Django/PostgreSQL parallel concurrent incrementing field value with transaction causes OperationalError - python-3.x

Question

I have model related to some bunch. Each record in one bunch should have it's own unique version (in order of creation records in DB).
As for me, the best way to do this is using transactions, but I faced a problem in parallel execution of transaction blocks. When I remove transaction.atomic() block, everything works, but versions are not updated after execution.
I wrote some banch of code to test concurrent incremention version of record in database:
def _save_instance(instance):
time = random.randint(1, 50)
sleep(time/1000)
instance.text = str(time)
instance.save()
def _parallel():
instances = MyModel.objects.all()
# clear version
print('-- clear old numbers -- ')
instances.update(version=None)
processes = []
for instance in instances:
p = Process(target=_save_instance, args=(instance,))
processes.append(p)
print('-- launching -- ')
for p in processes:
p.start()
for p in processes:
p.join()
sleep(1)
...
# assertions to check if versions are correct in one bunch
print('parallel Ok!')
save() method in MyModel is defined like this:
...
def save(self, *args, **kwargs) -> None:
with transaction.atomic():
if not self.number and self.banch_id:
max_number = MyModel.objects.filter(
banch_id=self.banch_id
).aggregate(max_number=models.Max('version'))['max_number']
self.version = max_number + 1 if max_number else 1
super().save(*args, **kwargs)
When I run my test code on random amount of records (30-300), I get an error:
django.db.utils.OperationalError: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
After that every process stacks and I can stop script only with KeyboardInterrupt.
Full process stack trace:
Process Process-14:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 86, in _execute
return self.cursor.execute(sql, params)
psycopg2.OperationalError: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/multiprocessing/process.py", line 258, in _bootstrap
self.run()
File "/usr/local/lib/python3.6/multiprocessing/process.py", line 93, in run
self._target(*self._args, **self._kwargs)
File "/app/scripts/test_concurrent_saving.py", line 17, in _save_instance
instance.save()
File "/app/apps/incident/models.py", line 385, in save
).aggregate(max_number=models.Max('version'))['max_number']
File "/usr/local/lib/python3.6/site-packages/django/db/models/query.py", line 384, in aggregate
return query.get_aggregation(self.db, kwargs)
File "/usr/local/lib/python3.6/site-packages/django/db/models/sql/query.py", line 503, in get_aggregation
result = compiler.execute_sql(SINGLE)
File "/usr/local/lib/python3.6/site-packages/django/db/models/sql/compiler.py", line 1152, in execute_sql
cursor.execute(sql, params)
File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 100, in execute
return super().execute(sql, params)
File "/usr/local/lib/python3.6/site-packages/raven/contrib/django/client.py", line 123, in execute
return real_execute(self, sql, params)
File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 68, in execute
return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 77, in _execute_with_wrappers
return executor(sql, params, many, context)
File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 86, in _execute
return self.cursor.execute(sql, params)
File "/usr/local/lib/python3.6/site-packages/django/db/utils.py", line 90, in __exit__
raise dj_exc_value.with_traceback(traceback) from exc_value
File "/usr/local/lib/python3.6/site-packages/django/db/backends/utils.py", line 86, in _execute
return self.cursor.execute(sql, params)
django.db.utils.OperationalError: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
What is the reason of such behavior?
I will be gratefull for any help or advice!