AWS services or capabilities described in AWS documentation might vary by Region. To see the differences applicable to the China Regions, see Getting Started with AWS services in China.

Error handling in Step Functions

Any state can encounter runtime errors. Errors can happen for various reasons:

By default, when a state reports an error, AWS Step Functions causes the execution to fail entirely.

Error names

Step Functions identifies errors in the Amazon States Language using case-sensitive strings, known as error names. The Amazon States Language defines a set of built-in strings that name well-known errors, all beginning with the States. prefix.

States can report errors with other names. However, these must not begin with the States. prefix.

As a best practice, ensure production code can handle AWS Lambda service exceptions (Lambda.ServiceException and Lambda.SdkClientException). For more information, see Handle Lambda service exceptions.

Retrying after an error

Task and Parallel states can have a field named Retry, whose value must be an array of objects known as retriers. An individual retrier represents a certain number of retries, usually at increasing time intervals.

A retrier contains the following fields.

This example of a Retry makes 2 retry attempts after waiting for 3 and 4.5 seconds.

"Retry": [ {
   "ErrorEquals": [ "States.Timeout" ],
   "IntervalSeconds": 3,
   "MaxAttempts": 2,
   "BackoffRate": 1.5
} ]

The reserved name States.ALL that appears in a retrier's ErrorEquals field is a wildcard that matches any error name. It must appear alone in the ErrorEquals array and must appear in the last retrier in the Retry array.

This example of a Retry field retries any error except States.Timeout.

"Retry": [ {
   "ErrorEquals": [ "States.Timeout" ],
   "MaxAttempts": 0
}, {
   "ErrorEquals": [ "States.ALL" ]
} ]

Complex retry scenarios

A retrier's parameters apply across all visits to the retrier in the context of a single-state execution.

Consider the following Task state.

"X": {
   "Type": "Task",
   "Resource": "arn:aws-cn:states:us-east-1:123456789012:task:X",
   "Next": "Y",
   "Retry": [ {
      "ErrorEquals": [ "ErrorA", "ErrorB" ],
      "IntervalSeconds": 1,
      "BackoffRate": 2.0,
      "MaxAttempts": 2
   }, {
      "ErrorEquals": [ "ErrorC" ],
      "IntervalSeconds": 5
   } ],
   "Catch": [ {
      "ErrorEquals": [ "States.ALL" ],
      "Next": "Z"
   } ]
}

This task fails five times in succession, outputting these error names: ErrorA, ErrorB, ErrorC, ErrorB, and ErrorB. The following occurs as a result:

Fallback states

Task and Parallel states can have a field named Catch. This field's value must be an array of objects, known as catchers.

A catcher contains the following fields.

When a state reports an error and either there is no Retry field, or if retries fail to resolve the error, Step Functions scans through the catchers in the order listed in the array. When the error name appears in the value of a catcher's ErrorEquals field, the state machine transitions to the state named in the Next field.

The reserved name States.ALL that appears in a catcher's ErrorEquals field is a wildcard that matches any error name. It must appear alone in the ErrorEquals array and must appear in the last catcher in the Catch array.

The following example of a Catch field transitions to the state named RecoveryState when a Lambda function outputs an unhandled Java exception. Otherwise, the field transitions to the EndState state.

"Catch": [ {
   "ErrorEquals": [ "java.lang.Exception" ],
   "ResultPath": "$.error-info",
   "Next": "RecoveryState"
}, {
   "ErrorEquals": [ "States.ALL" ],
   "Next": "EndState"
} ]

Error output

When Step Functions transitions to the state specified in a catch name, the object usually contains the field Cause. This field's value is a human-readable description of the error. This object is known as the error output.

In this example, the first catcher contains a ResultPath field. This works similarly to a ResultPath field in a state's top level, resulting in two possibilities:

Thus, in this example, for the first catcher the error output is added to the input as a field named error-info (if there isn't already a field with this name in the input). Then, the entire input is sent to RecoveryState. For the second catcher, the error output overwrites the input and only the error output is sent to EndState.

When a state has both Retry and Catch fields, Step Functions uses any appropriate retriers first, and only afterward applies the matching catcher transition if the retry policy fails to resolve the error.

Cause payloads and service integrations

A catcher returns a string payload as an output. When working with service integrations such as Amazon Athena or AWS CodeBuild, you may want to convert the Cause string to JSON. The following example of a Pass state with intrinsic functions shows how to convert a Cause string to JSON.

"Handle escaped JSON with JSONtoString": {
  "Type": "Pass",
  "Parameters": {
    "Cause.$": "States.StringToJson($.Cause)"
  },
  "Next": "Pass State with Pass Processing"
},

Examples using Retry and using Catch

The state machines defined in the following examples assume the existence of two Lambda functions: one that always fails and one that waits long enough to allow a timeout defined in the state machine to occur.

