Django race condition inside a transaction

Question

I have Django 1.11 app with PostgreSQL. Take a look at code below.

Is it possible to have race condition there? I am afraid I could get race condition on diff=-account.hours. Does transaction.atomic save from race condition?

from django.db import transaction def write_off(account_ids): accounts = Account.objects.filter(id__in=account_ids) for account in accounts: with transaction.atomic(): MyLog.objects.create( hours=0, operation_type=LOG_OPERATION_WRITE_OFF, diff=-account.hours, ) Account.objects.filter(pk=account.pk).update(hours=0)

From what it appears to me, your code should not work. account will be a QuerySet and not a single Account instance. Therefor it will not have .hours and .pk. — Klaus D.
– Klaus D., Commented Jan 22, 2018 at 14:40
And now explain where you see a possible race condition. Where is the race? — Klaus D.
– Klaus D., Commented Jan 22, 2018 at 14:48
@KlausD. in the line diff=-account.hours. I take account instance, then save account.hours to MyLog, and then set account.hours=0. Could the value of account.hours be changed after I create MyLog with outdated value? — v18o
– v18o, Commented Jan 22, 2018 at 14:54

Alasdair · Accepted Answer · 2018-01-22 15:02:09Z

The transaction.atomic() means that all your objects are created/saved in a single transaction, or none of them are. It does not prevent the accounts being modified by another request.

You could look at select_for_update:

def write_off(account_ids): with transaction.atomic(): accounts = Account.objects.select_for_update().filter(id__in=account_ids) for account in accounts: MyLog.objects.create( hours=0, operation_type=LOG_OPERATION_WRITE_OFF, diff=-account.hours, ) accounts.update(hours=0) # use fewer queries by updating all accounts at once

Updating all accounts at once is not a good idea, because if one update fails, all updates would be rolled back.
You can rearrange the code to suit your needs. Note that select_for_update must be inside an atomic block.

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