scoped_session(sessionmaker()) or plain sessionmaker() in sqlalchemy?

Question

I am using SQlAlchemy in my web project. What should I use - scoped_session(sessionmaker()) or plain sessionmaker() - and why? Or should I use something else?

## model.py
from sqlalchemy import *
from sqlalchemy.orm import *

engine = create_engine('mysql://dbUser:dbPassword@dbServer:dbPort/dbName',
pool_recycle=3600, echo=False)
metadata = MetaData(engine)
Session = scoped_session(sessionmaker())
Session.configure(bind=engine)
user = Table('user', metadata, autoload=True)

class User(object):
pass

usermapper = mapper(User, user)

## some other python file called abc.py
from models import *

def getalluser():
   session = Session()  
   session.query(User).all()
   session.flush()
   session.close()

## onemore file defg.py
from models import *

def updateuser():
   session = Session()  
   session.query(User).filter(User.user_id == '4').update({User.user_lname: 'villkoo'})
   session.commit()
   session.flush()
   session.close()

I create a session = Session() object for each request and I close it. Am I doing the right thing or is there a better way to do it?

Answer 1

Reading the documentation is recommended:

the scoped_session() function is provided which produces a thread-managed registry of Session objects. It is commonly used in web applications so that a single global variable can be used to safely represent transactional sessions with sets of objects, localized to a single thread.

In short, use scoped_session() for thread safety.

Answer 2

Scoped_session at every method will give you a thread of local session which you cannot obtain beforehand (like at the module level).It's not needed to open a new session in every method, You can use a global session , Create a session only when the global session is not available. i.e you can write a method which returns a session and add it to the init.py inside your package.

Answer 3

FYI, when using flask-sqlalchemy, the session object provided is by default a scoped session object.

http://flask-sqlalchemy.pocoo.org/2.3/quickstart/#road-to-enlightenment

Answer 4

I am looking into this myself, but I am not an expert.

My three points are:

1. SQLAlchemy docs provide a proposed approach using scoped_session, per Mr. Kluev's comment above, at this link: http://docs.sqlalchemy.org/en/rel_0_9/orm/session.html#using-thread-local-scope-with-web-applications.
2. At that web location, the SQLAlchemy docs also say that it is "...strongly recommended that the integration tools provided with the web framework itself be used, if available, instead of scoped_session."
3. Flask-SQLAlchemy, for example, appears to claim that it takes care of this: http://pythonhosted.org/Flask-SQLAlchemy/quickstart.html#a-minimal-application

Answer 5

Don't use scoped_session and don't use Flask-SQLAlchemy.

Just use Session = sessionmaker() held in a singleton/service class, and use session = Session() on every HTTP request to guarantee that a fresh connection is provided.

Thread Local storage is clumsy and involves holding state which doesn't play nicely with different web-server threading models. Better to stay stateless. See for example SqlAlchemy's documentation here mentioning not to forget to call .remove() if you are using scoped_session. Will anyone remember to do that?

Below is an excerpt from https://docs.sqlalchemy.org/en/14/orm/contextual.html#using-thread-local-scope-with-web-applications:

Using the above flow, the process of integrating the Session with the web application has exactly two requirements:

• Create a single scoped_session registry when the web application first starts, ensuring that this object is accessible by the rest of the application.

• Ensure that scoped_session.remove() is called when the web request ends, usually by integrating with the web framework’s event system to establish an “on request end” event.