Hello Craig,
do you or does anybody else see a problem in deferring verification and
update of versions to the end of the transaction? Compared 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:
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, 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,
Jörg
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!
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!
