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<@ULVA73B9P> if I have a long running script that ...
# ask-marvin
n
@Marvin if I have a long running script that is serving deployments how can I force it to terminate from within a flow. for example, if I had a deployment that was looking for specific condition, such as water level reaching 200mm, and wished to pause all deployments and terminate the long-running script once that condition was met how would I do that
m
To terminate a long-running script from within a Prefect flow when a specific condition is met, you would need to incorporate custom logic within your flow that monitors the condition and then triggers the termination. Here's a basic outline of how you can achieve this: 1. Monitor the Condition: Within your flow, continually check for the condition (e.g., water level reaching 200mm). 2. Initiate Termination: Once the condition is met, call a function to terminate the long-running script gracefully. Here's an example in Python:
Copy code
python
from prefect import flow, task
import os
import signal
import time

# Task to monitor water level
@task
def check_water_level():
    # Simulate checking the water level
    water_level = 210  # This would be replaced with actual sensor data
    return water_level

# Task to terminate the long-running script
@task
def terminate_script():
    pid = os.getpid()  # Get the current process ID
    os.kill(pid, signal.SIGTERM)  # Send the terminate signal

@flow
def monitoring_flow():
    while True:
        water_level = check_water_level()
        if water_level >= 200:
            terminate_script()
            break
        time.sleep(5)  # Check every 5 seconds

if __name__ == "__main__":
    monitoring_flow()
### Explanation - Task 
check_water_level
: Simulates fetching the water level. In a real scenario, you would replace this with actual sensor data fetching logic. - Task 
terminate_script
: Sends a terminate signal to the process running the script. - Flow 
monitoring_flow
: Continuously monitors the water level and triggers 
terminate_script
when the condition is met. ### Additional Resources You might find useful information and examples by examining the official Prefect documentation or relevant GitHub repositories. For example, the basic structure of a flow is explained in the code example provided here. Should you need further assistance or examples, feel free to ask!
n
@Marvin in the specific use case described I want to have a scheduled job running on a Cron Schedule "8 * * * * *" so that the job runs at 8 minutes past the hour every hour. Having created that deployment the script will 
serve()
it. Every 8 minutes a subprocess will be spawned to run the flow. I want to signal back to the parent process that we're finished once a flow run detects the right condition. Are signls the recommended way but using the parent pid rather than the subprocess pid?
m
To achieve the objective of running scheduled jobs, detecting a specific condition, and signaling back to terminate the parent process, you can utilize signals effectively in a Prefect flow. The following steps outline a method using a CronSchedule and signaling: 1. Set up the CronSchedule for your deployment. 2. Monitor the condition within the flow. 3. Signal the parent process to terminate upon detecting the condition. Here's an example: ### Step 1: Create and serve the deployment with a CronSchedule
Copy code
python
from prefect import flow, task
from prefect.deployments import Deployment
from prefect.server.schemas.schedules import CronSchedule

@flow
def monitoring_flow():
    water_level = check_water_level()
    if water_level >= 200:
        signal_terminate()

@task
def check_water_level():
    # Simulate checking water level
    water_level = 210  # Replace with actual logic
    return water_level

@task
def signal_terminate():
    import os, signal
    parent_pid = os.getppid()  # Parent process PID
    os.kill(parent_pid, signal.SIGTERM)  # Send SIGTERM to parent

deployment = Deployment.build_from_flow(
    flow=monitoring_flow,
    name="Water Level Monitoring",
    schedule=CronSchedule(cron="8 * * * *"),  # Runs at 8 minutes past the hour, every hour
)
deployment.apply()
### Step 2: Serve the Deployment In the script that serves deployments, you would do something like:
Copy code
python
from prefect.deployments import run_deployment

if __name__ == "__main__":
    run_deployment(name="Water Level Monitoring")
### Explanation - Flow 
monitoring_flow
: Checks the water level and signals termination if a condition is met. - Task 
check_water_level
: Simulates obtaining the water level. - Task 
signal_terminate
: Sends a termination signal (
SIGTERM
) to the parent process using its PID. ### Step 3: Handle SIGTERM in the Parent Process Ensure your parent process handles 
SIGTERM
appropriately. For example:
Copy code
python
import signal
import time

