10.2. Controlling concurrent access

Databases (and other transactional systems) attempt to ensure transaction isolation, meaning that, from the point of view of each concurrent transaction, it appears that no other transactions are in progress. Traditionally, this has been implemented with locking. A transaction may place a lock on a particular item of data in the database, temporarily preventing access to that item by other transactions. Some modern databases such as Oracle and PostgreSQL implement transaction isolation with multiversion concurrency control (MVCC) which is generally considered more scalable. We'll discuss isolation assuming a locking model; most of our observations are also applicable to multiversion concurrency, however.

How databases implement concurrency control is of the utmost importance in your Hibernate or Java Persistence application. Applications inherit the isolation guarantees provided by the database management system. For example, Hibernate never locks anything in memory. If you consider the many years of experience that database vendors have with implementing concurrency control, you'll see the advantage of this approach. On the other hand, some features in Hibernate and Java Persistence (either because you use them or by design) can improve the isolation guarantee beyond what is provided by the database.