Hi all,
Based on some comments here and in the SqlAlchemy IRC chat room, I've
updated my decorator to make some changes. Here is the updated version:
class ScopedSession(object):
SESSIONMAKER = None # this is the single sessionmaker instance
def __init__(self, engine, auto_commit=True):
assert engine is not None, "Must pass a valid engine parameter"
self._auto_commit = auto_commit
if ScopedSession.SESSIONMAKER is None:
ScopedSession.SESSIONMAKER =
scoped_session(sessionmaker(expire_on_commit=True, bind=engine))
def __call__(self, func):
@functools.wraps(func)
def wrapper(*args, **kwargs):
db_session = ScopedSession.SESSIONMAKER()
try:
results = func(db_session, *args, **kwargs)
db_session.commit()
# should we rollback for safety?
if not self._auto_commit:
db_session.rollback()
except:
db_session.rollback()
raise
finally:
# release the session back to the connection pool
db_session.close()
return results
return wrapper
This version changes the code to a class based decorator so that I can
create and use a single sessionmaker. It also calls the close() method of
the session at the end of the decorator to release the session back to the
connection pool.
In response to using SqlAlchemy in a thread (in Twisted), I also write
Pylons applications which use SqlAlchemy as the database backend. Since
every request is a thread in Pylons, SqlAlchemy runs in those threads
without a problem, so I'm not sure I see the problem running it in Twisted.
The threads.deferToThread(...) call is nothing fancy in Twisted, it gets a
thread from a thread pool, runs the passed function in that thread, and
returns a deferred that fires in the main Twisted thread when the thread
ends. This code looks like this:
@defer.inlineCallbacks
def update(data):
# this function does some db work
@ScopedSession(engine=engine)
def process(session):
# this method will be run in a thread and is passed a session by
the decorator
results = session.query(<use data parameter since it's in scope>)
return results
# call the internal process function in a thread
try:
results = yield threads.deferToThread(process)
except Exception, e:
# do something with exceptions
defer.returnValue(results)
Using @defer.inlineCallbacks makes calling asynchronous code easier to
write in Twisted than using the traditional callback model (in my opinion).
Defining the process() method internally gives it access to the parameters
within the scope it's defined within and allows me to simplify it's
parameters down to the session received from the @ScopedSession decorator.
process() returns the SqlAlchemy results normally and is received into
results when the yield returns from the thread (deferred fires). In the
above example the session is commited and closed by the @ScopedSession
decorator because I didn't override the default auto_commite=True parameter.
Any comments, criticisms or suggestions are welcome, and again, thanks in
advance!
Doug
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