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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.
Schedules can be inactiveWhen you create or edit a schedule, you can set the active property to False in Python (or false in a YAML file) to deactivate the schedule. This is useful to keep the schedule configuration but temporarily stop the schedule from creating new flow runs.

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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”.
Supported croniter featuresWhile Prefect supports most features of croniter for creating cron-like schedules, it does not support “R” random or “H” hashed keyword expressions or the schedule jittering possible with those expressions.
Cron properties include:
PropertyDescription
cronA valid cron string. (Required)
day_orBoolean indicating how croniter handles day and day_of_week entries. Default is True.
timezoneString name of a time zone. (See the IANA Time Zone Database for valid time zones.)

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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.
Daylight saving time considerationsIf the timezone is a DST-observing one, then the schedule adjusts itself appropriately.The cron rules for DST are based on schedule times, not intervals. This means that an hourly cron schedule fires on every new schedule hour, not every elapsed hour. For example, when clocks are set back, this results in a two-hour pause as the schedule will fire the first time 1 AM is reached and the first time 2 AM is reached, 120 minutes later.Longer schedules, such as one that fires at 9 AM every morning, adjust for DST automatically.

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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:
PropertyDescription
intervaldatetime.timedelta indicating the time between flow runs. (Required)
anchor_datedatetime.datetime indicating the starting or “anchor” date to begin the schedule. If no anchor_date is supplied, the current UTC time is used.
timezoneString name of a time zone, used to enforce localization behaviors like DST boundaries. (See the IANA Time Zone Database for valid time zones.)
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.
Daylight saving time considerationsIf the schedule’s anchor_date or timezone are provided with a DST-observing timezone, then the schedule adjusts itself appropriately. Intervals greater than 24 hours will follow DST conventions, while intervals of less than 24 hours will follow UTC intervals.For example, an hourly schedule will fire every UTC hour, even across DST boundaries. When clocks are set back, this results in two runs that appear to be scheduled for 1 AM local time, even though they are an hour apart in UTC time.For longer intervals, like a daily schedule, the interval schedule adjusts for DST boundaries so that the clock-hour remains constant. This means that a daily schedule that always fires at 9 AM will observe DST and continue to fire at 9 AM in the local time zone.

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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:
PropertyDescription
rruleString representation of an RRule schedule. See the rrulestr examples for syntax.
timezoneString name of a time zone. See the IANA Time Zone Database for valid time zones.
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'
RRule restrictions The max supported character length of an rrulestr is 6,500 charactersCOUNT is not supported. Please use UNTIL or the /deployments/{id}/runs endpoint to schedule a fixed number of flow runs.
Daylight saving time considerationsAs a calendar-oriented standard, RRules are sensitive to the initial timezone provided. A 9 AM daily schedule with a DST-aware start date maintains a local 9 AM time through DST boundaries. A 9 AM daily schedule with a UTC start date maintains a 9 AM UTC time.

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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).
Scheduler does not affect executionThe Prefect Scheduler service only creates new flow runs and places them in Scheduled states. It is not involved in flow or task execution.
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

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