def handle_sigterm(signum, frame):
    print("Received SIGTERM. Shutting down...")
    # Clean up resources here
    exit(0)

if __name__ == "__main__":
    signal.signal(signal.SIGTERM, handle_sigterm)

    while True:
        print("Running parent process...")
        time.sleep(30)  # Running an infinite loop for demonstration
### Conclusion This setup will allow the flow to run every 8 minutes, check the water level, and terminate the parent process if the condition is met, using signals to communicate between subprocesses. ### Additional Notes - You may need to adjust the actual logic for checking water levels as needed. - Ensure proper exception handling and cleanup procedures when terminating processes to avoid resource leaks. For further details or assistance, refer to the Prefect Documentation or relevant GitHub Issues.
n
marvin’s retrieval is broken right now so the syntax is old / wrong but if you do sigterm like that from somewhere within, it should kill your process
n
thanks @Nate, I'm used to Marvin's slightly wonky world view, I employed the concept and parenticide does the job nicely.
n
parenticide does the job nicely.
the FBI is knocking at your door
🤣 1
lol jk, glad its working!
testing something, hope you dont mind
@Marvin if I have a long running script that is serving deployments how can I force it to terminate from within a flow. for example, if I had a deployment that was looking for specific condition, such as water level reaching 200mm, and wished to pause all deployments and terminate the long-running script once that condition was met how would I do that
m
To force a long-running script that is serving deployments to terminate from within a flow, you need to use a custom signal to indicate that the condition has been met and then gracefully shut down the process. Here's a step-by-step method to achieve this: ### 1. Create a Flow to Monitor the Condition You can create a flow that continuously checks for the specific condition, such as a water level reaching 200mm. Once the condition is met, the flow can send a termination signal to the long-running script. ### 2. Set Up Signal Handling in the Script Ensure the script that is serving the deployments is configured to handle termination signals appropriately. ### 3. Deploy the Flow with a Schedule Deploy the flow using a 
CronSchedule
to run at the specified time. Here is an example in Python: ### Step 1: Create the Monitoring Flow
Copy code
python
from prefect import flow, task
import os
import signal

@task
def check_water_level():
    # Simulate checking water level
    water_level = 210  # Replace with actual logic to get the water level
    return water_level

@task
def signal_terminate():
    parent_pid = os.getppid()  # Parent process PID
    os.kill(parent_pid, signal.SIGTERM)  # Send SIGTERM to parent process

@flow
def monitoring_flow():
    water_level = check_water_level()
    if water_level >= 200:
        signal_terminate()

if __name__ == "__main__":
    monitoring_flow()
### Step 2: Handle SIGTERM in the Long-Running Script
Copy code
python
import signal
import time
from prefect.deployments import run_deployment

def handle_sigterm(signum, frame):
    print("Received SIGTERM. Shutting down...")
    # Perform any necessary cleanup here
    exit(0)

signal.signal(signal.SIGTERM, handle_sigterm)

if __name__ == "__main__":
    while True:
        print("Running parent process...")
        run_deployment(name="Water Level Monitoring")
        time.sleep(30)  # Check periodically
### Step 3: Deploy the Flow with a Schedule
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python
from prefect.deployments import Deployment
from prefect.server.schemas.schedules import CronSchedule

# Assume monitoring_flow is already defined as shown above
deployment = Deployment.build_from_flow(
    flow=monitoring_flow,
    name="Water Level Monitoring",
    schedule=CronSchedule(cron="8 * * * *"),  # Run at 8 minutes past the hour every hour
)
deployment.apply()
### Explanation - Flow 
monitoring_flow
: This flow checks the water level and sends a signal to terminate the parent process if the condition is met. - Task 
check_water_level
: Simulates obtaining the water level. - Task 
signal_terminate
: Sends a termination signal (
SIGTERM
) to the parent process using its PID. - Script Handling: The parent script that serves the deployments is configured to handle 
SIGTERM
appropriately, performing any necessary cleanup and shutting down gracefully. This approach ensures that the long-running script stops serving deployments and terminates once the specified condition (e.g., water level reaching 200mm) is met within the flow.
n
😢
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