What is the role of different transaction isolation levels in managing concurrent database access?

Transaction isolation levels determine how and when the changes made by one transaction become visible to other transactions, affecting concurrent access to the database.

Main isolation levels:

• Read Uncommitted: A transaction can see changes made by other transactions even if they are not yet committed. This can lead to "dirty" reads.

• Read Committed: A transaction only sees data that has been committed by other transactions. It prevents dirty reads but allows non-repeatable reads.

• Repeatable Read: Ensures that data read at the beginning of a transaction will not change until the transaction completes. It prevents non-repeatable reads but allows phantom reads.

• Serializable: The strictest level, providing complete transaction isolation as if transactions were executed sequentially. It prevents all anomalies but reduces concurrency.

Choosing an isolation level is a balance between data integrity and performance. For example, critical operations often use Serializable, while less sensitive ones may use Read Committed.