[ZODB-Dev] Undo differences between Z2 and Z3

[Jim Fulton]

David Pratt wrote:
> There seems to be enough of a difference between Z2 and Z3 undo to have 
> an effect on a backend like PGStorage from working on Z3. I believe the 
> objective in the storage api is that it should not matter to the backend 
> whether you are connected to Z2 or Z3. Can someone more familiar with 
> the history of undo in z2 and z3 explain the current situation?

Undo has a fairly complicated API, and I can easily believe that Z2 and
Z3 call that API in very different ways.

BTW, I strongly discourage use of Undo except in emergencies.
Sadly, except when undoing the last (non-undo) transactions in
a database can lead to inconsistency.  Undo should be used with
caution and should generally not be exposed to non-expert users.


> In Z3, before bringing up the contents view, there is a check of the 
> undo transactions for the principal. It uses z3's undo manager to 
> getPrincipalTransactions which eventually results in a lookup in the 
> undoLog in the external storage where it dies. Just a brief look at the 
> undo code in z3 gives me the feeling the format

Format of what?

 > between z2 and z3 may be
> incompatible which may be the problem.
> 
> The relevant portion of the traceback I receive is this:
> 
>   File "/usr/local/zope3/z3trunk/src/zope/tales/expressions.py", line 
> 199, in _eval
>     return ob()
>   File 
> "/usr/local/zope3/z3trunk/src/zope/app/publisher/browser/viewmeta.py", 
> line 419, in __call__
>     return meth(*a, **k)
>   File "/usr/local/zope3/z3trunk/src/zope/app/undo/browser.py", line 33, 
> in principalLastTransactionIsUndo
>     last=1)
>   File "/usr/local/zope3/z3trunk/src/zope/app/undo/__init__.py", line 
> 108, in getPrincipalTransactions
>     return self._getUndoInfo(context, principal, first, last)
>   File "/usr/local/zope3/z3trunk/src/zope/app/undo/__init__.py", line 
> 129, in _getUndoInfo
>     entries = self.__db.undoInfo(first, last, specification)
>   File "/usr/local/zope3/z3trunk/src/ZODB/UndoLogCompatible.py", line 
> 37, in undoInfo
>     return self.undoLog(first, last, filter)
>   File "/usr/local/zope3/z3trunk/src/ZODB/PGStorage/pgstorage.py", line 
> 478, in undoLog
>     d.update(cPickle.loads(ext))

You should report this to Shane.

It's possible that there is a bug in PGStorage in handling
extension data.  I can easily believe that Z2 doesn't use extension
data and perhaps z3 does.

Jim

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