[ZODB-Dev] SQL/ZODB integration (was Re: ZODB and new styleclasses?)

[Jim Fulton]

Phillip J. Eby wrote:

...

>>last read are deactivated.
>>
> 
> Right, and not all back-ends provide any kind of notification that lets you
> do this.  I suppose you could reasonably consider this a deficiency of the
> back-ends rather than of the ZODB framework.  :)  But it's nonetheless a
> requirement for many people to use such back-ends.  :)

This is a good point. I tend to take the invalidation protocol for granted.

I hope this doesn't prove fatal.

OTOH, I don't see how you can do an effective caching implimentation without
notification (invalidation). I wonder how other databases handle this.
Oracle does a lot of caching, I wonder if they have a notification model. I bet they
do. Does anyone know if Postgress caches on the client?

> 
> 
>>Note that we plan a major performance improvement that will cause
>>
> transactions to read
> 
>>data committed as of the transaction start time even if newer data has
>>
> been committed.
> 
>>This will assure consistency and serializability without producing as many
>>
> conflicts.
> 
> Eeek!  I can see this for CMS applications, but some applications really
> *need* pessimistic conflict detection, i.e. locking.

That's fair enough. That raises another possibly fatal issue.

I read a book once (Atomic Transactions, by William Weil) that gave a mathematical
proof that time-stamp protocols and locking protocols can be mixed in the same TM.

OTOH, I think few databases use locking protocols any more. Locking is certainly
hard to mix with object persistence. OTOH, I suspect you can get a pessimistic
time-stamp protocol.

> One thing that's illustrated very well by this little interchange is the
> need for a clear taxonomy and common working terminology in the new SIG,
> right from the start.  :)

Yes.

Jim



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