MapPT | include compass , legend , scale | Map library

Merge and persist don't need to immediately go to the database, so operations that depend on, triggers, sequencing and versions might need to call flush or commit to have those values set. Flush forces the context to synchronize with the database, and should set the values appropriately in managed objects. ID generation can be set on persist calls - this can happen when sequences allow for pre-allocation, but not usually when identity objects or triggers are used.

Since an EntityManager context represents a transaction, it is completely isolated from other contexts/transactions. Until the transaction commits, version and other changes cannot be picked up by other processes anyway, so it shouldn't matter when synchronization occurs to other processes. JPA states that most exceptions occur either on the persist/merge call or can be delayed until the context synchronizes with the database (flush/commit) depending on the nature of the operation. Optimistic locking is meant for systems where these collisions are infrequent and retries are less expensive than pessimistic locking every operation