Handling Error Conditions Using a State Machine

In this tutorial, you create an AWS Step Functions state machine with a Catch field, which 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.

Topics

Step 1: Create an IAM Role for Lambda
Step 2: Create a Lambda Function That Fails
Step 3: Test the Lambda Function
Step 4: Create a State Machine with a Catch Field
Step 5: Start a New Execution

Step 1: Create an IAM Role for Lambda

Both AWS Lambda and AWS Step Functions can execute code and access AWS resources (for example, data stored in Amazon S3 buckets). To maintain security, you must grant Lambda and Step Functions access to these resources.

Lambda requires you to assign an AWS Identity and Access Management (IAM) role when you create a Lambda function, in the same way Step Functions requires you to assign an IAM role when you create a state machine.

Sign in to the IAM console and choose Roles, Create role.
On the Select type of trusted entity page, under AWS service, select Lambda from the list, and then choose Next: Permissions.

Note

The role is automatically provided with a trust relationship that allows Lambda to use the role.
On the Attach permissions policy page, choose Next: Review.
On the Review page, enter MyLambdaRole for Role Name, and then choose Create role.

The IAM role appears in the list of roles.

Step 2: 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.

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

Choose Create a function.
In the Blueprints section, enter step-functions into the filter, and then choose the step-functions-error blueprint.
In the Basic information section, configure your Lambda function:
1. For Name, enter FailFunction.
2. For Role, select Choose an existing role.
3. For Existing role, choose the Lambda role that you created earlier.
  
  Note
  
  If the IAM role that you created doesn't appear in the list, the role might still need a few minutes to propagate to Lambda.

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


'use strict';

exports.handler = (event, context, callback) => {
    function CustomError(message) {
        this.name = 'CustomError';
        this.message = message;
    }
    CustomError.prototype = new Error();

    const error = new CustomError('This is a custom error!');
    callback(error);
};

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

Choose Create function.

When your Lambda function is created, note its Amazon Resource Name (ARN) in the upper-right corner of the page. For example:
```
arn:aws:lambda:us-east-1:123456789012:function:FailFunction
```

Step 3: Test the Lambda Function

Test your Lambda function to see it in operation.

On the FailFunction page, choose Test.
In the Configure test event dialog box, enter FailFunction for Event name, and then choose Create.
On the FailFunction page, Test your Lambda function.

The results of the test (the simulated error) are displayed at the bottom of the page.

Step 4: 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.

Open the Step Functions console and choose Create state machine.
On the Create a state machine page, choose Templates, and then choose Catch failure.
Name your state machine, for example, Catchfailure.
Note

State machine, execution, and activity names must be 1–80 characters in length, must be unique for your account and AWS Region, and must not contain any of the following:
- Whitespace
- Wildcard characters (? *)
- Bracket characters (< > { } [ ])
- Special characters (: ; , \ | ^ ~ $ # % & ` ")
- Control characters (\\u0000 - \\u001f or \\u007f - \\u009f).
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.

In the Code pane, add the ARN of the Lambda function that you created earlier to the Resource field, for example:


{
   "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 Retrying after an Error.

Note

Unhandled errors in Lambda are reported as Lambda.Unknown in the error output. These include out-of-memory errors, function timeouts, and hitting the concurrent Lambda invoke limit. You can match on Lambda.Unknown, States.ALL, or States.TaskFailed to handle these errors. For more information about Lambda Handled and Unhandled errors, see FunctionError in the AWS Lambda Developer Guide.

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.
Choose Next.
Create or enter an IAM role:
- To create an IAM role for Step Functions, select Create an IAM role for me, and enter a Name for your role.
- If you have previously created an IAM role with the correct permissions for your state machine, select Choose an existing IAM role. Select a role from the list, or provide an ARN for that role.
Note

If you delete the IAM role that Step Functions creates, Step Functions can't recreate it later. Similarly, if you modify the role (for example, by removing Step Functions from the principals in the IAM policy), Step Functions can't restore its original settings later.
Choose Create state machine.

Step 5: Start a New Execution

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

On the CatchStateMachine page, choose New execution.

The New execution page is displayed.
(Optional) To help identify your execution, you can specify an ID for it in the Enter an execution name box. If you don't enter an ID, Step Functions generates a unique ID 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.
Choose Start Execution.

A new execution of your state machine starts, and a new page showing your running execution is displayed.
In the Execution Details section, expand the Output section to view the output of your workflow.
To view your custom error message, select CreateAccount in the Visual workflow and expand the Output section.

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

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