Django multiprocessing and database connections

Multiprocessing copies connection objects between processes because it forks processes, and therefore copies all the file descriptors of the parent process. That being said, a connection to the SQL server is just a file, you can see it in linux under /proc//fd/.... any open file will be shared between forked processes. You can find more about forking here.

My solution was just simply close db connection just before launching processes, each process recreate connection itself when it will need one (tested in django 1.4):

from django import db
db.connections.close_all()
def db_worker():      
    some_paralell_code()
Process(target = db_worker,args = ())

Pgbouncer/pgpool is not connected with threads in a meaning of multiprocessing. It's rather solution for not closing connection on each request = speeding up connecting to postgres while under high load.

Update:

To completely remove problems with database connection simply move all logic connected with database to db_worker - I wanted to pass QueryDict as an argument... Better idea is simply pass list of ids... See QueryDict and values_list('id', flat=True), and do not forget to turn it to list! list(QueryDict) before passing to db_worker. Thanks to that we do not copy models database connection.

def db_worker(models_ids):        
    obj = PartModelWorkerClass(model_ids) # here You do Model.objects.filter(id__in = model_ids)
    obj.run()


model_ids = Model.objects.all().values_list('id', flat=True)
model_ids = list(model_ids) # cast to list
process_count = 5
delta = (len(model_ids) / process_count) + 1

# do all the db stuff here ...

# here you can close db connection
from django import db
db.connections.close_all()

for it in range(0:process_count):
    Process(target = db_worker,args = (model_ids[it*delta:(it+1)*delta]))   

For Python 3 and Django 1.9 this is what worked for me:

import multiprocessing
import django
django.setup() # Must call setup

def db_worker():
    for name, info in django.db.connections.databases.items(): # Close the DB connections
        django.db.connection.close()
    # Execute parallel code here

if __name__ == '__main__':
    multiprocessing.Process(target=db_worker)

Note that without the django.setup() I could not get this to work. I am guessing something needs to be initialized again for multiprocessing.

When using multiple databases, you should close all connections.

from django import db
for connection_name in db.connections.databases:
    db.connections[connection_name].close()

EDIT

Please use the same as @lechup mentionned to close all connections(not sure since which django version this method was added):

from django import db
db.connections.close_all()