Question

Seems like a common issue but I don’t know a straight forward answer for it-

environment

• Java EE environemnt (several JVMs)
• 2 phase commit setup
• Oracle DB

given

DB table with ‘AMOUNT’ column

logic

if SELECT SUM(AMOUNT) WHERE... + current amount < Const. ==> insert row

problem

race condition – i would like to insert the row only when i do ‘commit’. meanwhile, i dont want no other thread to access the data i used for my summation. i would also like to reduce the non accessed db data as much as possible in order not to detain other threads.

current solution

use of Oracle’s SELECT FOR UPDATE mechanism

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Does anyone have other solutions for dealing with this problem? Out of the box solutions would be welcomed as well.

Answer 1

JPA as part of Java EE provides locking mechanisms.

In general there are pessimistic and optimistic locking.

Pessimistic locking in fact locks the table row when data is read (the resulting SQL is a SELECT FOR UPDATE ... as your current solution), e.g.

Person person = em.find(Person.class, personPK, LockModeType.PESSIMISTIC_WRITE);

Optimistic locking uses a version column for each table:

public class Employee {
@ID int id;
@Version int version;
...

The pessimistic approach seems most suitable for your