[hibernate-dev] JTA synchronizations on WildFly

[Emmanuel Bernard]

So Gunnar, your workaround is fine today but will pose problems in the future if we decide that onRollback for ErrorHandler needs access to the session somehow to be able to compensate and undo the already executed operations:
- a closed session will do us no good
- there is a harder problem that Scott mentions below related to tx reaping in a different thread

Let’s try and find a solution for the first problem.

@Scott, I think what happens is the following:

- Hibernate registers a Sync when the EM is created (for the flush() callbacks and a few other things)
- Synchronization attached to the Hibernate Transaction objects are piggybacking on that initial Sync attachement
- Wildfly attach a “close session” Sync once it has the instance of EntityManager
- according to your rules, the close session afterTransaction Sync is called before the one Hibernate put
- that explains why Gunnar finds an already closed Tx.

But since Gunnar attaches its Sync when the Hibernate TransactionCoordinator creates the Hibernate transaction, which is when the EM is created, I can’t explain why the workaround works.

Emmanuel

> On 10 Mar 2015, at 14:39, Scott Marlow <smarlow at redhat.com> wrote:
> 
> Hi Gunnar,
> 
> Yes, this behaviour is expected since you registered an non-interposed 
> synchronization.  For what purpose are you registering the transaction 
> synchronization?  I would like to be aware of the synchronizations that 
> we register in WildFly.
> 
> The non-interposed sync beforeCompletion callback are invoked first, 
> then the interposed sync beforeCompletion calls, then the interposed 
> afterCompletion(int status) calls and finally, the non-interposed 
> afterCompletion(int status) calls.
> 
> The Synchronizations that are registered via the 
> javax.transaction.TransactionSynchronizationRegistry.registerInterposedSynchronization(Synchronization) 
> [2] are interposed.
> 
> Synchronizations that are registered via the 
> javax.transaction.Transaction.registerSynchronization(Synchronization) 
> [3] are non-interposed.
> 
> In WildFly, the transaction manager uses the registration order within 
> the interposed/non-interposed group.  Before completion syncs (within 
> their respective group), are run in registration order.  After 
> completion syncs (within their respective group), are run in reverse 
> registration order.
> 
> I believe that your workaround, mentioned below, of using 
> JtaPlatform#registerSynchronization() on WildFly, is registering your 
> synchronization as interposed via the TransactionSynchronizationRegistry 
> [2]. There might be a way to register a sync callback at the Hibernate 
> session level (which would also run as interposed sync on WildFly).
> 
> Not sure if you saw my email yesterday to Hibernate-dev ml.  You should 
> be aware that the afterCompletion(int status) callback, may be called 
> from a non-application thread when the WildFly tm reaper handles tx 
> timeout (this can happen while the application thread is still invoking 
> calls on the Hibernate session).  Because the Hibernate session is not 
> thread safe, we need to ensure that the Hibernate session 
> afterCompletion(int status) callback does not mutate the Hibernate 
> session (e.g. calling session.clear() what status == rolled back).
> 
> Scott
> 
> [2] 
> http://docs.oracle.com/javaee/5/api/javax/transaction/TransactionSynchronizationRegistry.html#registerInterposedSynchronization%28javax.transaction.Synchronization%29
> 
> [3] 
> http://docs.oracle.com/javaee/5/api/javax/transaction/Transaction.html#registerSynchronization%28javax.transaction.Synchronization%29
> 
> On 03/10/2015 09:03 AM, Gunnar Morling wrote:
>> Hi,
>> 
>> I'm trying to perform a specific action upon transaction rollback.
>> 
>> Assuming this could be done using a custom
>> javax.transaction.Synchronization, I tried to register a synchronization as
>> follows:
>> 
>>     TransactionImplementor transaction = ...; // e.g. a CMTTransaction
>>     transaction.registerSynchronization( new MySync() );
>> 
>> And indeed beforeCompletion() is invoked as expected. But afterCompletion()
>> never is. I debugged this a bit on WildFly and observed the following:
>> 
>> * Hibernate ORM registers RegisteredSynchronization with JTA.
>> RegisteredSynchronization manages (indirectly, through
>> TransactionCoordinator, SynchronizationRegistry etc.) those
>> synchronizations added through o.h.t.Transaction.registerSynchronization()
>> * WildFly (specifically, TransactionUtil [1]) registers its own
>> SessionSynchronization
>> for closing the entity manager/session
>> 
>> Now that second synchronization is called first, closing the session. Upon
>> SessionImpl#close(), the SynchronizationRegistry is cleared. Then when
>> afterComplection() is called on RegisteredSynchronization afterwards, any
>> previously registered delegate synchronizations are gone already and thus
>> do not get invoked.
>> 
>> I believe I found a workaround for my case by registering my
>> synchronization through JtaPlatform#registerSynchronization() (which
>> registers it with the actual JTA transaction).
>> 
>> My questions are:
>> 
>> * Is this behaviour expected?
>> * Is the work-around of doing the registration via JtaPlatform viable or
>> are there any drawbacks which I'm not aware of?
>> 
>> Thanks,
>> 
>> --Gunnar
>> 
>> [1]
>> https://github.com/wildfly/wildfly/blob/master/jpa/src/main/java/org/jboss/as/jpa/transaction/TransactionUtil.java
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