Dear all,
I'm starting a web project based on Python, and I've decided to rely on web.py
and SQLAlchemy.
My DB is for the moment a SQLite database, and I'm trying to figure out how the
framework deals with sessions / transactions. Your help would be greatly
appreciated !
Here's how I declare my global Session :
engine = create_engine('sqlite:///.....', echo = True, listeners=[MyListener()])
Session = scoped_session(sessionmaker(bind=engine))
Q1)
The documentation recommends to use that kind of scoped_session for a
web project : why, actually ? What if I use "regular" sessions ? What
would happen in that case if different users of my site would get their
own sessions ?
I've made a simple test : I declare 2 actions, 'add' and 'commit'.
'add' deliberately doesn't commit the Session because I want to understand the
behavior of the framework.
class Add:
def GET(self, name=None):
print Session.connection()
print engine.pool.__dict__
for conn in engine.pool._all_conns:
print "[CONN] %s" %conn
if prenom is not None :
user = User(name)
Session.add(user)
# No commit here
class Commit:
def GET(self):
Session.commit()
I ran several tests : I triggered the 'Add' action from different browsers,
adding 'Jack' then 'John'....
Sometimes 'Jack' appears in Session.new, sometimes not.
Sometimes Session.new is empty, sometimes not.
It seems to depend on the underlying connection of the session.
Q2) Why's that ? Shouldn't the session be somehow "global" ? Why doesn't it
encapsulate all my users ?
Q3)
Moreover, what happens to the connections I open when I add a user without
committing ?
Why are they recycled by the framework on the next call ? My
listeners indicates they're never checked-in (except on commit time)
Q4) When I run the "commit" action, it seems to randomly pick a connection and
flush the objects. Why's that ?
Besides , I'm trying to restrict pool_size to 1 in the engine.
Q5) It does not seem to change anything : more and more connections
are open when I run "add" multiple times. How could I actually restrict
the pool size ?
Q6) What is the best strategy when using scoped_session in a web project ?
Forgetting a commit seems to have very dangerous consequences (delayed flush).
Thanks very much for your help !
Franck
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