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Here is a simple example of a django view with a potential race condition:
# myapp/views.py from django.contrib.auth.models import User from my_libs import calculate_points def add_points(request): user = request.user user.points += calculate_points(user) user.save() The race condition should be fairly obvious: A user can make this request twice, and the application could potentially execute user = request.user simultaneously, causing one of the requests to override the other.
Suppose the function calculate_points is relatively complicated, and makes calculations based on all kinds of weird stuff that cannot be placed in a single update and would be difficult to put in a stored procedure.
So here is my question: What kind of locking mechanisms are available to django, to deal with situations similar to this?
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Database race condition with tasks and Django request handlers. Data races happen when two or more concurrent threads try to access the same memory address (or in this case, some specific data in a database) at the same time.
A race condition is an undesirable situation that occurs when a device or system attempts to perform two or more operations at the same time, but because of the nature of the device or system, the operations must be done in the proper sequence to be done correctly.
A race condition occurs when two threads access a shared variable at the same time. The first thread reads the variable, and the second thread reads the same value from the variable.
NO, get_or_create is not atomic. It first asks the DB if a satisfying row exists; database returns, python checks results; if it doesn't exist, it creates it. In between the get and the create anything can happen - and a row corresponding to the get criteria be created by some other code.
Django 1.4+ supports select_for_update, in earlier versions you may execute raw SQL queries e.g. select ... for update which depending on underlying DB will lock the row from any updates, you can do whatever you want with that row until the end of transaction. e.g.
from django.db import transaction @transaction.commit_manually() def add_points(request): user = User.objects.select_for_update().get(id=request.user.id) # you can go back at this point if something is not right if user.points > 1000: # too many points return user.points += calculate_points(user) user.save() transaction.commit() As of Django 1.1 you can use the ORM's F() expressions to solve this specific problem.
from django.db.models import F user = request.user user.points = F('points') + calculate_points(user) user.save() For more details see the documentation:
https://docs.djangoproject.com/en/1.8/ref/models/instances/#updating-attributes-based-on-existing-fields
https://docs.djangoproject.com/en/1.8/ref/models/expressions/#django.db.models.F
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