[ZODB-Dev] Migration of obsolete BTrees to ZOPE 2.8

[Tim Peters]

[Dieter Maurer]
> It is no problem as long as you migrate while you are still using a Zope
> 2.7 (ZODB 3.2) release.

I believe (but don't know) that the OP has many distinct, physically
distributed installations of their product, involving many distinct
databases.  I don't know how many "many" is ;-), or what kind of upgrade
strategy would be acceptable, but I'm pretty sure they can't nurse each
upgrade along "by hand".

> We added automatic migration code to our old "BTree/Intset" using code.

Right, and I think that's what most other people did too.

> But we may need an explicit conversion before we finally upgrade to Zope
> 2.8 -- to get very rarely used objects upgraded as well.

...

> The migration is trivial as long as "BTree" and "Intset" are still
> available. We use for example the following "BTree" migration.
>
> # backward (pre Zope 2.8) compatibility
> try:
>   from BTree import BTree as _BTreeOld
> # DM 2005-08-25: for unknown reasons, this can raise an "AttributeError"
> #  rather than an "ImportError"
> #except ImportError: _BTreeOld = None
> except (ImportError, AttributeError):
>   _BTreeOld = None
>   from BTrees.OOBTree import OOBTree as BTree
>
> 	...
>         tree = self._tree
>         if _BTreeOld is not None and isinstance(tree, _BTreeOld):
>             # still an oldy -- upgrade to modern structure
>             self._tree = BTree(tree)
>             msg = 'BTree upgraded'
>         else: msg = 'BTree was already modern'

Margie, does this help you?  Can you do migration "live" along these lines,
or do you need on offline conversion script, or ...?