Attempting to retry transactional method fails with constraint violation

Question

I'm trying to create unique random tokens and save them to a database. Because there are multiple servers doing this at the same time, the approach that has been taken is to attempt to save a randomly generated token, but create a new token and try again if it doesn't work.

We've discovered that the retry functionality is not working. It appears to be the case that the transaction has not rolled back. Transactions are annotation-driven, and we're using Hibernate as the JPA provider.

Our code looks something like this:

@Autowired EntityManager em;

Token save(Token t, int attempts) {
  if(attempts > SOME_MAX) {
    throw new RuntimeException("Could not generate Token");
  }

  t.setId(MyRandomNumberGenerator.next());
  try {
    saveToDb(t);
  catch(PersistenceException e) {
      t = save(t.clone(), ++attempts);
  }
  return t;
}

@Transactional(propogation=Propagation.REQUIRES_NEW)
boolean saveToDb(Token t) {
    em.persist(t);
    em.flush();
}

There is a transaction already going on when save gets called, and I've tried all sorts of stuff in the catch block of saveToDb to try and make sure the originating offending token is removed from the current context, but no matter what I try, each call to saveToDb throws a constraint violation exception.

What was intended to happen is that when saveToDb fails, the new transaction created by it is rolled back so that the save method can try again.

Is there a way to make this approach work, or should I scrap it and try something else?

score 2 · Accepted Answer · edited May 23 '17 at 11:43

2

I believe that if you call a transnational method directly from the same class, there is a chance the annotation is not even considered !

From Spring page

In proxy mode (which is the default), only external method calls coming in through the proxy are intercepted. This means that self-invocation, in effect, a method within the target object calling another method of the target object, will not lead to an actual transaction at runtime even if the invoked method is marked with @Transactional.

Have a look here.

edited May 23 '17 at 11:43

Community

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answered Feb 25 '15 at 01:29

Mooolo

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That was it! Putting the two methods in separate classes made it work. Thank you very much! – Jeremy Feb 25 '15 at 14:55

score -1 · Answer 2 · answered Feb 25 '15 at 00:38

-1

I believe that after the exception you need new EM as after exception the state of EM is not known.

answered Feb 25 '15 at 00:38

Zielu

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em is autowired and springy, so that's not really an option. I'll update the example. – Jeremy Feb 25 '15 at 00:43
switch on logging to fine in your persistance and see what statements are invoked, I believe the 'previous' attempts for tokens will be tried to inserted again. Maybe try EM.remove after transaction error. – Zielu Feb 25 '15 at 00:46
sorry I am not a spring person, don't know its transaction semantics. – Zielu Feb 25 '15 at 00:47

Attempting to retry transactional method fails with constraint violation

2 Answers2