Multiprogramming in Django, writing to the Database
- by Marcus Whybrow
Introduction
I have the following code which checks to see if a similar model exists in the database, and if it does not it creates the new model:
class BookProfile():
# ...
def save(self, *args, **kwargs):
uniqueConstraint = {'book_instance': self.book_instance, 'collection': self.collection}
# Test for other objects with identical values
profiles = BookProfile.objects.filter(Q(**uniqueConstraint) & ~Q(pk=self.pk))
# If none are found create the object, else fail.
if len(profiles) == 0:
super(BookProfile, self).save(*args, **kwargs)
else:
raise ValidationError('A Book Profile for that book instance in that collection already exists')
I first build my constraints, then search for a model with those values which I am enforcing must be unique Q(**uniqueConstraint). In addition I ensure that if the save method is updating and not inserting, that we do not find this object when looking for other similar objects ~Q(pk=self.pk).
I should mention that I ham implementing soft delete (with a modified objects manager which only shows non-deleted objects) which is why I must check for myself rather then relying on unique_together errors.
Problem
Right thats the introduction out of the way. My problem is that when multiple identical objects are saved in quick (or as near as simultaneous) succession, sometimes both get added even though the first being added should prevent the second.
I have tested the code in the shell and it succeeds every time I run it. Thus my assumption is if say we have two objects being added Object A and Object B. Object A runs its check upon save() being called. Then the process saving Object B gets some time on the processor. Object B runs that same test, but Object A has not yet been added so Object B is added to the database. Then Object A regains control of the processor, and has allready run its test, even though identical Object B is in the database, it adds it regardless.
My Thoughts
The reason I fear multiprogramming could be involved is that each Object A and Object is being added through an API save view, so a request to the view is made for each save, thus not a single request with multiple sequential saves on objects.
It might be the case that Apache is creating a process for each request, and thus causing the problems I think I am seeing. As you would expect, the problem only occurs sometimes, which is characteristic of multiprogramming or multiprocessing errors.
If this is the case, is there a way to make the test and set parts of the save() method a critical section, so that a process switch cannot happen between the test and the set?
