[jboss-as7-dev] JTA Synchronization.afterCompletion callback can occur in a background thread but JPA EntityManager must be accessed in single-threaded manner...

[Jason Greene]

On Feb 22, 2013, at 8:45 AM, Scott Marlow <smarlow at redhat.com> wrote:

> 
> One improvement that we talked to the JBossTM/TS team about, is adding a 
> TM policy that arranges for the Synchronization.afterCompletion to 
> always run in the application thread (some of the IRC discussion is 
> attached to HHH-7910).  The TM team doesn't think that they could do 
> that for both the TX timeout and tx propagating to a remote thread 
> (JBossTS) uses.
> 

This would be equivalent to trying to commit the transaction or trying to write to a database thats already been informed the TX was reverted. 

> One alternative solution, might be to create a top level container level 
> queue that the background thread Synchronization.afterCompletion defers 
> processing to.  As soon as the application thread returns control to the 
> top level, the queue is processed in FIFO order.

Well the problem is TX boundaries typically are the lifespan of the request, so the application thread would never fully give up control. The only thing you can really without relying on checks, or deferring to the db/tm, is to interrupt the application thread from the sync thread. However the EntityManager would have to be carefully coded to process thread interruption in a way that distinguishes between tx timeout and user requested interrupt. It's probably the same amount of code either way. 

--
Jason T. Greene
JBoss AS Lead / EAP Platform Architect
JBoss, a division of Red Hat