Question 21

Evaluating Statement I:

• The Strict Two-Phase Locking (Strict 2PL) protocol requires all exclusive (write) locks to be held until the transaction commits or aborts.
• Like basic 2PL, Strict 2PL ensures conflict serializability because the lock point ordering provides an equivalent serial schedule.
• In addition, because no uncommitted writes can be read or overwritten by other transactions, Strict 2PL generates schedules that are strict. Since strict schedules are a subset of cascadeless schedules, which are in turn a subset of recoverable schedules, every Strict 2PL schedule is guaranteed to be recoverable. Statement I is TRUE.

Evaluating Statement II:

• Basic Timestamp Ordering concurrency control generates conflict serializable schedules.
• Thomas' Write Rule modifies the write rule: if a write request arrives from a transaction with timestamp smaller than the read/write timestamp, the obsolete write is simply ignored without aborting the transaction.
• Ignoring obsolete writes allows schedules that are view serializable but not conflict serializable (due to blind writes). Statement II is TRUE.

Therefore, both statements I and II are TRUE.