using @Transactional for thread safety

Question

I have a question about how @Transactional annotation alone manages code and transactions execution. Given a correctly setup Spring application and the following code:

@Transactional
public void withdraw(int amount) {
    if(isEnoughFunds(amount)) {
        decreaseFunds(amount);
    }
}

Is it possible for the following scenario to occur:

funds == 100; amount == 100
thread A enters withdraw / transaction A starts
thread A executes isEnoughFunds which evaluates to true
thread B enters withdraw / transaction B starts
thread B executes isEnoughFunds which evaluates to true
thread A executes decreaseFunds / thread A locks db record
thread B waits for thread A to commit transaction and release write lock
thread A exits withdraw / transaction A commits
thread B executes decreaseFunds / thread B locks db record
thread B exits withdraw / transaction B commits
funds == -100

If this is possible how would you prevent that?

the annotation just says to wrap this in a transaction. the database isolation level determines how much interleaving occurs. — Nathan Hughes, Sep 26 '12 at 15:08

score 4 · Answer 1 · answered Sep 26 '12 at 16:58

4

Yes, it is possible, depending on the isolation level. To prevent it, you can explicitly obtain a read lock from the database before calling ifEnoughFunds(). The lock will be released at the end of the transaction. In this scenario, thread B will always wait for thread's A transaction to commit before checking.

answered Sep 26 '12 at 16:58

Assen Kolov

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4

can you explain this further with a code example how to do this in spring and hibernate? thank you! – Lahiru Ruhunage Dec 08 '13 at 08:52

using @Transactional for thread safety

1 Answers1