The Two-Phase Locking Protocol(Cont) TWO-phase locking does not ensure freedom from deadlocks Cascading roll-back is possible under two-phase locking To avoid this follow a modified protocol called strict two- phase locking(严格两段锁). Here a transaction must hold all its exclusive locks till it commits/aborts Rigorous two- phase locking(强两段锁) is even stricter: here all locks are held till commit/abort. In this protocol transactions can be serialized in the order in which they commit 标 Database System Concepts 3rd Edition 16.9 OSilberschatz, Korth and SudarshanDatabase System Concepts 3 16.9 ©Silberschatz, Korth and Sudarshan rd Edition The Two-Phase Locking Protocol (Cont.) Two-phase locking does not ensure freedom from deadlocks Cascading roll-back is possible under two-phase locking. To avoid this, follow a modified protocol called strict two￾phase locking（严格两段锁）. Here a transaction must hold all its exclusive locks till it commits/aborts. Rigorous two-phase locking（强两段锁） is even stricter: here all locks are held till commit/abort. In this protocol transactions can be serialized in the order in which they commit