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If they produce the same final state of the database. The simplest but least satisfactoryĭefinition involves comparing the effects of the schedules on the database. Several ways to define schedule equivalence. Question is: When are two schedules considered equivalent?
#Blind write in dbms serial
Serializable is equivalent to saying that it is cor-rect, because it isĮquivalent to a serial schedule, which is considered correct. More) of the serial schedules and hence are serializable, and those that are We can form twoĭisjoint groups of the nonserial schedules-those that are equivalent to one (or Transactions and many more possible nonserial schedules. Notice that there are n! possible serial schedules of n Is as follows: A schedule S of n transactions is serializable if it is equivalent to some serial schedule of the same n transactions. The concept used toĬharacterize schedules in this manner is that of serializability of a schedule. We would like to determine which of theĬorrect result and which may give erroneous results. However, some nonserial schedules give the correctĮxpected result, such as schedule D. The effect of T 1 on X is lost, overwritten by T 2, leading to the incorrect
#Blind write in dbms update
Update problem discussed in Section 3 transaction T 2 reads the value of X before it is changed by transaction T 1, so only the effect of T 2 on X is reflected in the database. Results X = 92 and Y = 93, in which the X value is erroneous, whereas schedule DĬ gives an erroneous result because of the lost Schedule C (which is the same as Figure 21.3(a)) gives the Schedules A or B gives the correct results. Sure enough, executing either of the serial We would expect the database values to be X After executing transactions T 1 and T 2, That the initial values of database items are X = 90 and Y = 90 and
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Illustrate our discussion, consider the schedules in Figure 21.5, and assume However, if we can determine which other schedules are equivalent to a serial sched-ule, we can allow these schedules to Hence, serial schedules are considered unacceptable in practice. Additionally, if someĬomplete all its operations before starting.
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Transaction, thus wasting valuable CPU processing time. I/O operation to complete, we cannot switch the CPU processor to another In a serial schedule, if a transaction waits for an Problem with serial schedules is that they limit concurrency by prohibiting To end in isolation from the operations of other transactions, we get a correct As long as every transaction is executed from beginning Hence, it does not matter which transaction is executed first. We can assume this because every transaction is assumed to be correct if One reasonable assumption we can make, if we consider the transactions to be independent, is that every serial schedule is considered correct. No interleaving occurs in a serial schedule. Therefore, in a serial schedule, only one transaction atĪ time is active-the commit (or abort) of the active transaction initiatesĮxecution of the next transaction. Schedule otherwise, the schedule is called nonserial.
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Schedules C and D in Figure 21.5(c) are called nonserial because each sequence interleaves operations from the twoĪ schedule S is serial if, for every transaction T participating in the schedule, all the operations of T are executed consecutively in the In a serial schedule, entire transactions are performed in serial order: T 1 and then T 2 in Figure 21.5(a), and T 2 and then T 1 in Figure 21.5(b). Serializability and discusses how it may be used in practice.Ī and B in Figure 21.5(a) and (b) are called serial because the operations of each transaction are executedĬonsecutively, without any interleaved operations from the other transaction. Interleaving of their operations in the schedules. Of schedules is used to identify which schedules are correct when transaction executions have Two possible schedules are shown inįigure 21.5(c). Individual operations of the transactions. If interleaving of operations isĪllowed, there will be many possible orders in which the system can execute the Shown in Figure 21.5(a) and (b), respectively. (in sequence) followed by all the operations of transaction T 1 (in sequence). (in sequence) followed by all the operations of transaction T 2 (in sequence). If no interleaving of operations is permitted, Users-for example, two airline reservations agents-submit to the DBMS transactions T 1Īpproxi-mately the same time. Such schedules are known as serializable schedules. Now we characterize the types of schedules that are alwaysĬoncurrent transactions are executing. Previous section, we characterized schedules based on their recoverability Characterizing Schedules Based on Serializability