How to execute a command at exact time once a day

Question

I am working on a Django web based project in which i need to build a application which work in the following sequence:

1) user open a page in which he need to enter a command and a time

2) Django application will execute that command at a given time on each day till user off the scheduler (by default it is True)

What i am facing the problem is that :

1) How should i execute the commands on a time but on each day. To save the commands and time i created a following model in my models.py

class commands(models.Model):
    username = models.ForeignKey(User)
    command = models.CharField(max_length=30)
    execution_time = models.DateField()

I have the same time but i am not getting the right way to execute it on each day at the given time

and is it possible to do with pytz library?

For executing the commands i am using paramiko library

PS: I don't want to use any external library

Answer 1

It seems to me that the proper way to do this would be write a Django custom command and execute it via cron. But you seem to be under luck as others have felt similar need and have written custom django apps. Take django-cron for example.

Answer 2

While you could have your django app add and remove cron jobs on the system, another more django-ish approach would be to use Celery. It is a task queue system that can run both synch and async tasks.

One specific feature of Celery is scheduled tasks: http://packages.python.org/celery/userguide/periodic-tasks.html

from datetime import timedelta

CELERYBEAT_SCHEDULE = {
    "runs-every-30-seconds": {
        "task": "tasks.add",
        "schedule": timedelta(seconds=30),
        "args": (16, 16)
    },
}

They also have a more granular version of the period task that replicates the scheduling of a crontab:

from celery.schedules import crontab

CELERYBEAT_SCHEDULE = {
    # Executes every Monday morning at 7:30 A.M
    'every-monday-morning': {
        'task': 'tasks.add',
        'schedule': crontab(hour=7, minute=30, day_of_week=1),
        'args': (16, 16),
    },
}

Celery by itself is stand-alone but there is the django-celery specific verison

The benefit of this solution is that you do not need to edit and maintain a system-level cron tab. This is a solution that is highly integrated into django for this exact use.

Also a huge win over using a cron is that Celery can scale with your system. If you were using a basic system crontab, then the tasks would be located on the server that hosts the application. But what if you needed to ramp up your site and run it on 5 web application nodes? You would need to centralize that crontab. If you are using Celery, you have a large number of options for how to transport and store tasks. It is inherently distributed, and available in sync to all your application servers. It is portable.

Answer 3

The solution for your problem is standard cron application (task planner on *nix systems). You can schedule a script using cron (by adding it to crontab).

If your script must perform in you Django application environment, it's possible to tell him to do that with setup_environment function. You can read more about standalone scripts for Django applications here.