Transactions

Isolation Levels

• READ UNCOMMITTED: Lowest isolation, may see uncommitted data
• READ COMMITTED: See only committed data (default in most)
• REPEATABLE READ: Same query returns consistent results
• SERIALIZABLE: Highest isolation, full isolation
• Snapshot Isolation: MVCC-based isolation

Concurrency Problems

• Dirty reads
• Non-repeatable reads
• Phantom reads
• Lost updates
• Write skew

Locking

• Shared locks (S locks)
• Exclusive locks (X locks)
• Deadlock detection and handling
• Lock escalation
• Optimistic vs pessimistic locking

Distributed Transactions

• Two-phase commit (2PC)
• Three-phase commit (3PC)
• Saga pattern
• Eventual consistency
• Compensation actions
• Outbox pattern

Best Practices

• Keep transactions short
• Avoid long-running operations
• Use appropriate isolation levels
• Handle deadlocks gracefully
• Test concurrency scenarios
• Monitor transaction metrics