@Marvin what is the proper way to wait for external state change in a flow? I have a task that needs to wait for some external database state to be modified after an external job is finished running. This can take upwards of several hours. What is the proper way of doing this?

To wait for an external state change in a Prefect flow, you can use a combination of retries and timeouts. Prefect does not have a built-in mechanism specifically designed to wait for external states. However, you can implement your own polling mechanism within a task, waiting and retrying until the external condition is met. Here's a way to handle it: ### Example Overview 1. Polling Task: A task that periodically checks the state of the external system. 2. Retries: Configure retries and retry delays to control the polling interval and duration. ### Example Code

python
from prefect import task, flow, get_run_logger
import time

@task(retries=20, retry_delay_seconds=900)  # Retries every 15 minutes for up to 5 hours
def wait_for_external_state_change():
    logger = get_run_logger()
    
    # Replace with your actual checking logic
    state = check_external_state()
    
    if state != "desired_state":
        <http://logger.info|logger.info>("External state not met. Checking again later.")
        raise Exception("External state not met yet.")
    
    <http://logger.info|logger.info>("External state condition met.")
    return state

def check_external_state():
    # Replace this with your actual logic to check the state of the external system
    # For example, query a database or make an API call
    ...
    return "current state"

@flow
def my_flow():
    result = wait_for_external_state_change()
    logger = get_run_logger()
    <http://logger.info|logger.info>(f"Final state: {result}")

if __name__ == "__main__":
    my_flow()

### Key Points - Retries and Delay: The

retries

parameter specifies the number of retry attempts, and

retry_delay_seconds

specifies the delay between retries. - State Checking: Replace

check_external_state

with your actual logic for checking the state of the external system. ### Sources 1. State Change Hooks 2. AWS Client Waiter Example This approach allows your flow to wait for several hours, periodically checking for the desired external state.

I don't have any reason to limit the number of retries.

depending on the type of thing you're waiting for, you could suspend the flow run and have something external resume it. if you persist task results it'll pick back up from the last unfinished task