[ZODB-Dev] Re: [Zope-dev] Re: sessions in the presence of conflicts

[Jeremy Hylton]

On 12/15/05, Tim Peters <tim at zope.com> wrote:
> [Chris McDonough]
> > Note that changing the transientobject conflict resolution algorithm
> > won't get rid of all write conflict errors, because the BTree-based
> > indexes in the transient object container will still conflict during a
> > "bucket split" and other situations that I can't exactly recall
> > (they're documented in the BTrees source code).
>
> A more readable account is here:
>
>     http://www.zope.org/Wikis/ZODB/BTreeConflictResolution
>
> BTrees are mappings too, and looks like Dennis is trying to apply "similar"
> conflict-resolution rules to session mapping objects.
>
> > Conflict resolution algorithms are difficult and any algorithm will
> > have DWIM-y tradeoffs, so it's useful to keep it as simple as possible.
>
> Or no more complex as is actually helpful ;-)
>
> > ...
>
> [Dennis Allison]
> > I have yet to figure out how to map a TransientObject "state" back
> > to the object it represents, but it clearly is possible.
>
> I didn't see a response to that bit yet, so:  "the state" of an object P is
> whatever P.__getstate__() returns.  Given such a return value `state`, and
> some object Q of the same type as P, Q.__setstate__(state) gives Q the same
> state P had.  What state "means" is entirely up to the type's __setstate__()
> and __getstate__() implementations (if any).  Objects deriving from
> Persistent inherit (by default) implementations that retrieve and update an
> instance's __dict__.  BTrees.Length is a good example of a class that
> overrides these methods, using an integer as "the state".

It may also be worth mentioning that the state is for a single
persistent object.  If you have two classes, A and B, and both inherit
from Persistent and each A has a reference to a B, then the state of A
available for conflict resolution can not see the state of B.  In the
state that _p_resolveConflict() sees, you will see a reference to a
unique PersistentReference object.

Jeremy