Björn Lindqvist Björn Lindqvist - 3 years ago 183
Python Question

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
counter.save()


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?

Answer Source

New in Django 1.1

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

or using an F expression:

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