Schedule Global Tasks#
This guide addresses how to schedule tasks that run independently of any user visiting an application.
The pn.state.schedule_task functionality allows scheduling global tasks at certain times or on a specific schedule. This is distinct from periodic callbacks, which are scheduled per user session. Global tasks are useful for performing periodic actions like updating cached data, performing cleanup actions or other housekeeping tasks, while periodic callbacks should be reserved for making periodic updates to an application.
The different contexts in which global tasks and periodic callbacks run also has implications on how they should be scheduled. Scheduled task must not be declared in the application code itself, i.e. if you are serving panel serve app.py the callback you are scheduling must not be declared in the app.py. It must be defined in an external module or in a separate script declared as part of the panel serve invocation using the --setup commandline argument.
Scheduling using pn.state.schedule_task is idempotent, i.e. if a callback has already been scheduled under the same name subsequent calls will have no effect. By default the starting time is immediate but may be overridden with the at keyword argument. The period may be declared using the period argument or a cron expression (which requires the croniter library). Note that the at time should be in local time but if a callable is provided it must return a UTC time. If croniter is installed a cron expression can be provided using the cron argument.
Here is a simple example of a task scheduled at a fixed interval:
import panel as pn
import datetime as dt
import asyncio
async def task():
print(f'Task executed at: {dt.datetime.now()}')
pn.state.schedule_task('task', task, period='1s')
await asyncio.sleep(3)
pn.state.cancel_task('task')
Note that while both async and regular callbacks are supported, asynchronous callbacks are preferred if you are performing any I/O operations to avoid interfering with any running applications.
If you have the croniter library installed you may also provide a cron expression, e.g. the following will schedule a task to be repeated at 4:02 am every Monday and Friday:
pn.state.schedule_task('task', task, cron='2 4 * * mon,fri')
