[ZODB-Dev] savepoint bug

[Jim Fulton]

Edward Moy wrote:
> So I retrieved ZODB/branches/tim-transactions and see "savepoint" in  
> the code, but (still as a ZODB beginner) can't figure out how to call  
> it.  Given a file-based persistent database, can you give me a few  
> lines on how to use the savepoints?

I don't think this code is close to working.  Tim and I got a start
but then had to work on other things.

Jim

> Thanks,
> ------------------------------------------------------------------------ --
> 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.
> 
> 
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