What are transaction isolation levels and what anomalies do they prevent?

Transaction isolation levels determine the degree to which changes made by concurrent transactions are isolated from each other in a relational database. Under heavy load, multiple users can access the same data simultaneously. Without clear isolation control, this can lead to a violation of information integrity and the appearance of various anomalies.

To combat these problems, the standard introduces four isolation levels. The weakest level is called Read Uncommitted; it allows all types of anomalies but provides maximum performance. The Read Committed level prevents dirty reads by ensuring that a transaction sees only committed changes.

The stricter Repeatable Read level excludes both dirty and non-repeatable reads, guaranteeing the stability of read data within a single transaction. The maximum isolation level, known as Serializable, completely eliminates all anomalies, including phantom reads, by modeling strictly sequential execution of all transactions, although the price for this is a severe reduction in parallelism.

Developers must consciously choose the isolation level for each task. Too weak a level can lead to critical errors in financial calculations, while an excessively strict level can cause locks and reduce the scalability of the web application.