Handling errors in Step Functions workflows

All states, except Pass and Wait states, can encounter runtime errors. Errors can happen for various reasons, such as the following:

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 error names can't 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 transient Lambda service exceptions.

Retrying after an error

Task, Parallel, and Map 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.

When one of these states reports an error and there's a Retry field, Step Functions scans through the retriers in the order listed in the array. When the error name appears in the value of a retrier's ErrorEquals field, the state machine makes retry attempts as defined in the Retry field.

If your redriven execution reruns a Task workflow state, Parallel workflow state, or Inline Map state, for which you have defined retries, the retry attempt count for these states is reset to 0 to allow for the maximum number of attempts on redrive. For a redriven execution, you can track individual retry attempts of these states using the console. For more information, see Retry behavior of redriven executions in Restarting state machine executions with redrive in Step Functions.

A retrier contains the following fields:

Retry field examples

This section includes the following Retry field examples.

Example 1 – Retry with BackoffRate

The following example of a Retry makes two retry attempts with the first retry taking place after waiting for three seconds. Based on the BackoffRate you specify, Step Functions increases the interval between each retry until the maximum number of retry attempts is reached. In the following example, the second retry attempt starts after waiting for three seconds after the first retry.

"Retry": [ {
   "ErrorEquals": [ "States.Timeout" ],
   "IntervalSeconds": 3,
   "MaxAttempts": 2,
   "BackoffRate": 1
} ]

Example 2 – Retry with MaxDelaySeconds

The following example makes three retry attempts and limits the wait time resulting from BackoffRate at 5 seconds. The first retry takes place after waiting for three seconds. The second and third retry attempts take place after waiting for five seconds after the preceding retry attempt because of the maximum wait time limit set by MaxDelaySeconds.

"Retry": [ {
    "ErrorEquals": [ "States.Timeout" ],
    "IntervalSeconds": 3,
    "MaxAttempts": 3,
    "BackoffRate":2,
    "MaxDelaySeconds": 5,
    "JitterStrategy": "FULL"
} ]

Without MaxDelaySeconds, the second retry attempt would take place six seconds after the first retry, and the third retry attempt would take place 12 seconds after the second retry.

Example 3 – Retry all errors except States.Timeout

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. The name States.TaskFailed also acts a wildcard and matches any error except for States.Timeout.

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

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

Example 4 – Complex retry scenario

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

Fallback states

Task, Map and Parallel states can each 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 name States.TaskFailed also acts a wildcard and matches any error except for States.Timeout.

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

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

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"
},

State machine 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 Node.js Lambda function that always fails, returning the message error. In the state machine examples that follow, this Lambda function is named FailFunction. For information about creating a Lambda function, see Step 1: Create a Lambda function section.

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

This is a definition of a Node.js 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. It retries this function 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: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: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, it catches the error 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: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: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 Task state that times out, based on the timeout value specified in TimeoutSeconds. Step Functions retries the Lambda function invocation in this Task state 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: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: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
      }
   }
}