# Scheduling flow runs

If a flow has a schedule attached, then the Prefect backend can automatically create new flow runs according to that schedule.

TIP

Scheduling in this manner is nothing more than a convenient way to generate new runs; you can still create ad-hoc runs alongside the auto-scheduled ones (even if they have the same start time).

Flow schedules can be defined when your flow is registered using the Prefect Core library or after registration using the Prefect API or UI. For details on defining a schedule at registration time, see the Core scheduling documentation. The Core documentation will also be useful for understanding the basic building blocks of schedules, but if you'd like to venture forward here's a quick summary:

  • Flows can have as many schedules as you want
  • Flows have two types of schedules: CronClock (datetime based) and IntervalClock (every x seconds)
  • Schedules are independent; if schedules overlap, multiple flow runs will be created
  • Schedules can set flow parameter values

If you're looking to run a flow once in the future instead of creating a recurring schedule you can create a flow run with a specified start time. See the flow run creation documentation for details.

# Creating flow schedules GQL

Flow schedules are attached to the flow group and can be set using the set_flow_group_schedule mutation.

Existing schedules

Setting flow group schedules will remove any existing schedules.

Scheduling in the Prefect UI

The UI provides a friendly interface for setting schedules in the flow group settings page. The UI will preserve existing schedules and generate CRON clocks for you.

In this example, two interval clocks are set for a flow group. The first runs every hour and sets the parameter name to "Marvin 1hr". The second runs every two hours and sets the parameter name to "Marvin 2hr".

mutation {
  set_flow_group_schedule(
    input: {
      flow_group_id: "<flow-group-id>", 
      interval_clocks: [
        {interval: 3600, parameter_defaults: "{\"name\": \"Marvin 1hr\"}"}
        {interval: 7200, parameter_defaults: "{\"name\": \"Marvin 2hr\"}"}
      ]
    }
  ) {
    success
  }
}

In the next example, a single cron clock is set for a flow group. This clock will run the flow at 12:00 PM Monday, Wednesday, and Friday. The parameters will retain their default values.

mutation {
  set_flow_group_schedule(
    input: {
      flow_group_id: "947c0966-a7b2-452b-a7da-2051415dbddf", 
      cron_clocks: [{cron: "0 12 * * 1,3,5"}]
    }
  ) {
    success
  }
}

Scheduling with parameters

Prefect cannot auto-schedule flows that have required parameters, because the scheduler won't know what value to use. You will get an error if you try to turn on scheduling for such a flow.

To resolve this, provide a default value for your parameters in your flow code or the schedule clock:

from prefect import Parameter

x = Parameter('x', default=1)
{..., parameter_defaults: "{\"x\": 1}"}

# Scheduled flow run creation

Flows with an active schedule will have some of their flow runs created ahead of time. These flow runs will have a scheduled start time and will not begin executing until then. When a flow schedule is updated, these generated runs will be deleted and new runs will be created. Scheduled flow run creation is handled by the Prefect Scheduler.

# Querying for flow schedules GQL

To query for schedules on a flow group, you can select the schedule property.

query {
  flow_group(where: {id: {_eq: "<flow-group-id>"}}) {
    schedule
  }
}

Example return value

{
  "data": {
    "flow_group": [
      {
        "schedule": {
          "type": "Schedule",
          "clocks": [
            {
              "cron": "0 0 * * 1,3,5",
              "type": "CronClock",
              "start_date": {
                "dt": "2021-05-07T10:24:07.143675",
                "tz": "America/Chicago"
              }
            },
            {
              "type": "IntervalClock",
              "interval": 3600000000,
              "start_date": {
                "dt": "2021-05-07T10:24:07.143675",
                "tz": "America/Chicago"
              },
              "parameter_defaults": {
                "name": "Marvin"
              }
            }
          ]
        }
      }
    ]
  }
}

# Toggling flow schedules GQL

You can turn auto-scheduling on or off at any time:

mutation {
  set_schedule_active(input: {
    flow_id: "<flow-id>"
  }) {
    success
  }
}
mutation {
  set_schedule_inactive(input: {
    flow_id: "<flow-id>"
  }) {
    success
  }
}