[ZODB-Dev] savepoint bug

[Edward Moy]

Glad to hear that you're working on it.  I'll try to make some time  
(probably next week, since I'm at a conference this week) to take a  
look.

------------------------------------------------------------------------ 
--
Edward Moy
Apple Computer, Inc.
emoy at apple.com

(This message is from me as a reader of this list, and not a statement
from Apple.)

On Jun 29, 2004, at 10:08 PM, Tim Peters wrote:

> [Edward Moy tries out his savepoint patch, and finds mysterious
>  behavior; Jeremy Hylton cogitates, and finds savepoint bugs by
>  raw brain power, but isn't sure they're the cause of Edward's
>  problems
> ]
>
> FYI:  savepoints didn't make it into 3.3b1, and since the feature set  
> was
> frozen with the first beta, won't be in 3.3 final either.
>
> Jim and I were looking for places to simplify ZODB code today, and  
> wondered
> whether implementing real savepoints might allow substantial reduction  
> in
> the distributed, confusing mass of current special-casing for the  
> feebler
> "subtransactions".  I think they would, but since we don't know for  
> sure yet
> we're doing exploratory work on a branch  
> (ZODB/branches/tim-transactions).
>
> Jim concocted problem scenarios that I think were akin to Edward's,  
> and also
> raised nasty questions about what happens if resource managers join a
> transaction at various times between savepoint creations.  We're  
> playing
> with an implementation on the branch that looks like it should solve  
> these
> problems.  A linked sequence of currently-active savepoints is  
> maintained,
> and rollback works by paying attention to all savepoints in the  
> sequence,
> from the most recent back to the target savepoint.  This means the work
> involved in a rollback will be proportional to the number of  
> still-active
> savepoints created after the rollback savepoint; in return, the  
> rollback
> machinery can know about every change made at and after the rollback  
> target.
>
> This is a novel approach, trying to do something obviously correct  
> instead
> of something so clever it's not obviously wrong <wink>.  BTW, note the
> change in terminology:  these are SavePoint objects now, not Rollback
> objects.  They have a rollback() method, though.
>
> I have high hopes that this will work out for ZODB 3.4, but no  
> promises at
> this time.