Enterprise JavaBeans, Fourth Edition

Isolation and Database Locking

Transaction isolation (the “I” in ACID) is a critical part of any transactional system. This section explains isolation conditions, database locking, and transaction isolation levels. These concepts are important when deploying any transactional system.

Dirty, Repeatable, and Phantom Reads

Transaction isolation is defined in terms of isolation conditions called dirty reads, repeatable reads, and phantom reads. These conditions describe what can happen when two or more transactions operate on the same data.^[48] To illustrate these conditions, let’s think about two separate client applications using their own instances of the TravelAgent EJB to access the same data—specifically, a cabin record with the primary key of 99. These examples revolve around the RESERVATION table, which is accessed by both the bookPassage( ) method (through the Reservation EJB) and the listAvailableCabins( ) method (through JDBC). (It might be a good idea to go back to Chapter 11 and review how the RESERVATION table is accessed through these methods. This will help you to understand how two transactions executed by two different clients can impact each other.) Assume that both methods have a transaction attribute of Required.

Dirty reads

A dirty read occurs when a transaction reads uncommitted changes made by a previous transaction. If the first transaction is rolled back, the data read by the second transaction becomes invalid because the rollback undoes the changes. The ...