Multiprogramming in Django, writing to the Database

Question

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?

Answer 1

You need to use synchronization on the save method. I haven't tried this yet, but here's a decorator that can be used to do so.

Answer 2

Based on what you've described, it seems reasonable to assume that multiple Apache processes are a source of problems. Are you able to replicate if you limit Apache to a single worker process?

Maybe the suggestions in this thread will help: http://stackoverflow.com/questions/1123200/how-to-lock-a-critical-section-in-django

An alternative approach could be utilizing a queue. You'd just stick your objects to be saved into the queue and have another process doing the actual save. That way you could guarantee that objects were processed sequentially. This wouldn't work well if your application depends on having the object saved by the time the response is returned unless you also had the request processes wait on the result (watching a finished queue for example).

Updated You may find this info useful. Mr. Dumpleton does a much better job of laying out the considerations than I could attempt to summarize here:

http://code.google.com/p/modwsgi/wiki/ProcessesAndThreading

http://code.google.com/p/modwsgi/wiki/ConfigurationGuidelines especially the Defining Process Groups section.

http://code.google.com/p/modwsgi/wiki/QuickConfigurationGuide Delegation to Daemon Process section

http://code.google.com/p/modwsgi/wiki/IntegrationWithDjango Find the section of text toward the bottom of the page that begins with:

Now, traditional wisdom in respect of Django has been that it should perferably only be used on single threaded servers. This would mean for Apache using the single threaded 'prefork' MPM on UNIX systems and avoiding the multithreaded 'worker' MPM.

and read until the end of the page.

Answer 3

I have found a solution that I think might work:

import threading

def save(self, *args, **kwargs):

    lock = threading.Lock()
    lock.acquire()
    try:
        # Test and Set Code
    finally:
        lock.release()

It doesn't seam to break the save method like that decorator and thus far I have not seen the error again.

Unless anyone can say that this is not a correct solution, I think this works.

Update

The accepted answer was the inspiration for this change.

I seams I was under the impressions that locks were some sort of special voodoo that were exempt by normal logic. Here the lock = threading.Lock() is run each time, thus instantiating a new unlocked lock which may always be acquired.

I needed a single central lock for the purpose, but were could that go unless I had a thread running all the time holding the lock? The answer seamed to be to use file locks explained in this answer to the StackOverflow question mentioned in the accepted answer.

The following is that solution modified to suit my situation:

The Code

Th following is my modified DjangoLock. I wished to keep locks relative to the Django root, to do this I put a custom variable into the settings.py file.

# locks.py

import os
import fcntl
from django.conf import settings

class DjangoLock:

    def __init__(self, filename):
        self.filename = os.path.join(settings.LOCK_DIR, filename)
        self.handle = open(self.filename, 'w')

    def acquire(self):
        fcntl.flock(self.handle, fcntl.LOCK_EX)

    def release(self):
        fcntl.flock(self.handle, fcntl.LOCK_UN)

    def __del__(self):
        self.handle.close()

And now the additional LOCK_DIR settings variable:

# settings.py

import os
PATH = os.path.abspath(os.path.dirname(__file__))

# ...

LOCK_DIR = os.path.join(PATH, 'locks')

That will now put locks in a folder named locks relative to the root of the Django project. Just make sure you give apache write access, in my case:

sudo chown www-data locks

And finally the usage is much the same as before:

import locks

def save(self, *args, **kwargs):

    lock = locks.DjangoLock('ClassName')
    lock.acquire()
    try:
        # Test and Set Code
    finally:
        lock.release()

This is now the implementation I am using and it seams to be working really well. Thanks to all who have contributed to the process of arriving at this end.