Enterprise JavaBeans 3.1, 6th 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.^[21] When two users execute data access methods concurrently, different problems can surface or be avoided entirely, depending on the isolation level used by the database. Transaction isolation seeks to avoid these situations.

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 second transaction will not be aware that the data it has read has become invalid.

Repeatable reads

A repeatable read occurs when the data read is guaranteed to look the same if read again during the same transaction. Repeatable reads are guaranteed in one of two ways: either the data read is locked against changes, or it is a snapshot that doesn’t reflect changes. If the data is locked, it cannot be changed by any other transaction until the current transaction ends. If the data is a ...