What is the role of each transaction isolation level in a database?

Transaction isolation levels in a database determine how and when the changes made by one transaction become visible to other transactions. The main isolation levels:

1. Read Uncommitted

   • A transaction can see changes made by other transactions even if they have not been committed.
   • Dirty reads are possible.

2. Read Committed

   • A transaction only sees changes that have been committed.
   • It prevents dirty reads but allows non-repeatable reads.

3. Repeatable Read

   • Ensures that data read during a transaction does not change until the transaction completes.
   • It prevents dirty and non-repeatable reads but allows phantom reads.

4. Serializable

   • The strictest isolation level.
   • Transactions are executed as if they were executed sequentially.
   • It prevents all types of read anomalies.

Choosing an isolation level affects the balance between performance and data integrity. For example, stricter levels of isolation reduce concurrency but provide greater consistency.