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:

Hello Jörg

 

One common portable solution to this is to acquire locks during the conflict detection step (using SELECT for UPDATE instead of simple SELECT).

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.

This is noise. Doesn't apply to optimistic scenarios. Eric's comments are spot on.

Craig

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).

I 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.

Craig

 

Best Regards

....: Eric Samson, Founder & CTO, xcalia

Service your Data!


De : Jörg von Frantzius [mailto:joerg.von.frantzius@artnology.com]
Envoyé : mardi 14 mars 2006 12:20
À : jdo-dev@db.apache.org
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,
Jörg



Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
P.S. A good JDO? O, Gasp!


Craig Russell

Architect, Sun Java Enterprise System http://java.sun.com/products/jdo

408 276-5638 mailto:Craig.Russell@sun.com

P.S. A good JDO? O, Gasp!