Hi there, Today I had a discussion with Jasper Spaans about how to go about improving megrok.rdb, Grok's relational database integration which aims to integrate Grok with SQLAlchemy. We started developing megrok.rdb at the Grokkerdam sprint a few weeks ago. We reread the discussion surrounding zope.sqlalchemy for ideas on how to go about integration and configuration. I think these discussions reach wider than just Grok's concerns. Note that I'm not proposing we fold any of these ideas into zope.sqlalchemy itself, which should remain as free of policy as possible; it could become (yet another) extension. Let me run our current thinking by the list. What would be nice in Zope applications (and we think would be good for Grok) would be per-instance database configuration. That is, we want to be able to install multiple instances of the same application and then configure each of them with a different database URN and it should all work, each talking to their own database. Michael Bayer's suggestion involves the use of scoped sessions. He proposed the following code: Session = scoped_session() # start of request engine = get_appropriate_engine() Session(bind=engine) try: # do request finally: Session.remove() Let's go through the steps. First it makes a scoped session object, it then configures it with the right engine at the start of the request (it can do this on a per-class level), and then at the end of the request it removes the Session again, which results in the actual session being closed. Our get_appropriate_engine() would probably look the engine up as a local utility, as Laurence suggested. There is a bit of question about engine configuration, though. If we want to support the use case of looking up the engine URL in a persistent datastore (for instance one URL per location), we have a question of ordering. We cannot do it too early; at the start of the transaction there isn't a ZODB yet to talk to so we can't look up a local utility. We can try doing it just in time: _Session = scoped_session() def Session(*args, **kw): engine = get_appropriate_engine() _Session.bind(bind=engine) return _Session(*args, **kw) Here get_appropriate_engine() could do a component.getUtility() and look up the engine for us, possibly in an application-local way. There's still the question of how this engine got configured in the first place. How does it know the database URL? How does the engine get created after the database URL is known (this might be quite late in the game; it could be stored in the ZODB). It then starts to look more and more attractive to do something similar like collective.lead's IDatabase utility, which can be stored persistently in the ZODB and has a getEngine() method which actually gets the engine (creating it if necessary). If we use sqlalchemy.ext.declarative, we also need to make the declarative extension of SQLALchemy load up the tables at the right point in time. We would also like a way to hook into matters and register some of our own tables and mappers manually. We figured perhaps the utility could fire an event that you can then write a handler for. This way there's less need to subclass the utility just to change some configuration (this is what collective.lead currently requires you to do). If a persistent local utility is in play, it shouldn't fire the configuration event during its own creation, as that would mean it'd only be fired once ever. We want to fire it just after engine creation. I guess the database utility can remain quite simple. Its main tasks would be: * allow access to the engine (creating it the first time) * fire the event for additional configuration when the engine is first created * maintain or somehow obtain the database URL. This could be retrieved from the ZODB if it's a local utility, or it could be hardcoded into a global utility, or it could be retrieved from some config file by a global utility. We could have an expanded variety which also configures things using the SQLAlchemy declarative extension. We still have the question of the 'remove()' bit in Michael's code. We looked at ScopedSession's remove() method, and it looks like it removes the session from the thread-local storage, and it actually closes the session. Closing the session should be taken care of: zope.sqlalchemy's integration with Zope's transaction machinery will close the session. What about the registry cleanup that remove() appears to do? Is this currently being done by zope.sqlalchemy? Should it be? Anyway, a whole lot of abstract talk. I still hope to get some feedback on this. Regards, Martijn