SQLAlchemy session issues with celery - sqlalchemy

Question

I have scheduled a few recurring tasks with celery beat for our web app
The app itself is build using pyramid web framework. Using the zopetransaction extension to manage session
In celery, I am using the app as a library. I am redefining session in models with a function.
It works well but once in a while, it raises InvalidRequestError: This session is in 'prepared' state; no further SQL can be emitted within this transaction
I am not sure what is wrong and why it issues these warnings.
Sample code:
in tasks.py
def initialize_async_session():
import sqlalchemy
from webapp.models import Base, set_dbsession, engine
Session = sqlalchemy.orm.scoped_session(
sqlalchemy.orm.sessionmaker(autocommit=True, autoflush=True)
)
Session.configure(bind=engine)
session = Session()
set_dbsession(session)
Base.metadata.bind = engine
return session
#celery.task
def rerun_scheduler():
log.info("Starting pipeline scheduler")
session = initialize_async_session()
webapp.sheduledtask.service.check_for_updates(session)
log.info("Ending pipeline scheduler")
In models.py in webapp
DBSession = scoped_session(sessionmaker(bind=engine, expire_on_commit=False,
extension=ZopeTransactionExtension()))
def set_dbsession(db_session=None):
"""
This function sets the db session
"""
global DBSession
if db_session:
DBSession = db_session
log.info("session changed to {0}".format(db_session))
UPDATE:
traceback:
Traceback (most recent call last):
File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
self.run()
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/edgem_common-0.0-py2.7.egg/common/utils.py", line 54, in new_function
result = f(*args, **kwargs)
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/edgem_common-0.0-py2.7.egg/common/utils.py", line 100, in new_function
result = f(*args, **kwargs)
File "/home/ubuntu/modwsgi/env/mvc-service/webapp/webapp/data/mongo_service.py", line 1274, in run
self.table_params.set_task_status_as_finished()
File "/home/ubuntu/modwsgi/env/mvc-service/webapp/webapp/mem_objects.py", line 33, in set_task_status_as_finished
task = Task.get_by_id(self.task_id)
File "/home/ubuntu/modwsgi/env/mvc-service/webapp/webapp/models.py", line 162, in get_by_id
return DBSession.query(cls).filter(cls.id == obj_id).first()
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2156, in first
ret = list(self[0:1])
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2023, in __getitem__
return list(res)
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2227, in __iter__
return self._execute_and_instances(context)
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2240, in _execute_and_instances
close_with_result=True)
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2231, in _connection_from_session
**kw)
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 777, in connection
close_with_result=close_with_result)
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 781, in _connection_for_bind
return self.transaction._connection_for_bind(engine)
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 289, in _connection_for_bind
self._assert_is_active()
File "/home/ubuntu/modwsgi/env/local/lib/python2.7/site-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 217, in _assert_is_active
"This Session's transaction has been rolled back "
InvalidRequestError: This Session's transaction has been rolled back by a nested rollback() call. To begin a new transaction, issue Session.rollback() first.
#########################################################################
[2013-05-30 14:32:57,782: WARNING/PoolWorker-3] Exception in thread Thread-4:
Traceback (most recent call last):
File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
self.run()
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/edgem_common-0.0-py2.7.egg/common/utils.py", line 54, in new_function
result = f(*args, **kwargs)
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/edgem_common-0.0-py2.7.egg/common/utils.py", line 100, in new_function
result = f(*args, **kwargs)
File "/home/ranjith/wksp/mvc-service/webapp/webapp/data/mongo_service.py", line 1274, in run
self.table_params.set_task_status_as_finished()
File "/home/ranjith/wksp/mvc-service/webapp/webapp/mem_objects.py", line 33, in set_task_status_as_finished
task = Task.get_by_id(self.task_id)
File "/home/ranjith/wksp/mvc-service/webapp/webapp/models.py", line 166, in get_by_id
return DBSession.query(cls).filter(cls.id == obj_id).first()
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2145, in first
ret = list(self[0:1])
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2012, in __getitem__
return list(res)
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2216, in __iter__
return self._execute_and_instances(context)
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2229, in _execute_and_instances
close_with_result=True)
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/query.py", line 2220, in _connection_from_session
**kw)
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 798, in connection
close_with_result=close_with_result)
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 802, in _connection_for_bind
return self.transaction._connection_for_bind(engine)
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 281, in _connection_for_bind
self._assert_active()
File "/home/ranjith/wksp/env/local/lib/python2.7/site-packages/SQLAlchemy-0.8.1-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 181, in _assert_active
"This session is in 'prepared' state; no further "
InvalidRequestError: This session is in 'prepared' state; no further SQL can be emitted within this transaction.

Answer 1

I believe the problem is that you are attempting to use the SQLAlchemy session in your Celery task.
The first thing I recommend doing is creating two separate scoped sessions, one for your Celery application and another one for your web application. Next, I would make sure your Celery database session is only configured once during Celery initialization. You can use the Celery worker_init.connect to make sure it creates the database during Celery startup (http://hynek.me/articles/using-celery-with-pyramid/).
It is very important that your web application does not use the same database session as your Celery application.
Something like this for your tasks.py file:
from celery import Celery
from celery.signals import worker_init
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
Session = sqlalchemy.orm.scoped_session(
sqlalchemy.orm.sessionmaker(autocommit=True, autoflush=True))
#worker_init.connect
def initialize_session():
some_engine = create_engine('database_url')
Session.configure(bind=some_engine)
#celery.task
def rerun_scheduler():
log.info("Starting pipeline scheduler")
webapp.sheduledtask.service.check_for_updates(Session)
log.info("Ending pipeline scheduler")

Answer 2

Cross posting my answer to a very similar stack overflow:
What's the proper way to use SQLAlchemy Sessions with Celery?
This solved the issue for me:
Sqlalchemy pools connections by default in a non-threadsafe manner,
Celery forks processes by default: one or the other needs to be changed.
Turn off Sqlalchemy pooling
Sql Alchemy Docs
from sqlalchemy.pool import NullPool
engine = create_engine(
SQLALCHEMY_DATABASE_URL, poolclass=NullPool
)