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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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verify that, with rigorous two-phase locking, transactions can be serialized in the order in which they commit Most database systems implement either strict or rigorous two-phase locking Consider the following two transactions, for which we have shown only some of the signi cant read and write operations: T8 : read(a1 ); read(a2 ); read(an ); write(a1 ) T9 : read(a1 ); read(a2 ); display(a1 + a2 ) If we employ the two-phase locking protocol, then T8 must lock a1 in exclusive mode Therefore, any concurrent execution of both transactions amounts to a serial execution Notice, however, that T8 needs an exclusive lock on a1 only at the end of its execution, when it writes a1 Thus, if T8 could initially lock a1 in shared mode, and then could later change the lock to exclusive mode, we could get more concurrency, since T8 and T9 could access a1 and a2 simultaneously This observation leads us to a re nement of the basic two-phase locking protocol, in which lock conversions are allowed We shall provide a mechanism for upgrading a shared lock to an exclusive lock, and downgrading an exclusive lock to a shared lock We denote conversion from shared to exclusive modes by upgrade, and from exclusive to shared by downgrade Lock conversion cannot be allowed arbitrarily Rather, upgrading can take place in only the growing phase, whereas downgrading can take place in only the shrinking phase Returning to our example, transactions T8 and T9 can run concurrently under the re ned two-phase locking protocol, as shown in the incomplete schedule of Figure 169, where only some of the locking instructions are shown T8 lock-S (a 1 ) lock-S (a 2 ) lock-S (a 2 ) lock-S (a 3 ) lock-S (a 4 ) unlock(a 1 ) unlock(a 2 ) lock-S ( an ) upgrade (a 1 ) Figure 169 Incomplete schedule with a lock conversion T9 lock-S (a 1 )
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Lock-Based Protocols
Note that a transaction attempting to upgrade a lock on an item Q may be forced to wait This enforced wait occurs if Q is currently locked by another transaction in shared mode Just like the basic two-phase locking protocol, two-phase locking with lock conversion generates only con ict-serializable schedules, and transactions can be serialized by their lock points Further, if exclusive locks are held until the end of the transaction, the schedules are cascadeless For a set of transactions, there may be con ict-serializable schedules that cannot be obtained through the two-phase locking protocol However, to obtain con ictserializable schedules through non-two-phase locking protocols, we need either to have additional information about the transactions or to impose some structure or ordering on the set of data items in the database In the absence of such information, two-phase locking is necessary for con ict serializability if Ti is a non-two-phase transaction, it is always possible to nd another transaction Tj that is two-phase so that there is a schedule possible for Ti and Tj that is not con ict serializable Strict two-phase locking and rigorous two-phase locking (with lock conversions) are used extensively in commercial database systems A simple but widely used scheme automatically generates the appropriate lock and unlock instructions for a transaction, on the basis of read and write requests from the transaction: When a transaction Ti issues a read(Q) operation, the system issues a lockS(Q) instruction followed by the read(Q) instruction When Ti issues a write(Q) operation, the system checks to see whether Ti already holds a shared lock on Q If it does, then the system issues an upgrade(Q) instruction, followed by the write(Q) instruction Otherwise, the system issues a lock-X(Q) instruction, followed by the write(Q) instruction All locks obtained by a transaction are unlocked after that transaction commits or aborts
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