Hello Eric,
thanks a lot for your answer, that's exactly what I was looking for!
Regards,
Jörg
Eric Samson schrieb:
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).
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).
Best 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]
*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
