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Pessimistic vs Optimistic locking

The pessimistic locking is not really a locking. It is easy to introduce optimistic locking in our code, all we need to do is add the @Version annotation to our entities. Pessimism means that we assume that there will be no conflict and everything will complete normally. But if they don't, then an OptimisticLockException is thrown and a transaction is marked for roll back. An alternative way to activate optimistic locking is annotating an entity with OptimisticLockType.ALL and activate dynamic update. This annotation tells hibernate to check the state of all entity fields before and after persisting it and detect any change. Instead we could use OptimisticLockType.DIRTY and an OptimisticLockException is thrown only if the same field has been changed before persisting by another unit of work. It depends what are the business demands to choose one or the other. Another option is to use OPTIMISTIC_FORCE_INCREASE which means that after a read, we force an increase of the entity version.

The pessimistic lock is actually locking a row of a table. In case we attempt to acquire a lock but a this row is already locked, an exception is thrown after a timeout. This approach can lead to deadlocks in cases where two units of work need to update the same data and each of them holds a lock on some of them.
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