[ZODB-Dev] [Enhancement Proposal] Garanteed lifetime for volatile variables

[Jim Fulton]

Dieter Maurer wrote:
> Jim Fulton wrote at 2006-10-6 12:10 -0400:
>> ...
>> I'm a little uneasy about baking this policy so deeply into
>> the infrastructure.  I wonder if the use case can be handled
>> another way.
>>
>> A persistent object can override _p_deactivate.  For example:
>>
>>     def _p_deactivate(self):
>>         pass
>>
>> prevents an object from being turned into a ghost unless it is
>> invalidated.
> 
> I think that
> this will not work -- unless more drastical changes are done to
> "persistent":
> 
>    When we invalidate an object, it must get deactivated.
> 
>    Now, currently invalidating a cache entry calls
>    the object's "_p_invalidate" which calls
>    "Per_set_changed(obj, NULL)" which uses "_p_deactivate"
>    for the deactivation.
> 
>    With a definition like the above, the object will never
>    again be deactivated -- not even when it is invalidated.

That is clearly a bug that is easily fixed.

> 
>> You could implement your sticky attribute at the application level:
>>
>>
>>     def _p_deactivate(self):
>>         if getattr(self, '_p_sticky', False):
>>            return
>>         Persistent._p_deactivate(self)
>>
>> You could provide any policy you want, without making the policy part
>> of ZODB.
> 
> But, such an object would never again be deactivated (unless
> "_p_sticky" is reset). In my proposal, such objects may be
> invalidated at transaction boundaries.
> 
> This may not be a big problem, as there are probably not too many
> objects that have "_p_sticky" defined permanently
> ("Shared.DC.ZRDB.Connection" and
> "Products.CMFCore.Skinnable.ObjectManager" would be examples).

Good point.  I left off that you could register a callback on transaction
commit to clear the sticky flag.

Jim

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