Using Cron Scheduling to automatically run background jobs

Cron scheduling is nothing new. It is basically a concept in which the system keeps executing lines of code every few seconds, minutes, or hours. It is required in many large applications which require some automation in their working. I had to use it for two purposes in our Open-Event application.

  • To automatically delete the items in the trash after a period of 30 days.
  • To automatically send after event mails to the speakers and organizers when an event gets completed

1. Delete items in Trash system

So the deleted items get stored in the trash of the admin. However if the items are not deleted or no action is performed on them then they should be deleted automatically if they stay in the trash for more than 30 days. I have used a framework – apscheduler (Advanced Python Scheduler) for this.The code is like this

from apscheduler.schedulers.background import BackgroundScheduler

def empty_trash():
 with app.app_context():
 print 'HELLO'
 events = Event.query.filter_by(in_trash=True)
 users = User.query.filter_by(in_trash=True)
 sessions = Session.query.filter_by(in_trash=True)
 for event in events:
 if datetime.now() - event.trash_date >= timedelta(days=30):
 DataManager.delete_event(event.id)

 for user in users:
 if datetime.now() - user.trash_date >= timedelta(days=30):
 transaction = transaction_class(Event)
 transaction.query.filter_by(user_id=user.id).delete()
 delete_from_db(user, "User deleted permanently")

 for session in sessions:
 if datetime.now() - session.trash_date >= timedelta(days=30):
 delete_from_db(session, "Session deleted permanently")


trash_sched = BackgroundScheduler(timezone=utc)
trash_sched.add_job(empty_trash, 'cron', day_of_week='mon-fri', hour=5,
                    minute=30)
trash_sched.start()

The trash_sched is initialized first. It is given an instance of BackgroundScheduler() meaning it will always run in the background as long as the application server is running.

The second line defines the trigger which is given to the scheduler meaning at what time intervals do we want the scheduler to execute the function.

trash_sched.add_job(empty_trash, 'cron', day_of_week='mon-fri', hour=5, 
                    minute=30)

Here the scheduler adds the function to the job list. The function is empty_trash and the trigger given here is ‘cron’. The following line:

day_of_week='mon-fri', hour=5, minute=30

sets the time at which the sheduler executes the job. The above line means that the job will be executed every day from Monday to Friday at 5:30 AM. Now coming to the function which is to be executed:

def empty_trash():
    with app.app_context():
        events = Event.query.filter_by(in_trash=True)
        users = User.query.filter_by(in_trash=True)
        sessions = Session.query.filter_by(in_trash=True)
        for event in events:
            if datetime.now() - event.trash_date >= timedelta(days=30):
                DataManager.delete_event(event.id)

        for user in users:
            if datetime.now() - user.trash_date >= timedelta(days=30):
                transaction = transaction_class(Event)
                transaction.query.filter_by(user_id=user.id).delete()
                delete_from_db(user, "User deleted permanently")

        for session in sessions:
            if datetime.now() - session.trash_date >= timedelta(days=30):
                delete_from_db(session, "Session deleted permanently")

There are three items in the trash: events, users and sessions. We get all the items in the trash by the query.all() method. Each model: Events, Users and Sessions has a column

trash_date = db.Column(db.DateTime)

The date on which the item is moved to the trash is stored in the trash_date column. And the following line:

 if datetime.now() - event.trash_date >= timedelta(days=30)

checks if the item has been in the trash for more than 30 days. If yes then it is deleted automatically. This function is constantly executed by the apscheduler according to the time settings given by us. Thus we do not need to manually delete the trash after 30 days.

2. Send After Event Mails

This is similar to the trash emptying function.

from apscheduler.schedulers.background import BackgroundScheduler

def send_after_event_mail():
 with app.app_context():
 events = Event.query.all()
 for event in events:
 upcoming_events = DataGetter.get_upcoming_events(event.id)
 organizers = DataGetter.get_user_event_roles_by_role_name(event.id, 'organizer')
 speakers = DataGetter.get_user_event_roles_by_role_name(event.id, 'speaker')
 if datetime.now() > event.end_time:
 for speaker in speakers:
 send_after_event(speaker.user.email, event.id, upcoming_events)
 for organizer in organizers:
 send_after_event(organizer.user.email, event.id, upcoming_events)

#logging.basicConfig()
sched = BackgroundScheduler(timezone=utc)
sched.add_job(send_after_event_mail, 'cron', day_of_week='mon-fri', hour=5, minute=30)
#sched.start()

The scheduler settings are the same as the trash scheduler settings. The function returns all the events and checks whether the event’s end_date has come or not. This check is performed against the present date by the following line:

if datetime.now() > event.end_time

If yes then the speakers and organizers for that event are obtained and after event mails are sent to them automatically.

In this way the whole system is automated. 🙂

Implementing Admin Trash in Open Event

So last week I had the task of implementing a trash system for the Admin. It was observed that sometimes a user may delete an item and then realize that the item needs to be restores. Thus a trash system works well in this case. Presently the items that are being moved to the trash are:

  • Deleted Users
  • Deleted Events
  • Deleted Sessions

So it works like this. I added a column in_trash to the tables User, Event and Sessions to mark whether the item is in the trash or not

in_trash = db.Column(db.Boolean, default=False)

So depending on whether the value is True or False the item will be in the trash of the admin. Thus for a normal user on deleting an event, user or session a message would flash that the item is deleted and the item would not be shown in the table list of the user. However it would not be deleted from the database.

trash4.png

trash5.png

Thus for the user the item is deleted. The item’s in_trash property is set to True and it gets moved to the trash. The items are displayed in the “Deleted Items” section of the Admin panel

trash1trash2trash3

The items deleted are displayed in the trash and as soon as they deleted in the trash they are deleted from the database permanently. A message will flash for the Admin when it is deleted

trash11

trash10.png

Thus the trash is implemented. 🙂

Two more things are left:

  • To restore items from trash
  • To automatically delete the items in trash after an inactivity of 30 days

This will soon be implemented 🙂