According to the Doctrine 2 ORM Documentation on optimistic locking:
Database transactions are fine for concurrency control during a single request. However, a database transaction should not span across requests, the so-called "user think time". Therefore a long-running "business transaction" that spans multiple requests needs to involve several database transactions. Thus, database transactions alone can no longer control concurrency during such a long-running business transaction. Concurrency control becomes the partial responsibility of the application itself.
Doctrine has integrated support for automatic optimistic locking via a version field. In this approach any entity that should be protected against concurrent modifications ...
