Play 1.2.3 framework - Right way to commit transaction - jpa

Question

We have a HTTP end-point that takes a long time to run and can also be called concurrently by users. As part of this request, we update the model inside a synchronized block so that other (possibly concurrent) requests pick up that change.
E.g.
MyModel m = null;
synchronized (lockObject) {
m = MyModel.findById(id);
if (m.status == PENDING) {
m.status = ACTIVE;
} else {
//render a response back to user that the operation is not allowed
}
m.save(); //Is not expected to be called unless we set m.status = ACTIVE
}
//Long running operation continues here. It can involve further changes to instance "m"
The reason for the synchronized block is to ensure that even concurrent requests get to pick up the latest status. However, the underlying JPA does not commit my changes (m.save()) until the request is complete. Since this is a long-running request, I do not want to wait until the request is complete and still want to ensure that other callers are notified of the change in status. I tried to call "m.em().flush(); JPA.em().getTransaction().commit();" after m.save(), but that makes the transaction unavailable for the subsequent action as part of the same request. Can I just given "JPA.em().getTransaction().begin();" and let Play handle the transaction from then on? If not, what is the best way to handle this use-case?
UPDATE:
Based on the response, I modified my code as follows:
MyModel m = null;
synchronized (lockObject) {
m = MyModel.findById(id);
if (m.status == PENDING) {
m.status = ACTIVE;
} else {
//render a response back to user that the operation is not allowed
}
m.save(); //Is not expected to be called unless we set m.status = ACTIVE
}
new MyModelUpdateJob(m.id).now();
And in my job, I have the following line:
doJob() {
MyModel m = MyModel.findById(id);
print m.status; //This still prints the old status as-if m.save() had no effect...
}
What am I missing?

Answer 1

Put your update code in a job an call
new MyModelUpdateJob(id).now().get();
thus the update will be done in another transaction that is commited at the end of the job

Answer 2

ouch, as soon as you add more play servers, you will be in trouble. You may want to play with optimistic locking in your example or and I advise against it pessimistic locking....ick.
HOWEVER, looking at your code, maybe read the article Building on Quicksand. I am not sure you need a synchronized block in that case at all...try to go after being idempotent.
In your case if
1. user 1 and user 2 both call that method and it is pending, then it goes to active(Idempotent)
If user 1 or user 2 wins, well that would be like you had the synchronization block anyways.
I am sure however you have a more complex scenario not shown here, BUT READ that article Building on Quicksand as it really changes the traditional way of thinking and is how google and amazon and very large scale systems operate.
Another option for distributed transactions across play servers is zookeeper which the big large nosql guys use BUT only as a last resort ;) ;)
later,
Dean