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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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T3 : lock-X(B); read(B); B := B 50; write(B); lock-X(A); read(A); A := A + 50; write(A); unlock(B); unlock(A) Figure 165 Transaction T3
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Suppose that the values of accounts A and B are $100 and $200, respectively If these two transactions are executed serially, either in the order T1 , T2 or the order T2 , T1 , then transaction T2 will display the value $300 If, however, these transactions are executed concurrently, then schedule 1, in Figure 164 is possible In this case, transaction T2 displays $250, which is incorrect The reason for this mistake is that the transaction T1 unlocked data item B too early, as a result of which T2 saw an inconsistent state The schedule shows the actions executed by the transactions, as well as the points at which the concurrency-control manager grants the locks The transaction making a lock request cannot execute its next action until the concurrency-control manager grants the lock Hence, the lock must be granted in the interval of time between the lock-request operation and the following action of the transaction Exactly when within this interval the lock is granted is not important; we can safely assume that the lock is granted just before the following action of the transaction We shall therefore drop the column depicting the actions of the concurrency-control manager from all schedules depicted in the rest of the chapter We let you infer when locks are granted Suppose now that unlocking is delayed to the end of the transaction Transaction T3 corresponds to T1 with unlocking delayed (Figure 165) Transaction T4 corresponds to T2 with unlocking delayed (Figure 166) You should verify that the sequence of reads and writes in schedule 1, which lead to an incorrect total of $250 being displayed, is no longer possible with T3 and T4
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T4 : lock-S(A); read(A); lock-S(B); read(B); display(A + B); unlock(A); unlock(B) Figure 166 Transaction T4
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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V Transaction Management
16 Concurrency Control
The McGraw Hill Companies, 2001
Lock-Based Protocols
T3 lock-X (B) read(B) B := B 50 write(B)
lock-S (A) read(A) lock-S (B) lock-X(A) Figure 167 Schedule 2
Other schedules are possible T4 will not print out an inconsistent result in any of them; we shall see why later Unfortunately, locking can lead to an undesirable situation Consider the partial schedule of Figure 167 for T3 and T4 Since T3 is holding an exclusive-mode lock on B and T4 is requesting a shared-mode lock on B, T4 is waiting for T3 to unlock B Similarly, since T4 is holding a shared-mode lock on A and T3 is requesting an exclusive-mode lock on A, T3 is waiting for T4 to unlock A Thus, we have arrived at a state where neither of these transactions can ever proceed with its normal execution This situation is called deadlock When deadlock occurs, the system must roll back one of the two transactions Once a transaction has been rolled back, the data items that were locked by that transaction are unlocked These data items are then available to the other transaction, which can continue with its execution We shall return to the issue of deadlock handling in Section 166 If we do not use locking, or if we unlock data items as soon as possible after reading or writing them, we may get inconsistent states On the other hand, if we do not unlock a data item before requesting a lock on another data item, deadlocks may occur There are ways to avoid deadlock in some situations, as we shall see in Section 1615 However, in general, deadlocks are a necessary evil associated with locking, if we want to avoid inconsistent states Deadlocks are de nitely preferable to inconsistent states, since they can be handled by rolling back of transactions, whereas inconsistent states may lead to real-world problems that cannot be handled by the database system We shall require that each transaction in the system follow a set of rules, called a locking protocol, indicating when a transaction may lock and unlock each of the data items Locking protocols restrict the number of possible schedules The set of all such schedules is a proper subset of all possible serializable schedules We shall present several locking protocols that allow only con ict-serializable schedules Before doing so, we need a few de nitions Let {T0 , T1 , , Tn } be a set of transactions participating in a schedule S We say that Ti precedes Tj in S, written Ti Tj , if there exists a data item Q such that Ti has held lock mode A on Q, and Tj has held lock mode B on Q later, and comp(A,B) = false If Ti Tj , then that precedence implies that in any equivalent serial schedule, Ti must appear before Tj Observe that this graph is similar to the precedence
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