Thank you, Simon, for clarifying this and pointing out that part of the SQLAlchemy docs... somehow I missed that part :-).
On Mar 26, 2010, at 7:30 AM, King Simon-NFHD78 wrote: > I think that point should be clarified, so that people don't later > come > across this post and just accept it without understanding. > > I imagine that SQLALchemy is used in a lot of threaded applications. > For > example, it is the recommended ORM in web frameworks such as Pylons > and > TurboGears, which work fine in threaded environments. However, > typically > in these libraries a web request is handled by a single thread, and > all > the SQLAlchemy operations occur within the scope of that request. As > long as you don't share a Session instance between the threads, you > won't have any problems. SQLAlchemy provides a ScopedSession class > which > helps in these situations, as you can call the constructor many > times on > a single thread and always get the session instance back for that > thread. Sessions themselves aren't thread-safe. > > When an instance is loaded from the database, it is linked to the > session that loaded it. This means that when you have lazy-loading > properties on that instance (such as related classes, or deferred > column > properties), they will be automatically loaded when they are accessed, > in the same session. > > This will cause a problem if you load an instance in thread A, hand > the > object off to thread B, and then thread B accesses one of these > lazy-loading properties. The load will occur in thread A's session, > which might be in the middle of doing something else. > > The solution to this is either to eager-load all the attributes you > think you are going to need before handing the instance off to another > thread (difficult), or (probably better) to detach (expunge) the > instance from thread A's session. Thread B should then merge the > object > into its own session (using the load=False flag so that it doesn't > needlessly requery the database). > > The Session docs at http://www.sqlalchemy.org/docs/session.html > explain > the lifecycle of loaded instances. > > I haven't actually done any of this - I've only ever used SA from TG > and > command-line scripts, but I think the principles are about right. I > hope > that helps, > > Simon _______________________________________________ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
