Add section about periodic tasks

README.md

@@ -1031,6 +1031,79 @@ Meaning, you can end up with `tasks/domain_a.py` and `tasks/domain_b.py`. All yo
 
 The general rule of thumb is - split your tasks in a way that'll make sense to you.
 
+### Periodic Tasks
+
+Managing periodic tasks is quite important, especially when you have tens, or hundreds of them.
+
+We use [Celery Beat](https://docs.celeryproject.org/en/latest/userguide/periodic-tasks.html) + `django_celery_beat.schedulers:DatabaseScheduler` + [`django-celery-beat`](https://github.com/celery/django-celery-beat) for our peridoic tasks.
+
+The extra thing that we do is to have a management command, called `setup_periodic_tasks`, which holds the definition of all periodic tasks within the system. This command is located in the `tasks` app, discussed above.
+
+Here's how `project.tasks.management.commands.setup_periodic_tasks.py` looks like:
+
+```python
+from django.core.management.base import BaseCommand
+from django.db import transaction
+
+from django_celery_beat.models import IntervalSchedule, CrontabSchedule, PeriodicTask
+
+from project.app.tasks import some_periodic_task
+
+
+class Command(BaseCommand):
+    help = f"""
+    Setup celery beat periodic tasks.
+
+    Following tasks will be created:
+
+    - {some_periodic_task.name}
+    """
+
+    @transaction.atomic
+    def handle(self, *args, **kwargs):
+        print('Deleting all periodic tasks and schedules...\n')
+
+        IntervalSchedule.objects.all().delete()
+        CrontabSchedule.objects.all().delete()
+        PeriodicTask.objects.all().delete()
+
+        periodic_tasks_data = [
+            {
+                'task': some_periodic_task
+                'name': 'Do some peridoic stuff',
+                # https://crontab.guru/#15_*_*_*_*
+                'cron': {
+                    'minute': '15',
+                    'hour': '*',
+                    'day_of_week': '*',
+                    'day_of_month': '*',
+                    'month_of_year': '*',
+                },
+                'enabled': True
+            },
+        ]
+
+        for periodic_task in periodic_tasks_data:
+            print(f'Setting up {periodic_task["task"].name}')
+
+            cron = CrontabSchedule.objects.create(
+                **periodic_task['cron']
+            )
+
+            PeriodicTask.objects.create(
+                name=periodic_task['name'],
+                task=periodic_task['task'].name,
+                crontab=cron,
+                enabled=periodic_task['enabled']
+            )
+```
+
+Few key things:
+
+* We use this task as part of a deploy procedure.
+* We always put a link to [`crontab.guru`](https://crontab.guru) to explain the cron. Otherwhise it's unreadable.
+* Everything is in one place.
+
 ## Inspiration
 
 The way we do Django is inspired by the following things: