https://realpython.com/blog/python/asynchronous-tasks-with-django-and-celery/

## Install Celery
```
$ pip install celery
```

## Install celery Broker
```
$ sudo apt-get install rabbitmq-server
```

## Django Settings
```
BROKER_URL = 'amqp://localhost'
CELERY_RESULT_BACKEND = 'rpc://'
CELERY_RESULT_PERSISTENT = False
CELERY_ACCEPT_CONTENT = ['application/json']
CELERY_TASK_SERIALIZER = 'json'
CELERY_RESULT_SERIALIZER = 'json'
CELERY_TIMEZONE = 'America/Sao_Paulo'
```

Inside your app, create a file called `celery.py`, and add this:
```
from __future__ import absolute_import

import os

from celery import Celery

# set the default Django settings module for the 'celery' program.
os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'sendmail.settings')

from django.conf import settings  # noqa

app = Celery('sendmail')

# Using a string here means the worker will not have to
# pickle the object when using Windows.
app.config_from_object('django.conf:settings')
app.autodiscover_tasks(lambda: settings.INSTALLED_APPS)


@app.task(bind=True)
def debug_task(self):
    print('Request: {0!r}'.format(self.request))
```

Now, ad another file to the app called `tasks.py`, with the following content:
```
import os
from celery.schedules import crontab
from celery.task import periodic_task
from celery.utils.log import get_task_logger

from core.utils import send_mail
import configparser
from django.conf import settings


try:
    config = configparser.ConfigParser()
    file = os.path.join(settings.BASE_DIR, 'schedule.ini')
    config.read(file)
    hour = config['DEFAULT']['hour']
    minute = config['DEFAULT']['minute']
    day = config['DEFAULT']['day']
except Exception as e:
    hour = 8
    minute = 30
    day = 'mon'

logger = get_task_logger(__name__)
@periodic_task(
    run_every=(crontab(hour=hour, minute=minute, day_of_week=day)),
    name="task_send_mail",
    ignore_result=True
)
def task_send_mail():
    """
    Saves latest image from Flickr
    """
    send_mail()
    logger.info("Sent e-mails ;)")
```

# Supervisor
```
$ sudo apt-get install supervisor
```

Now we need to add the supervisor config files at `/etc/supervisor/conf.d`:
```
$ sudo nano /etc/supervisor/conf.d/mail_celery.conf
```
Paste the following content to the file (edit the paths to match your server):
```
; ==================================
;  celery worker supervisor example
; ==================================

; the name of your supervisord program
[program:mailcelery]

; Set full path to celery program if using virtualenv
command=/home/maumau/.virtualenvs/sendmail/bin/celery worker -A sendmail --loglevel=INFO

; The directory to your Django project
directory=/home/maumau/Workspace/projects/sendmail

; If supervisord is run as the root user, switch users to this UNIX user account
; before doing any processing.
user=maumau

; Supervisor will start as many instances of this program as named by numprocs
numprocs=1

; Put process stdout output in this file
stdout_logfile=/var/log/celery/mail_worker.log

; Put process stderr output in this file
stderr_logfile=/var/log/celery/mail_worker.log

; If true, this program will start automatically when supervisord is started
autostart=true

; May be one of false, unexpected, or true. If false, the process will never
; be autorestarted. If unexpected, the process will be restart when the program
; exits with an exit code that is not one of the exit codes associated with this
; process’ configuration (see exitcodes). If true, the process will be
; unconditionally restarted when it exits, without regard to its exit code.
autorestart=true

; The total number of seconds which the program needs to stay running after
; a startup to consider the start successful.
startsecs=10

; Need to wait for currently executing tasks to finish at shutdown.
; Increase this if you have very long running tasks.
stopwaitsecs = 600

; When resorting to send SIGKILL to the program to terminate it
; send SIGKILL to its whole process group instead,
; taking care of its children as well.
killasgroup=true

; if your broker is supervised, set its priority higher
; so it starts first
priority=998
```

And then...
```
$ sudo nano /etc/supervisor/conf.d/mail_celerybeat.conf
```
With the following content:
```
; ================================
;  celery beat supervisor example
; ================================

; the name of your supervisord program
[program:mailcelerybeat]

; Set full path to celery program if using virtualenv
command=/home/maumau/.virtualenvs/sendmail/bin/celerybeat -A sendmail --loglevel=INFO

; The directory to your Django project
directory=/home/maumau/Workspace/projects/sendmail

; If supervisord is run as the root user, switch users to this UNIX user account
; before doing any processing.
user=maumau

; Supervisor will start as many instances of this program as named by numprocs
numprocs=1

; Put process stdout output in this file
stdout_logfile=/var/log/celery/mail_beat.log

; Put process stderr output in this file
stderr_logfile=/var/log/celery/mail_beat.log

; If true, this program will start automatically when supervisord is started
autostart=true

; May be one of false, unexpected, or true. If false, the process will never
; be autorestarted. If unexpected, the process will be restart when the program
; exits with an exit code that is not one of the exit codes associated with this
; process’ configuration (see exitcodes). If true, the process will be
; unconditionally restarted when it exits, without regard to its exit code.
autorestart=true

; The total number of seconds which the program needs to stay running after
; a startup to consider the start successful.
startsecs=10

; if your broker is supervised, set its priority higher
; so it starts first
priority=999
```

Now, to run supervisor run:
```
$ sudo supervisord
```
It will load the `.conf` files we just created.

To start/stop and status use the following commands:

```
$ sudo supervisorctl stop mailcelery
$ sudo supervisorctl start mailcelery
$ sudo supervisorctl status mailcelery
```