Schedule flow runs

Prefect allows you to specify schedules on which your flows run. You can add one or more schedules to any deployed flow. Schedules tell Prefect when and how to create new flow runs. You can add a schedule to a deployed flow in the Prefect UI, in the CLI through the prefect deployment schedule command, or the prefect.yaml configuration file.

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Schedule types

Prefect supports three types of schedules:

• Cron is most appropriate for users who are already familiar with cron from previous use.
• Interval is best suited for deployments that run at some consistent cadence that isn’t related to absolute time.
• RRule is best suited for deployments that rely on calendar logic for simple recurring schedules, irregular intervals, exclusions, or day-of-month adjustments.

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Cron

A Cron schedule uses a standard cron string to define when flow runs should occur. This is ideal for schedules tied to specific clock times and dates (e.g., “every Monday at 9:00 AM”). You may also provide a timezone to enforce daylight saving time (DST) behaviors.

Prefect’s cron scheduling is based on the croniter library, allowing it to interpret familiar cron expressions. For example, the expression 0 9 * * MON-FRI means “at 9:00 AM on every Monday through Friday”.

Cron properties include:

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How the day_or property works

The day_or property defaults to True, matching the behavior of cron.

In this mode, if you specify a day (of the month) entry and a day_of_week entry, the schedule runs a flow on both the specified day of the month and on the specified day of the week.

The “or” in day_or refers to the two entries treated like an OR statement. The schedule should include both, as in the SQL statement:

SELECT * FROM employees WHERE first_name = 'Ford' OR last_name = 'Prefect';

For example, with day_or set to True, the cron schedule * * 3 1 2 runs a flow every minute on the third day of the month and on Tuesday (the second day of the week) in January (the first month of the year).

With day_or set to False, the day (of the month) and day_of_week entries are joined with the more restrictive AND operation, as in the SQL statement:

SELECT * from employees WHERE first_name = 'Zaphod' AND last_name = 'Beeblebrox';

For example, the same schedule, when day_or is False, runs a flow on every minute on the third Tuesday in January. This behavior matches fcron instead of cron.

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Interval

As the name suggests, an Interval schedule is best for deployments that need to run at a consistent cadence not strictly tied to absolute clock time (e.g., “every 10 minutes” or “every 3 hours”). It creates new flow runs on a regular interval, measured in seconds, which can be anchored to a specific anchor_date.

schedule:
  interval: 600
  timezone: America/Chicago

Interval properties include:

The anchor_date does not indicate a “start time” for the schedule, but a fixed point to compute intervals. If the anchor date is in the future, then schedule dates are computed by subtracting the interval from it.

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RRule

For complex scheduling needs that go beyond simple cron expressions or fixed intervals, RRule (Recurrence Rule) is the most powerful option. It’s ideal for deployments that rely on calendar-based logic, such as “the last weekday of each month,” “every other Tuesday,” or schedules with specific exclusions.

RRule scheduling supports iCal recurrence rules, providing a rich syntax for defining a wide array of repetitive schedules, from yearly down to every minute.

RRules are appropriate for any kind of calendar-date manipulation, including simple repetition, irregular intervals, exclusions, week day or day-of-month adjustments, and more. RRules can represent complex logic like:

• The last weekday of each month
• The fourth Thursday of November
• Every other day of the week

RRule properties include:

You may find it useful to use an RRule string generator such as the iCalendar.org RRule Tool to help create valid RRules.

schedule:
  rrule: 'FREQ=WEEKLY;BYDAY=MO,WE,FR;UNTIL=20240730T040000Z'

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How scheduling works

Prefect’s Scheduler service evaluates each deployment’s schedules and creates new runs appropriately. It starts automatically when prefect server start is run and it is a built-in service of Prefect Cloud.

The Scheduler creates the fewest runs that satisfy the following constraints, in order:

• No more than 100 runs will be scheduled.
• Runs will not be scheduled more than 100 days in the future.
• At least three runs will be scheduled.
• Runs will be scheduled until at least one hour in the future.

These behaviors are adjustable through the relevant settings with the prefect config view --show-defaults command:

PREFECT_API_SERVICES_SCHEDULER_DEPLOYMENT_BATCH_SIZE='100'
PREFECT_API_SERVICES_SCHEDULER_ENABLED='True'
PREFECT_API_SERVICES_SCHEDULER_INSERT_BATCH_SIZE='500'
PREFECT_API_SERVICES_SCHEDULER_LOOP_SECONDS='60.0'
PREFECT_API_SERVICES_SCHEDULER_MIN_RUNS='3'
PREFECT_API_SERVICES_SCHEDULER_MAX_RUNS='100'
PREFECT_API_SERVICES_SCHEDULER_MIN_SCHEDULED_TIME='1:00:00'
PREFECT_API_SERVICES_SCHEDULER_MAX_SCHEDULED_TIME='100 days, 0:00:00'

See the Settings docs for more information on altering your settings.

These settings mean that if a deployment has an hourly schedule, the default settings will create runs for the next four days (or 100 hours). If it has a weekly schedule, the default settings will maintain the next 14 runs (up to 100 days in the future).

If you change a schedule, previously scheduled flow runs that have not started are removed, and new scheduled flow runs are created to reflect the new schedule.

To remove all scheduled runs for a flow deployment, you can remove the schedule.

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Further reading

• How to create schedules