The Two-Phase Locking Protocol (Cont.) T T Two-phase locking is not a necessary condition for serializability lock-X(B) There are conflict serializable read(B) schedules that cannot be obtained B:=B-50 if the two-phase locking protocol is write(B) used. unlock(B) In the absence of extra information lock-S(4) (e.g.,ordering of access to data),two- read() phase locking is necessary for conflict unlock() serializability in the following sense: lock-S(B) Given a transaction Ti that does not follow two-phase locking,we read(B) can find a transaction T;that uses unlock(B) two-phase locking,and a schedule display(A +B) for Ti and Ti that is not conflict lock-X(4) serializable. read() A:=A+50 write() unlock() Database System Concepts-7th Edition 18.11 @Silberschatz,Korth and SudarshanDatabase System Concepts - 7 18.11 ©Silberschatz, Korth and Sudarshan th Edition The Two-Phase Locking Protocol (Cont.) ▪ Two-phase locking is not a necessary condition for serializability • There are conflict serializable schedules that cannot be obtained if the two-phase locking protocol is used. ▪ In the absence of extra information (e.g., ordering of access to data), two￾phase locking is necessary for conflict serializability in the following sense: • Given a transaction Ti that does not follow two-phase locking, we can find a transaction Tj that uses two-phase locking, and a schedule for Ti and Tj that is not conflict serializable