Celery Task needs to run every N time but need to finish first

Question

so i am using django + celery(rabbitMQ). I wrote several tasks for download files, and send request to notify "hey, i got the file" response. I want to run these tasks run in parallel. Hence i used, celery's group and chain combo. Now the main problem, when i run task it works and all the tasks were called and i just can't detect whether all tasks finished or not. Is it possible to know that all the relevant tasks finished?

PS:// I thought using celery beat does not suit for this job. Because current tasks might not done when celery beat executes.

Here is my snippet, i call this task at first.

@shared_task(name="getOrders", bind=True,  retry_backoff=10, autoretry_for=(exceptions.RequestException,), retry_kwargs={"max_retries": 3, "countdown": 5})
def getOrders(self):
    url = "https://example.com/orders"
    orders = fetchData(url)  # fetch function
    if orders.status_code == requests.codes.ok:
        tree = etree.fromstring(orders.content)
        jobs = group([download_order.si(order_id) for order_id in tree.xpath('order/text()')])
        chain(jobs, notify.s()).apply_async()
        self.retry(countdown=900, max_retries=2)

download task is:

@shared_task(name="download_order", acks_late=True, autoretry_for=(exceptions.RequestException,), retry_kwargs={"max_retries": 3, "countdown": 5},)
def download_order(order_id):
    """Download order"""
    print("\n===========\nstarting download on {}\n=========\n".format(order_id))
    url = "https://example.com/order/{}.xml".format(order_id)
    get_xml = fetchData(url)
    if fulfillment_request.status_code == request.codes.ok:
        print("\n=======\nxml fetched\n========\n")
        tree = etree.fromstring(get_xml.content)
        for product in find_product(tree):
            uuid = product.find("UUID").text
            for v in product.iterfind("product"):
                file_type = "Video"
                file_id = v.find('files/file/id').text
                url = "https://example.com/download?order_id={}&type={}&task_id={}".format(
                    order_id, file_type, file_id
                )
                path = "{}/{}/{}.mp4".format(
                    settings.VIDEO_DIRECTORY, order_id, file_id
                )
                chain(
                    download.si(url, path),
                    orderReceived.si(order_id, file_type, file_id)
                ).delay()

@shared_task(name="notify")
def notify(id):
   print("notifying", id)

@shared_task(name="order_received")
def orderReceived(order_id, ftype, file_id):
   print("order received", order_id, ftype, file_id)