Graph-Based Protocols (Cont.) The tree protocol ensures conflict serializability as well as freedom from deadlock. Unlocking may occur earlier in the tree-locking protocol than in the two- phase locking protocol. shorter waiting times,and increase in concurrency protocol is deadlock-free,no rollbacks are required Drawbacks Protocol does not guarantee recoverability or cascade freedom Need to introduce commit dependencies to ensure recoverability Transactions may have to lock data items that they do not access. increased locking overhead,and additional waiting time potential decrease in concurrency Schedules not possible under two-phase locking are possible under tree protocol,and vice versa. Database System Concepts-5th Edition,Oct 5,2006 16.18 @Silberschatz,Korth and SudarshanDatabase System Concepts - 5 16.18 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Graph-Based Protocols (Cont.) The tree protocol ensures conflict serializability as well as freedom from deadlock. Unlocking may occur earlier in the tree-locking protocol than in the two￾phase locking protocol. shorter waiting times, and increase in concurrency protocol is deadlock-free, no rollbacks are required Drawbacks Protocol does not guarantee recoverability or cascade freedom  Need to introduce commit dependencies to ensure recoverability Transactions may have to lock data items that they do not access.  increased locking overhead, and additional waiting time  potential decrease in concurrency Schedules not possible under two-phase locking are possible under tree protocol, and vice versa