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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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V Transaction Management
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16 Concurrency Control
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The McGraw Hill Companies, 2001
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graph that we used in Section 159 to test for con ict serializability Con icts between instructions correspond to noncompatibility of lock modes We say that a schedule S is legal under a given locking protocol if S is a possible schedule for a set of transactions that follow the rules of the locking protocol We say that a locking protocol ensures con ict serializability if and only if all legal schedules are con ict serializable; in other words, for all legal schedules the associated relation is acyclic
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1612 Granting of Locks
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When a transaction requests a lock on a data item in a particular mode, and no other transaction has a lock on the same data item in a con icting mode, the lock can be granted However, care must be taken to avoid the following scenario Suppose a transaction T2 has a shared-mode lock on a data item, and another transaction T1 requests an exclusive-mode lock on the data item Clearly, T1 has to wait for T2 to release the shared-mode lock Meanwhile, a transaction T3 may request a shared-mode lock on the same data item The lock request is compatible with the lock granted to T2 , so T3 may be granted the shared-mode lock At this point T2 may release the lock, but still T1 has to wait for T3 to nish But again, there may be a new transaction T4 that requests a shared-mode lock on the same data item, and is granted the lock before T3 releases it In fact, it is possible that there is a sequence of transactions that each requests a shared-mode lock on the data item, and each transaction releases the lock a short while after it is granted, but T1 never gets the exclusive-mode lock on the data item The transaction T1 may never make progress, and is said to be starved We can avoid starvation of transactions by granting locks in the following manner: When a transaction Ti requests a lock on a data item Q in a particular mode M , the concurrency-control manager grants the lock provided that 1 There is no other other transaction holding a lock on Q in a mode that con icts with M 2 There is no other transaction that is waiting for a lock on Q, and that made its lock request before Ti Thus, a lock request will never get blocked by a lock request that is made later
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1613 The Two-Phase Locking Protocol
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One protocol that ensures serializability is the two-phase locking protocol This protocol requires that each transaction issue lock and unlock requests in two phases: 1 Growing phase A transaction may obtain locks, but may not release any lock 2 Shrinking phase A transaction may release locks, but may not obtain any new locks
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
V Transaction Management
16 Concurrency Control
The McGraw Hill Companies, 2001
Lock-Based Protocols
Initially, a transaction is in the growing phase The transaction acquires locks as needed Once the transaction releases a lock, it enters the shrinking phase, and it can issue no more lock requests For example, transactions T3 and T4 are two phase On the other hand, transactions T1 and T2 are not two phase Note that the unlock instructions do not need to appear at the end of the transaction For example, in the case of transaction T3 , we could move the unlock(B) instruction to just after the lock-X(A) instruction, and still retain the two-phase locking property We can show that the two-phase locking protocol ensures con ict serializability Consider any transaction The point in the schedule where the transaction has obtained its nal lock (the end of its growing phase) is called the lock point of the transaction Now, transactions can be ordered according to their lock points this ordering is, in fact, a serializability ordering for the transactions We leave the proof as an exercise for you to do (see Exercise 161) Two-phase locking does not ensure freedom from deadlock Observe that transactions T3 and T4 are two phase, but, in schedule 2 (Figure 167), they are deadlocked Recall from Section 1562 that, in addition to being serializable, schedules should be cascadeless Cascading rollback may occur under two-phase locking As an illustration, consider the partial schedule of Figure 168 Each transaction observes the two-phase locking protocol, but the failure of T5 after the read(A) step of T7 leads to cascading rollback of T6 and T7 Cascading rollbacks can be avoided by a modi cation of two-phase locking called the strict two-phase locking protocol This protocol requires not only that locking be two phase, but also that all exclusive-mode locks taken by a transaction be held until that transaction commits This requirement ensures that any data written by an uncommitted transaction are locked in exclusive mode until the transaction commits, preventing any other transaction from reading the data Another variant of two-phase locking is the rigorous two-phase locking protocol, which requires that all locks be held until the transaction commits We can easily T5 lock-X(A) read(A) lock-S(B) read(B) write(A) unlock(A) T6 T7
lock-X(A) read(A) read write(A) unlock(A) lock-S (A) read(A) Figure 168 Partial schedule under two-phase locking
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