Re: [dev] Better tpc_finish (was Re: [dev] RFC IMVCCStorage everywhere (Move ZODB's MVCC to storage layer))

Le 06/16/16 22:29, Jim Fulton a écrit :
>> You can find the monkey-patch at
>>   https://lab.nexedi.com/nexedi/neoppod/blob/v1.6.3/neo/client/__init__.py#L35
>>
>> https://lab.nexedi.com/nexedi/neoppod/blob/v1.6.3/ZODB.patch
>> contains other changes:
>> - update of IStorage.tpc_finish doc string
>> - update of existing unit tests
> 
> This is the 21st Century. You should start submitting pull requests. :)

Sure, when I'm actually decided to work on this. And I also realized that our patch was not ready.

> I'm with you.  I'm planning to make ZEO5 like NEO in this regard and
> there's even a new comment in ZODB that alludes to this.

:)

> Let's change ZODB5 to make this easier/saner for NEO and future ZEO.
> 
> Let's think of how we **want** this to work and work out a transition
> plan.  I'd like to do this in a way that doesn't break existing
> storages if practical.
> 
> Here's a straw man:
> 
> - store, undo, and tpc_vote are no longer required to return serials,
> except when they want to signal oids who's conflicts have been
> resolved.
> 
> - tpc_finish may optionally return a tid. If it does, then that tid
> should be applied to modified objects who's without resolved
> conflicts.
> 
> - If tpc_finish doesn't return a tid, then store, undo, and tpc_vote
> must return serials for all stored objects.

I don't understand what you mean, because that's already what our patch does:
- it does not touch store, undo, or tpc_vote
- tpc_finish is not required to return a tid

> - There are some special cases where undo or tpc_vote should return
> serials for without conflicts, because objects were modified via unto.
> The serials in this case may be None, as long as tpc_finish returns a
> tid.

About the expected failure in NEO when checking undo (previous mail), it was actually a misuse of ZODB API in the test. The undo was done directly at the storage level and obviously oids were automatically invalidated.

Anyway, since undo is not done via a connection, there's nothing to change here, and tpc_finish is not required to return a tid.

> Eventually, it will be expected for tpc_finish to return a tid.
> 
> Thoughts?

Except for the TODO I listed in a previous mail, the patch looks acceptable.
We can go further by adding deprecation warnings if we expected tpc_finish to return a tid.
The return type of tpc_vote and store can also be simplified to a simple list and a boolean respectively.

Julien

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