Question

I use celery in my application to run periodic tasks. Let’s see simple example below

from myqueue import Queue
@perodic_task(run_every=timedelta(minutes=1))
def process_queue():
    queue = Queue()
    uid, questions = queue.pop()
    if uid is None:
        return

    job = group(do_stuff(q) for q in questions)
    job.apply_async()

def do_stuff(question):
    try:
        ...
    except:
        ...
        raise

As you can see in the example above, i use celery to run async task, but (since it’s a queue) i need to do queue.fail(uid) in case of exception in do_stuff or queue.ack(uid) otherwise. In this situation it would be very clear and usefull to have some callback from my task in both cases – on_failure and on_success.

I saw some documentation, but never seen practices of using callbacks with apply_async. Is it possible to do that?

Answer 1

Subclass the Task class and overload the on_success and on_failure functions:

from celery import Task


class CallbackTask(Task):
    def on_success(self, retval, task_id, args, kwargs):
        '''
        retval – The return value of the task.
        task_id – Unique id of the executed task.
        args – Original arguments for the executed task.
        kwargs – Original keyword arguments for the executed task.
        '''
        pass
        
    def on_failure(self, exc, task_id, args, kwargs, einfo):
        '''
        exc – The exception raised by the task.
        task_id – Unique id of the failed task.
        args – Original arguments for the task that failed.
        kwargs – Original keyword arguments for the task that failed.
        '''
        pass

Use:

@celery.task(base=CallbackTask)  # this does the trick
def add(x, y):
    return x + y