Atomic increment of a counter in django

I'm trying to atomically increment a simple counter in Django. My code looks like this:

from models import Counter from django.db import transaction @transaction.commit_on_success def increment_counter(name): counter = Counter.objects.get_or_create(name = name)[0] counter.count += 1

If I understand Django correctly, this should wrap the function in a transaction and make the increment atomic. But it doesn't work and there is a race condition in the counter update. How can this code be made thread-safe?

-------------Problems Reply------------

New in Django 1.1

Counter.objects.get_or_create(name = name)
Counter.objects.filter(name = name).update(count = F('count')+1)


counter = Counter.objects.get_or_create(name = name)
counter.count = F('count') +1

In Django 1.4 there is support for SELECT ... FOR UPDATE clauses, using database locks to make sure no data is accesses concurrently by mistake.

Keeping it simple and building on @Oduvan's answer:

counter, created = Counter.objects.get_or_create(name = name,
if not created:
counter.count = F('count') +1

The advantage here is that if the object was created in the first statement, you don't have to do any further updates.

Django 1.7

from django.db.models import F

counter, created = Counter.objects.get_or_create(name = name)
counter.count = F('count') +1

Or if you just want a counter and not a persistent object you can use itertools counter which is implemented in C. The GIL will provide the safety needed.


Category:python Views:10 Time:2009-10-21

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