Hi, [I posted this to zodb-dev, but it seems that list isn't working at the moment(?) so I thought I'd try here too] I have a package (plone.registry) that currently has a persistent structure like this: Registry(Persistent) | +--> Records(Persistent) | +--> BTree of Record(Persistent) | +--> PersistentField(Persistent) That is, a Registry is a persistent object containing a persistent Records object that in turn contains a BTree of persistent Record objects that contain a persistent PersistentField and a primitive value. This is quite inefficient, though, because it results in a lot of object loads. On any given request, some of our projects load a dozen or more values from the registry. Each is just a simple primitive, but we need to load the full shebang to make it work. Now, I'd like to move to this structure: Registry(Persistent) | +--> Records | +--> BTree of Field | +--> BTree of values Here, there's only one Persistent object, plus the two BTrees: one holding all the fields and one holding all the values. Records no longer needs to be persistent (its attributes become part of the parent Registry's _p_jar). Fields no longer need to be persistent either, since they are in effect immutable objects. Values are primitives anyway. I've done this (in a branch) and it works for new sites. However, I'm having a nightmare trying to migrate old sites. As soon as I access anything that uses the registry, I get ZODB errors, because the persistent structure is now different. In particular, it's trying to read a value into e.g. a Records object that used to derive from Persistent, but now no longer does. What is the best way to manage this type of migration? In terms of API compatibility, I'd really like to keep plone.registry.Record as the name and module path of the record, since it is used by the API. The difference is that before it was persisted and returned by an API on the Registry. Now, it's constructed as needed on the fly from the internal data structure. The same applies to the various field types that derive from PersistentField, which are now Persistent, but won't be. There's code and documentation out there that use these. I'm less worried about the Records object, which was always an implementation detail, and the BTree-of-records, which will never have been accessed directly. Cheers, Martin