Scheduling remote cron jobs

A common requirement is to perform some task at a given time every day or week automatically. Modal facilitates this through function schedules.

Basic scheduling

Let’s say we have a Python module heavy.py with a function, perform_heavy_computation().

# heavy.py
def perform_heavy_computation():
    ...

@app.local_entrypoint()
def main():
    perform_heavy_computation.remote()

To schedule this function to run once per day, we create a Modal App and attach our function to it with the @app.function decorator and a schedule parameter:

# heavy.py
import modal

app = modal.App()  # Note: prior to April 2024, "app" was called "stub"

@app.function(schedule=modal.Period(days=1))
def perform_heavy_computation():
    ...

To activate the schedule, deploy your app, either through the CLI:

modal deploy --name daily_heavy heavy.py

Or programmatically:

if __name__ == "__main__":
    modal.runner.deploy_app(app)

When you make changes to your function, just rerun the deploy command to overwrite the old deployment.

Note that when you redeploy your function, modal.Period resets, and the schedule will run X hours after this most recent deployment.

If you want to run your function at a regular schedule not disturbed by deploys, modal.Cron (see below) is a better option.

Monitoring your scheduled runs

To see past execution logs for the scheduled function, go to the Apps section on the Modal web site.

Schedules currently cannot be paused. Instead the schedule should be removed and the app redeployed. Schedules can be started manually on the app’s dashboard page, using the “run now” button.

Schedule types

There are two kinds of base schedule values - modal.Period and modal.Cron.

modal.Period lets you specify an interval between function calls, e.g. Period(days=1) or Period(hours=5):

# runs once every 5 hours
@app.function(schedule=modal.Period(hours=5))
def perform_heavy_computation():
    ...

modal.Cron gives you finer control using cron syntax:

# runs at 8 am (UTC) every Monday
@app.function(schedule=modal.Cron("0 8 * * 1"))
def perform_heavy_computation():
    ...

For more details, see the API reference for Period, Cron and Function