Antoine Pitrou wrote: >>I'd be happy to explain how >>ZODB solves those problems, if you're interested. > > > Well, yes, I'm interested :) > (I don't anything about Zope internals though, and I've never even used > it)
Ok. Quoting your list: > To apply the same thing to Python you would at least need : > 1. a way to define a subset of the current bag of reachable objects > which has to stay consistent w.r.t. transactions that are applied > to it (of course, you would have several such subsets in any > non-trivial application) ZODB holds a tree of objects. When you add an attribute to an object managed by ZODB, you're expanding the tree. Consistency comes from several features: - Each thread has its own lazy copy of the object tree. - The application doesn't see changes to the object tree except at transaction boundaries. - The ZODB store keeps old revisions, and the new MVCC feature lets the application see the object system as it was at the beginning of the transaction. - If you make a change to the object tree that conflicts with a concurrent change, all changes to that copy of the object tree are aborted. > 2. a way to start and end a transaction on a bag of objects (begin / > commit / rollback) ZODB includes a transaction module that does just that. In fact, the module is so useful that I think it belongs in the standard library. > 3. a precise definition of the semantics of "consistency" here : for > example, only one thread could modify a bag of objects at any given > time, and other threads would continue to see the frozen, > stable version of that bag until the next version is committed by the > writing thread As mentioned above, the key is that ZODB maintains a copy of the objects per thread. A fair amount of RAM is lost that way, but the benefit in simplicity is tremendous. You also talked about the risk that applications would accidentally pull a lot of objects into the tree just by setting an attribute. That can and does happen, but the most common case is already solved by the pickle machinery: if you pickle something global like a class, the pickle stores the name and location of the class instead of the class itself. Shane _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
