Can you list the existing levels of transaction isolation in database management systems?

In database management systems (DBMS), four main levels of transaction isolation are usually distinguished, which determine how and when changes made by one transaction become visible to others:

1. Read Uncommitted

   • A transaction can see changes made by other transactions even if they are not yet 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

   • Guarantees that if a transaction reads data, subsequent reads of the same data will yield the same result.
   • It prevents non-repeatable reads but allows phantom reads.

4. Serializable

   • The strictest level, where transactions are executed as if they were sequential.
   • It prevents phantom reads, ensures maximum integrity, but reduces concurrency.

Choosing an isolation level is a balance between performance and data integrity.