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Celery Documentation, Release 2.4.7
8.7.10 Can I send some tasks to only some servers?
Answer: Yes. You can route tasks to an arbitrary server using AMQP, and a worker can bind to as many queues as it
wants.
See Routing Tasks for more information.
8.7.11 Can I change the interval of a periodic task at runtime?
Answer: Yes. You can use the Django database scheduler, or you can override PeriodicTask.is_due or turn PeriodicTask.run_every into a property:
class MyPeriodic(PeriodicTask):
def run(self):
# ...
@property
def run_every(self):
return get_interval_from_database(...)
8.7.12 Does celery support task priorities?
Answer: No. In theory, yes, as AMQP supports priorities. However RabbitMQ doesn’t implement them yet.
The usual way to prioritize work in Celery, is to route high priority tasks to different servers. In the real world this
may actually work better than per message priorities. You can use this in combination with rate limiting to achieve a
highly responsive system.
8.7.13 Should I use retry or acks_late?
Answer: Depends. It’s not necessarily one or the other, you may want to use both.
Task.retry is used to retry tasks, notably for expected errors that is catchable with the try: block. The AMQP transaction
is not used for these errors: if the task raises an exception it is still acknowledged!.
The acks_late setting would be used when you need the task to be executed again if the worker (for some reason)
crashes mid-execution. It’s important to note that the worker is not known to crash, and if it does it is usually an
unrecoverable error that requires human intervention (bug in the worker, or task code).
In an ideal world you could safely retry any task that has failed, but this is rarely the case. Imagine the following task:
@task()
def process_upload(filename, tmpfile):
# Increment a file count stored in a database
increment_file_counter()
add_file_metadata_to_db(filename, tmpfile)
copy_file_to_destination(filename, tmpfile)
If this crashed in the middle of copying the file to its destination the world would contain incomplete state. This is not a
critical scenario of course, but you can probably imagine something far more sinister. So for ease of programming we
have less reliability; It’s a good default, users who require it and know what they are doing can still enable acks_late
(and in the future hopefully use manual acknowledgement)
In addition Task.retry has features not available in AMQP transactions: delay between retries, max retries, etc.
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Chapter 8. Frequently Asked Questions