Triggering Workflows with Schedule in Hatchet

In addition to cron triggers and event-based triggers, Hatchet provides the ability to schedule workflows to run at specific times. This feature is useful when you need to execute a workflow at a precise moment, such as sending a reminder email at a specific hour or a one-time maintenance task at a predetermined time.

Configuring Schedule Triggers

To trigger a workflow at a specific time, you need to configure the on property in the workflow definition using the schedule option. Here's an example of how to define a schedule trigger:

const myWorkflow: Workflow = {
  id: "my-workflow",
  description: "A workflow triggered by a schedule",
  on: {
    schedule: "2023-06-15T09:30:00Z",
  },
  steps: [
    // Define your workflow steps here
  ],
};

In this example, the on property is set to an object with a schedule property. The schedule property specifies the exact date and time when the workflow should be triggered. The date and time should be provided in the ISO 8601 format (YYYY-MM-DDTHH:mm:ssZ).

Delaying Events Using Child Workflows

In addition to triggering workflows at specific times using schedule triggers, Hatchet also provides the ability to delay events by leveraging child workflows. This approach allows you to schedule a workflow to run at a future time and proxy relevant data to it.

Here's an example of how you can delay events using child workflows in Hatchet:

from datetime import datetime, timedelta
from hatchet_sdk import Hatchet, Context
 
hatchet = Hatchet(debug=True)
 
@hatchet.workflow(on_events=["printer:schedule"])
class PrintSchedule:
    @hatchet.step()
    def schedule(self, context: Context):
        now = datetime.now()
        print(f"The time is \t {now.strftime("%H:%M:%S")}")
        future_time = now + timedelta(seconds=15)
        print(f"Scheduling for \t {future_time.strftime("%H:%M:%S")}")
 
        # schedule the run in the future
        hatchet.client.admin.schedule_workflow(
            'PrintPrinter',
            [future_time],
            context.workflow_input()
        )
 
@hatchet.workflow()
class PrintPrinter:
    @hatchet.step()
    def step1(self, context: Context):
        now = datetime.now()
        print(f"Printed at \t {now.strftime("%H:%M:%S")}")
        print(f"Message \t {context.workflow_input()['message']}")

In this example, we have two workflows: PrintSchedule and PrintPrinter. The PrintSchedule workflow is triggered by the printer:schedule event. When this workflow is executed, it performs the following steps:

It retrieves the current time using datetime.now() and prints it.
It calculates a future time by adding a specified delay (in this case, 15 seconds) to the current time using timedelta.
It schedules the PrintPrinter workflow to run at the calculated future time using hatchet.client.admin.schedule_workflow(). The future time is passed as an argument, and the input data for the child workflow is obtained from context.workflow_input().

By scheduling the PrintPrinter workflow to run at a future time, you effectively delay the execution of the event.

This approach provides flexibility in delaying events and passing relevant data to the delayed workflow. You can customize the delay duration based on your specific requirements and include any necessary data in the workflow input.

Remember to register both workflows with the Hatchet worker and start the worker to enable the execution of the workflows.

Scheduling Considerations

When using schedule triggers, there are a few considerations to keep in mind:

Time Zone: Schedule triggers use the UTC (Coordinated Universal Time) time zone by default. If you want to specify the schedule in a different time zone, you need to include the time zone offset in the ISO 8601 format (2023-06-15T09:30:00+02:00 for UTC+2).
Execution Time: Hatchet makes a best-effort attempt to start the workflow at the exact scheduled time. However, there may be slight delays due to system load or other factors. The actual execution time may vary slightly from the scheduled time.
One-Time Execution: Schedule triggers are designed for one-time execution. Once a workflow is triggered by a schedule, it will not be triggered again automatically. If you need recurring executions, consider using cron triggers instead.

Scheduling Best Practices

When working with schedule triggers, consider the following best practices:

Scheduling Granularity: Be mindful of the granularity of your scheduled workflows. Avoid scheduling workflows too frequently or at overly precise times, as it may lead to increased system load and potential delays.
Time Zone Awareness: Ensure that you consider the appropriate time zone when specifying the schedule. Use UTC or include the time zone offset to avoid confusion and ensure consistent execution across different regions.
Scheduling Window: Configure an appropriate scheduling window based on your workflow requirements. Consider factors such as data availability, dependencies, and system load when determining the window size.
Error Handling: Implement proper error handling and retry mechanisms in your workflows to handle any potential failures or exceptions that may occur during scheduled executions.

Conclusion

Schedule triggers in Hatchet provide a convenient way to trigger workflows at specific times. By configuring the on property with a schedule value, you can define the exact date and time when a workflow should be executed.

Remember to consider time zones, execution times, one-time execution, and scheduling windows when working with schedule triggers. Follow best practices such as choosing appropriate scheduling granularity, handling errors, and implementing monitoring and alerting.

With Hatchet's scheduling capabilities, you can automate time-sensitive tasks, generate reports at specific intervals, and execute workflows with precise timing requirements.

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