What are transaction isolation levels and how do they affect concurrent data access?

Transaction isolation levels determine how changes made by one transaction are visible to other concurrently running operations. In multi-user systems, databases face concurrency issues such as dirty reads, non-repeatable reads, and phantom reads. The SQL standard defines four main isolation levels that help developers find a balance between performance and data integrity.

The first level, Read Uncommitted, is the weakest and allows a transaction to read data that has not yet been committed by another parallel operation. If the first transaction rolls back, the second will read inconsistent or non-existent data. The second level, Read Committed, solves this problem by allowing only committed data to be read; however, non-repeatable reads are still possible here, where a repeated query within the same transaction returns rows modified by another process.

The third level, Repeatable Read, guarantees that data read once will remain unchanged throughout the transaction, preventing non-repeatable reads. Nevertheless, phantoms—new rows added by a concurrent transaction matching a query condition—can still appear at this level. The fourth and strictest level, Serializable, completely isolates transactions from each other by executing them effectively sequentially, which eliminates any anomalies but significantly reduces system throughput.

When choosing an isolation level, one should be guided by the business logic of the application. For most web applications, Read Committed or Repeatable Read is used by default. It is important to test the system under load, as excessively strict isolation levels can lead to a high number of deadlocks and slow down the database.