Gatekeeper loop

gatekeeper.py

class NotReadyError(Exception):
    """
    Custom exception signaling to the Step Function
    that it cannot move to the next state. Instead, 
    the Retry block is triggered, pausing the process.
        
    You can pass details about the progress as a
    string when initializing it. It will be shown
    in the SF console.
    """
    pass

def is_processing_done():
    """
    Function checking if whatever external work the
    pipeline is waiting on has finished already.
    """
    return False

def main(event, context):
    if event.get('force'):
        return

    if not is_processing_done():
        raise NotReadyError()

stepfunction.json

{
    "Comment": "Step Function demonstrating the gatekeeper loop",
    "StartAt": "GatekeeperState",
    "States": {
        "GatekeeperState": {
            "Type": "Task",
            "Resource": "",
            "Next": "DoneState",
            "ResultPath": null,
            "Retry": [{
                "ErrorEquals": ["NotReadyError"],
                "IntervalSeconds": 30,
                "BackoffRate": 1.0,
                "MaxAttempts": 50
            }],
            "Catch": [{
                "ErrorEquals": ["NotReadyError"],
                "ResultPath": "$.exception",
                "Next": "ForceGatekeeperState"
            }]
        },
        "ForceGatekeeperState": {
            "Type": "Pass",
            "Result": true,
            "ResultPath": "$.force",
            "Next": "GatekeeperState"
        },
        "DoneState": {
            "Type": "Task",
            "Resource": "",
            "End": true
        }
    }
}