[Zope] Re: Zope will not start after failed 2.8.1 upgrade

[Tim Peters]

[hpinson at indepthl.com]
> Well, fortunatly I had last nights backup of the Data.fs.  Restoring
> that allowed Zope to be reverted to 2.7.7 and restart.
>
> So here is my question.
>
> Normally I have been able to upgrade Zope, and if the upgrade had
> problems, revert to a prior version by simple redirecting the startup
> and python to the earlier version. Zope 2.8.1 was different. The
> upgrade, at least on the surface, appeared to alter something in the
> Data.fs, which prevented reversion to 2.7.7.
>
> I would be very interested in any insights into why this is.  Was I
> experience a Zope issue or a Python 2.3.5 issue?  The logs have not
> provided me an insight expect that the Mapping module was missing
> after the revert, which I don't think was an accurate assessment of
> the problem.

I doubt anyone can guess with certainty unless they're able to do
exactly what you did.

My uncertain guess is that a ZODB change is most likely.  ZODB 3.4
(which Zope 2.8 uses) is radically different from ZODB 3.2 (which Zope
2.7 uses) in several ways.

A relatively minor way that may be relevant here is that the
PersistentMapping type moved, from
ZODB.PersistentMapping.PersistentMapping (3.2) to
persistent.mapping.PersistentMapping (3.4).  The database root object
is of this type.  3.4 can load an old (3.2) Data.fs because it slams
3.2's 'ZODB.PersistentMapping' into sys.modules as an alias for 3.4's
persistent.mapping, but if the root object is written out again, it
will reference the correct-for.3.4 class path
(persistent.mapping.PersistentMapping), and no pre-3.4 version of ZODB
will be able to load that again (unless you slam your own aliases into
sys.modules, going in the other direction).

If so (and I don't _know_ -- I'm guessing here), you're suffering a
consequence of that some classes moved.  Because some classes did
move, it's intended that you be able to move from 2.7 to 2.8, but it's
not intended that you can move back again.

In any case, it's always very dangerous to change things without
ensuring you have a Data.fs backup first.  I'm glad to hear you never
got burned before, but you should view that as luck more than as an
entitlement <0.5 wink>.