This is a definition of a Lambda function that always fails, returning the message error. In the state machine examples that follow, this Lambda function is named FailFunction.

exports.handler = (event, context, callback) => {
    callback("error");
};

This is a definition of a Lambda function that sleeps for 10 seconds. In the state machine examples that follow, this Lambda function is named sleep10.

exports.handler = (event, context, callback) => {
    setTimeout(function(){
    }, 11000);
};

Handling a failure using Retry

This state machine uses a Retry field to retry a function that fails and outputs the error name HandledError. The function is retried twice with an exponential backoff between retries.

{
   "Comment": "A Hello World example of the Amazon States Language using an AWS Lambda function",
   "StartAt": "HelloWorld",
   "States": {
      "HelloWorld": {
         "Type": "Task",
         "Resource": "arn:aws-cn:lambda:us-east-1:123456789012:function:FailFunction",
         "Retry": [ {
            "ErrorEquals": ["HandledError"],
            "IntervalSeconds": 1,
            "MaxAttempts": 2,
            "BackoffRate": 2.0
         } ],
      "End": true
      }
   }
}

This variant uses the predefined error code States.TaskFailed, which matches any error that a Lambda function outputs.

{
   "Comment": "A Hello World example of the Amazon States Language using an AWS Lambda function",
   "StartAt": "HelloWorld",
   "States": {
      "HelloWorld": {
         "Type": "Task",
         "Resource": "arn:aws-cn:lambda:us-east-1:123456789012:function:FailFunction",
         "Retry": [ {
            "ErrorEquals": ["States.TaskFailed"],
            "IntervalSeconds": 1,
            "MaxAttempts": 2,
            "BackoffRate": 2.0
         } ],
         "End": true
      }
   }
}

Handling a failure using Catch

This example uses a Catch field. When a Lambda function outputs an error, the error is caught and the state machine transitions to the fallback state.

{
   "Comment": "A Hello World example of the Amazon States Language using an AWS Lambda function",
   "StartAt": "HelloWorld",
   "States": {
      "HelloWorld": {
         "Type": "Task",
         "Resource": "arn:aws-cn:lambda:us-east-1:123456789012:function:FailFunction",
         "Catch": [ {
            "ErrorEquals": ["HandledError"],
            "Next": "fallback"
         } ],
         "End": true
      },
      "fallback": {
         "Type": "Pass",
         "Result": "Hello, AWS Step Functions!",
         "End": true
      }
   }
}

This variant uses the predefined error code States.TaskFailed, which matches any error that a Lambda function outputs.

{
   "Comment": "A Hello World example of the Amazon States Language using an AWS Lambda function",
   "StartAt": "HelloWorld",
   "States": {
      "HelloWorld": {
         "Type": "Task",
         "Resource": "arn:aws-cn:lambda:us-east-1:123456789012:function:FailFunction",
         "Catch": [ {
            "ErrorEquals": ["States.TaskFailed"],
            "Next": "fallback"
         } ],
      "End": true
      },
      "fallback": {
         "Type": "Pass",
         "Result": "Hello, AWS Step Functions!",
         "End": true
      }
   }
}

Handling a timeout using Retry

This state machine uses a Retry field to retry a function that times out. The function is retried twice with an exponential backoff between retries.

{
   "Comment": "A Hello World example of the Amazon States Language using an AWS Lambda function",
   "StartAt": "HelloWorld",
   "States": {
      "HelloWorld": {
         "Type": "Task",
         "Resource": "arn:aws-cn:lambda:us-east-1:123456789012:function:sleep10",
         "TimeoutSeconds": 2,
         "Retry": [ {
            "ErrorEquals": ["States.Timeout"],
            "IntervalSeconds": 1,
            "MaxAttempts": 2,
            "BackoffRate": 2.0
         } ],
         "End": true
      }
   }
}

Handling a timeout using Catch

This example uses a Catch field. When a timeout occurs, the state machine transitions to the fallback state.

{
   "Comment": "A Hello World example of the Amazon States Language using an AWS Lambda function",
   "StartAt": "HelloWorld",
   "States": {
      "HelloWorld": {
         "Type": "Task",
         "Resource": "arn:aws-cn:lambda:us-east-1:123456789012:function:sleep10",
         "TimeoutSeconds": 2,
         "Catch": [ {
            "ErrorEquals": ["States.Timeout"],
            "Next": "fallback"
         } ],
         "End": true
      },
      "fallback": {
         "Type": "Pass",
         "Result": "Hello, AWS Step Functions!",
         "End": true
      }
   }
}