from celery import Celery

app = Celery('tasks', broker='redis://localhost', backend='redis://localhost')

# for demo purpose restrict the number of prefetched tasks to 1
# this effectively limits the number of rate limited tasks submitted to 4
# before the worker gets blocked. Leaving it at the default value (4) means
# you need 4 * 4 = 16 tasks before the worker stops receiving new tasks
# That is the number of rate limited tasks required to block the worker
# increases however it does not solve the problem (though setting the prefetch
# multiplier to some very large number e.g. sys.maxint may be a work around
# in some scenarios)
# https://docs.celeryproject.org/en/stable/userguide/configuration.html#worker-prefetch-multiplier
app.conf.worker_prefetch_multiplier = 1

@app.task
def A(x, y):
    return x + y

@app.task
def B(x, y):
    return x - y