Can you explain the principles and mechanisms underlying transaction isolation levels in database systems?

Transaction isolation levels in databases determine how much a transaction can see the changes made by other transactions, and help control concurrency issues:

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 committed data.
   • It prevents dirty reads but allows non-repeatable reads.

3. Repeatable Read

   • Ensures that data read at the beginning of a transaction does not change until the transaction ends.
   • It prevents non-repeatable reads but allows phantom reads.

4. Serializable

   • The strictest isolation level.
   • Transactions are executed as if they were sequential, one after another.
   • It prevents phantom reads but reduces concurrency and performance.

Implementation mechanisms:

• Locks: transactions lock rows or tables to prevent conflicts.
• MVCC: the database stores multiple versions of data, allowing transactions to see a consistent state without locking.

Choosing an isolation level is a trade-off between data integrity and system performance.