Callback Patterns & Task Tokens in AWS Step Functions

1. Introduction

AWS Step Functions enable the coordination of distributed applications and microservices using visual workflows. Callback patterns and task tokens are essential for managing asynchronous tasks within these workflows.

2. Callback Patterns

Callback patterns in Step Functions allow workflows to pause and wait for an external signal before resuming. This is particularly useful when dealing with long-running tasks.

Note: Callback patterns help avoid blocking resources while waiting for responses, thus optimizing costs and resource utilization in your application.

2.1 How Callback Patterns Work

When a task is initiated using a callback pattern, the workflow transitions to the WAITING state while awaiting an external event to resume its execution.


{
    "Comment": "A simple example of a callback pattern",
    "StartAt": "TaskWithCallback",
    "States": {
        "TaskWithCallback": {
            "Type": "Task",
            "Resource": "arn:aws:lambda:us-east-1:123456789012:function:MyFunction",
            "TimeoutSeconds": 300,
            "HeartbeatSeconds": 60,
            "ResultPath": "$.result",
            "Catch": [
                {
                    "ErrorEquals": ["States.Timeout"],
                    "ResultPath": "$.error-info",
                    "Next": "HandleTimeout"
                }
            ],
            "Next": "WaitForCallback"
        },
        "WaitForCallback": {
            "Type": "Wait",
            "Seconds": 30,
            "Next": "CompleteTask"
        },
        "CompleteTask": {
            "Type": "Succeed"
        }
    }
}

3. Task Tokens

Task tokens are unique identifiers that Step Functions generate for each task that allows external systems to communicate back to the workflow. They are essential for resuming a workflow after asynchronous tasks are completed.

3.1 Using Task Tokens

Task tokens can be passed to external services (like Lambda or SNS) that will eventually signal back to the Step Function to continue execution.


{
    "Comment": "Example of using Task Tokens",
    "StartAt": "InvokeExternalService",
    "States": {
        "InvokeExternalService": {
            "Type": "Task",
            "Resource": "arn:aws:lambda:us-east-1:123456789012:function:MyService",
            "Parameters": {
                "Token.$": "$$.Task.Token"
            },
            "Next": "WaitForResponse"
        },
        "WaitForResponse": {
            "Type": "Wait",
            "SecondsPath": "$.waitTime",
            "Next": "HandleResponse"
        },
        "HandleResponse": {
            "Type": "Succeed"
        }
    }
}

4. Best Practices

Always handle errors and timeouts gracefully using the Catch field in your state definitions.
Use CloudWatch Logs to monitor your Step Functions executions and debug issues.
Limit the number of waiting states to avoid excessive resource usage.
Implement retries for tasks that may fail due to transient issues.

5. FAQ

What are the benefits of using callback patterns?

Callback patterns help manage asynchronous workflows better, reduce resource blocking, and improve overall application performance.

How do I debug callback patterns in Step Functions?

You can use CloudWatch Logs and the AWS Step Functions console to track the state transitions and identify any errors in the workflow.

What happens if a task fails with a task token?

If a task fails while using a task token, you can catch the error and implement a fallback or retry mechanism to handle the failure appropriately.