, {
               "ErrorEquals": ["States.ALL"],
               "Next": "CatchAllFallback"
            } ],
            "End": true
         },
         "CustomErrorFallback": {
            "Type": "Pass",
            "Result": "This is a fallback from a custom Lambda function exception",
            "End": true
         },
         "ReservedTypeFallback": {
            "Type": "Pass",
            "Result": "This is a fallback from a reserved error code",
            "End": true
         },
         "CatchAllFallback": {
            "Type": "Pass",
            "Result": "This is a fallback from any error code",
            "End": true
         }
      }
   }

   This is a description of your state machine using the Amazon States Language. It defines a single Task state named CreateAccount. For more information, see State Machine Structure.

   For more information about the syntax of the Retry field, see Examples using Retry and using Catch.

   Note

   Unhandled errors in Lambda are reported as Lambda.Unknown in the error output. These include out-of-memory errors and function timeouts. You can match on Lambda.Unknown, States.ALL, or States.TaskFailed to handle these errors. When Lambda hits the maximum number of invocations, the error is Lambda.TooManyRequestsException. For more information about Lambda Handled and Unhandled errors, see FunctionError in the AWS Lambda Developer Guide.

5. Use the graph in the Visual Workflow pane to check that your Amazon States Language code describes your state machine correctly.

   If you don't see the graph, choose in the Visual Workflow pane.

6. Choose Next.

7. Enter a Name for your state machine, such as Catchfailure.

8. In Permissions, choose Create new role.

9. Choose Create state machine.

Step 4: Start a New Execution

After you create your state machine, you can start an execution.

1. On the Catchfailure page, choose Start execution.

   The Start execution dialog box is displayed.

2. (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.

   Note

   Step Functions allows you to create state machine, execution, and activity names 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.

3. Choose Start Execution.

   A new execution of your state machine starts, and a new page showing your running execution is displayed.

4. Go to the Execution output tab to view the output of your workflow.

   

5. To view your custom error message, select CreateAccount in the Graph inspector pane and choose the Step output tab.

   
   Note

   You can preserve the state input with the error by using ResultPath. See Use ResultPath to Include Both Error and Input in a Catch.