DB transaction or Java DAO's method synchronization?

Question

I have Java-based web server, and I also have DAO singleton object with method, whose SQL operations\' logic must be synchronized in some way in order to guarantee data integri

Answer 1

Yes, using transactions is better. With synchronizing in your code, locking on the class, the scope of that lock is your classloader, and standing up a second instance of your application will invalidate your locking, because the two instances are using different locks.

With database transactions you can have multiple instances of your application and the database treats all the transactions the same.

Also with databases you have options like dialing down the isolation level to no higher than what you need for that transaction, or using row-level locking. Those are harder to implement in code and you're still stuck with not being able to deploy a second instance.

Answer 2

Depends deeply in what is what you want to synchronize, synchronization is about resources, if you have more than one database in your code, and the data integrity problem is distributed, you need a transaction context, not only declaring it but knowing how to manage it properly. Assuming you have a single database and assuming your problem is integrity caused by a possible inconsistency of a SELECT clause with a UPDATE or INSERT clause happening later in the method, The right solution would be a DB transaction and the use of a SELECT FOR UPDATE clause. If your problem is about UPDATE/INSERT of different tables in the same operation you may have two resources, one is including CONSTRAINTS, this is the preferred method, but in some cases is not possible. In the case that a CONTRAINT is not possible, consider a redesign of your DATAMODEL as managing this kind of problems synchronyzing app code is the worst solution, but even so is a solution.