run code after transaction commit in Django

Question

Is there any way to run some code after transaction commit in Django?

I need to send some messages to a rabbitmq server for offline processing, but the message gets to the consumer before the Django transaction is commited.

My message is sent in the post_save signal of the model. What I'm looking for is a similar mechanism, using signals or something else, that would execute code after the commit (and do nothing if the transaction fails).

I haven't found any generic way of doing it in Django. Do you have any ideas?

Answer 1

UPDATE 2: django-transaction-hooks was merged into Django core and released in Django version 1.9.

UPDATE: django-transaction-hooks solves this problem.

I don't believe there is a clean way to do this; at least I can't think of one. You could monkeypatch django.db.transaction.commit to send a custom signal; not pretty but I think it would work.

Might also be interesting to raise this use-case on the django-developers mailing list. The devs are generally averse to adding new signals, but you might have a good case here (and a rebuttal from a core dev might include a useful suggestion of how to resolve your situation). You're more likely to get responses if you wait until after 1.1 comes out, though.

Answer 2

Hope this may help someone using Django 1.9 or later. Since 1.9 on_commit is available.

So basically you would be doing it like this:

from django.db import transaction

transaction.on_commit(
    lambda: send_msg_to_rabbitmqp(param1, param2, ...)
)

If you wish to keep post_save, you can still use on_commit:

@receiver(pre_save, sender=MyModel)
def my_handler(sender, instance, created, **kwargs):
    transaction.on_commit(
        lambda: send_msg_to_rabbitmqp(instance.id)
    )

Answer 3

I have implemented transaction signals (post_commit and post_rollback) by monkey patching django: http://gist.github.com/247844

Answer 4

One possibility would be to subclass the transaction middleware so that it sends a custom signal on commit. Your code could listen for that signal, rather than post_save.

Answer 5

Have a look at django-celery-transactions for a solution to this.

I've recently finished splitting-out and refactoring the underlying signals code code into a stand-alone app django-db-signals.