Update tasks in Celery with RabbitMQ

问题

I'm using Celery in my django project to create tasks to send email at a specific time in the future. User can create a Notification instance with notify_on datetime field. Then I pass value of notify_on as a eta.

class Notification(models.Model):
    ...
    notify_on = models.DateTimeField()


def notification_post_save(instance, *args, **kwargs):
    send_notification.apply_async((instance,), eta=instance.notify_on)

signals.post_save.connect(notification_post_save, sender=Notification)

The problem with that approach is that if notify_on will be changed by the user, he will get two(or more) notifications instead of one.

The question is how do I update the task associated with a specific notification, or somehow delete the old one and create new.

回答1:

First of all, by using post_save, we can't fetch the old data. So, here I'm overriding the save() method of the Notification model. Apart from that,create a field to store the celery task_id.

from celery.task.control import revoke


class Notification(models.Model):
    ...
    notify_on = models.DateTimeField()
    celery_task_id = models.CharField(max_length=100)

    def save(self, *args, **kwargs):
        pre_notify_on = Notification.objects.get(pk=self.pk).notify_on
        super().save(*args, **kwargs)
        post_notify_on = self.notify_on
        if not self.celery_task_id:  # initial task creation
            task_object = send_notification.apply_async((self,), eta=self.notify_on)
            Notification.objects.filter(pk=self.pk).update(celery_task_id=task_object.id)
        elif pre_notify_on != post_notify_on:
            # revoke the old task
            revoke(self.celery_task_id, terminate=True)
            task_object = send_notification.apply_async((self,), eta=self.notify_on)
            Notification.objects.filter(pk=self.pk).update(celery_task_id=task_object.id)

Reference

Cancel an already executing task with Celery?
Django: How to access original (unmodified) instance in post_save signal

回答2:

I think there is no need for deleting the previous tasks. You just have to validate that the task that is executing is the lasted one. For that create a new field called checksum that is a UUID field update that field everytime you change notify_on. Check this checksum in the task where you are sending the email.

class Notification(models.Model):
    checksum = models.UUIDField(default=uuid.uuid4)
    notify_on = models.DateTimeField()

def notification_post_save(instance, *args, **kwargs):
    send_notification.apply_async((instance.id, str(instance.checksum)),eta=instance.notify_on)

signals.post_save.connect(notification_post_save, sender=Notification)


@shared_task 
def send_notification(notification_id, checksum):
    notification = Notification.objects.get(id=notification_id)
    if str(notification.checksum) != checksum:
        return False
    #send email

Also please don't send signal everytime on notification object save just send this when notify_on changes. You can also check this Identify the changed fields in django post_save signal

来源：https://stackoverflow.com/questions/55337206/update-tasks-in-celery-with-rabbitmq

标签

django

RabbitMQ

celery