Hi Jörg, On Mar 14, 2006, at 11:34 AM, Jörg von Frantzius wrote:
Craig L Russell schrieb:Hi Jörg, On Mar 14, 2006, at 10:42 AM, Jörg von Frantzius wrote:Hello Craig,do you or does anybody else see a problem in deferring verification and update of versions to the end of the transaction?In my experience, deferring verification and update of versions to the end of the transaction is best practice. Of course, if you use the flush method the implementation has no choice.Is flush() required to update version columns?
No, commit() will do it. But a flush() is required to report version exceptions.
Craig
CraigCompared to JPOX' current implementation, in my opinion this would decrease database deadlock probability a lot. At the moment, locking would be dispersed over time of the transaction, as verification happens with every update request issued. For implications on the number of statements, please see the said JPOX issue <http://www.jpox.org/servlet/jira/browse/CORE-2743>.Regards, Jörg Craig L Russell schrieb:Oops, On Mar 14, 2006, at 9:53 AM, Craig L Russell wrote:Hi Eric, On Mar 14, 2006, at 4:01 AM, Eric Samson wrote:This would typically involve a user configuration setting, either globally or on a per-class basis. It's been implemented in e.g. the SunOne application server CMP as policy "lock-when- loaded" on a persistent class.Hello JörgOne common portable solution to this is to acquire locks during the conflict detection step (using SELECT for UPDATE instead of simple SELECT).This is noise. Doesn't apply to optimistic scenarios. Eric's comments are spot on.CraigI don't follow this. The UPDATE statement only updates one row at a time so I don't know why this would be an issue. This is the recommended solution from me.Another approach could be to perform the conflict detection and the update at the same time (statement like “UPDATE WHERE pk=OID AND TS=ts”), but it raises some concerns for the conflict resolution (most JDBC drivers are not able to indicate which rows raised an exception).CraigBest Regards **...****.:**** Eric Samson, Founder & CTO, x****calia****** //Service your Data!//----------------------------------------------------------------- -------*De :* Jörg von Frantzius [mailto:[EMAIL PROTECTED]*Envoyé :* mardi 14 mars 2006 12:20 *À :* [email protected] <mailto:[email protected]> *Cc :* JDO Expert Group*Objet :* Optimistic locking - not 100% reliable without triggers?Hi,JPOX optimistic version verification entirely takes place within the VM, by reading a version column and comparing it with an expected value. When it verfies OK, JPOX proceeds and updates the version column with a new value. That means verification and update of version do not happen atomically, at least not on the database level, unless at least REPEATABLE_READ is used.Now if two threads or processes want to update the same row, and happen to verify the row's version at the same time, it is theoretically possible that they both decide to update it, i.e. none of them will receive a JDOOptimisticVerificationException. Using READ_UNCOMMITTED instead of READ_COMMITTED for verifying the version column will increase chances of detecting a conflict, but still a conflict can remain undetected.In Oracle's suggestion for implementing optimistic locking, the process will write the same optimistic version that it had previously read, and a trigger on the database will do the verification and increment the version if it had not been so yet. I guess that the trigger executes atomically, so conflicts will always be detected.Am I wrong here somewhere or do we really need triggers to have 100% reliability of conflict detection?Thanks for any hints, JörgCraig RussellArchitect, Sun Java Enterprise System http://java.sun.com/ products/jdo408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!Craig RussellArchitect, Sun Java Enterprise System http://java.sun.com/ products/jdo408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!Craig RussellArchitect, Sun Java Enterprise System http://java.sun.com/products/ jdo408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!
Craig Russell Architect, Sun Java Enterprise System http://java.sun.com/products/jdo 408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!
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