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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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T15 read (B) B := B 50 write(B)
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read (A) read (A) display(A + B) A := A + 50 write(A) display (A + B) Figure 1613 Schedule 3
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a transaction is found to be getting restarted repeatedly, con icting transactions need to be temporarily blocked to enable the transaction to nish The protocol can generate schedules that are not recoverable However, it can be extended to make the schedules recoverable, in one of several ways: Recoverability and cascadelessness can be ensured by performing all writes together at the end of the transaction The writes must be atomic in the following sense: While the writes are in progress, no transaction is permitted to access any of the data items that have been written Recoverability and cascadelessness can also be guaranteed by using a limited form of locking, whereby reads of uncommitted items are postponed until the transaction that updated the item commits (see Exercise 1622) Recoverability alone can be ensured by tracking uncommitted writes, and allowing a transaction Ti to commit only after the commit of any transaction that wrote a value that Ti read Commit dependencies, outlined in Section 1615, can be used for this purpose
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1623 Thomas Write Rule
We now present a modi cation to the timestamp-ordering protocol that allows greater potential concurrency than does the protocol of Section 1622 Let us consider schedule 4 of Figure 1614, and apply the timestamp-ordering protocol Since T16 starts before T17 , we shall assume that TS(T16 ) < TS(T17 ) The read(Q) operation of T16 succeeds, as does the write(Q) operation of T17 When T16 attempts its write(Q) operation, we nd that TS(T16 ) < W-timestamp(Q), since W-timestamp(Q) = TS(T17 ) Thus, the write(Q) by T16 is rejected and transaction T16 must be rolled back Although the rollback of T16 is required by the timestamp-ordering protocol, it is unnecessary Since T17 has already written Q, the value that T16 is attempting to write is one that will never need to be read Any transaction Ti with TS(Ti ) < TS(T17 ) that attempts a read(Q) will be rolled back, since TS(Ti ) < W-timestamp(Q) Any
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
V Transaction Management
16 Concurrency Control
The McGraw Hill Companies, 2001
Validation-Based Protocols
T16 read(Q) write(Q) Figure 1614
T17 write(Q)
Schedule 4
transaction Tj with TS(Tj ) > TS(T17 ) must read the value of Q written by T17 , rather than the value written by T16 This observation leads to a modi ed version of the timestamp-ordering protocol in which obsolete write operations can be ignored under certain circumstances The protocol rules for read operations remain unchanged The protocol rules for write operations, however, are slightly different from the timestamp-ordering protocol of Section 1622 The modi cation to the timestamp-ordering protocol, called Thomas write rule, is this: Suppose that transaction Ti issues write(Q) 1 If TS(Ti ) < R-timestamp(Q), then the value of Q that Ti is producing was previously needed, and it had been assumed that the value would never be produced Hence, the system rejects the write operation and rolls Ti back 2 If TS(Ti ) < W-timestamp(Q), then Ti is attempting to write an obsolete value of Q Hence, this write operation can be ignored 3 Otherwise, the system executes the write operation and sets W-timestamp(Q) to TS(Ti ) The difference between these rules and those of Section 1622 lies in the second rule The timestamp-ordering protocol requires that Ti be rolled back if Ti issues write(Q) and TS(Ti ) < W-timestamp(Q) However, here, in those cases where TS(Ti ) R-timestamp(Q), we ignore the obsolete write Thomas write rule makes use of view serializability by, in effect, deleting obsolete write operations from the transactions that issue them This modi cation of transactions makes it possible to generate serializable schedules that would not be possible under the other protocols presented in this chapter For example, schedule 4 of Figure 1614 is not con ict serializable and, thus, is not possible under any of two-phase locking, the tree protocol, or the timestamp-ordering protocol Under Thomas write rule, the write(Q) operation of T16 would be ignored The result is a schedule that is view equivalent to the serial schedule <T16 , T17 >
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