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Silberschatz,Korth and Sudarshan,Bo Zhou15.1Database System ConceptsTransaction ConceptnE.g.Transaction to transfer$50 from account A to account B:1.read(A)4.read(B)2.A:=A 505.B:=B+503.write(A)6.write(BnA transaction is a unit of program execution that accesses and possibly updates various data items.A transaction must see a consistent database.During transaction execution the database may be inconsistent.When the transaction is committed,the database must be consistent.nTwo main issues to deal with:Failures of various kinds,such as hardware failures and system crashesConcurrent execution of multiple transactionsSilberschatz,Korth and Sudarshan,Bo Zhou15.2Database System ConceptsACID PropertiesnAtomicity.Either all operations of the transaction are properly reflected in the database or none are.Commit a transactionRollback a transactionnConsistency.Execution of a transaction in isolation preserves the consistency of the database.nIsolation.Although multiple transactions may execute concurrently,each transaction must be unaware of other concurrently executing transactions.Intermediate transaction results must be hidden from other concurrently executed transactions.nDurability.After a transaction completes successfully,the changes it has made to the database persist,even if there are system failures.To preserve integrity of data,the database system must ensure:Silberschatz,Korth and Sudarshan,Bo Zhou15.3Database System ConceptsExample of Fund TransfernTransaction to transfer$50 from account A to account B:1.read(A)2.A:=A 503.write(A)4.read(B)5.B:=B+506.write(B)nConsistency requirement the sum of A and B is unchanged by the execution of the transaction.nAtomicity requirement if the transaction fails after step 3 and before step 6,the system should ensure that its updates are not reflected in the database,else an inconsistency will result.Failure could be due to software or hardwareSilberschatz,Korth and Sudarshan,Bo Zhou15.4Database System ConceptsExample of Fund Transfer(Cont.)nDurability requirement once the user has been notified that the transaction has completed(i.e.,the transfer of the$50 has taken place),the updates to the database by the transaction must persist despite failures.nIsolation requirement if between steps 3 and 6,another transaction is allowed to access the partially updated database,it will see an inconsistent database(the sum A+B will be less than it should be).Can be ensured trivially by running transactions serially,that is one after the other.However,executing multiple transactions concurrently has significant benefits,as we will see.Silberschatz,Korth and Sudarshan,Bo Zhou15.5Database System ConceptsConcurrent ExecutionsnMultiple transactions are allowed to run concurrently in the system.Advantages are:increased processor and disk utilization,leading to better transaction throughput:one transaction can be using the CPU while another is reading from or writing to the diskreduced average response time for transactions:short transactions need not wait behind long ones.nConcurrency control schemes mechanisms to achieve isolation,i.e.,to control the interaction among the concurrent transactions in order to prevent them from destroying the consistency of the databaseSilberschatz,Korth and Sudarshan,Bo Zhou15.6Database System ConceptsConsistency problems of concurrent Consistency problems of concurrent Execution without controlExecution without controlnLost modification:nDirty Read:T1T21Read(x)2Read(x)3X=x+14Write(x)X=x*25Write(x)T1T21Write(T)2Read(T)3rollbackSilberschatz,Korth and Sudarshan,Bo Zhou15.7Database System ConceptsConsistency problems(Cont.)nNon repeatable readT1T21Read(x)2Write(x)3Read(x)X?Silberschatz,Korth and Sudarshan,Bo Zhou15.8Database System ConceptsSchedulesnSchedules sequences that indicate the chronological order in which instructions of concurrent transactions are executeda schedule for a set of transactions must consist of all instructions of those transactionsmust preserve the order in which the instructions appear in each individual transaction.nSome notions:Serial ScheduleEquivalent scheduleSerializable ScheduleSilberschatz,Korth and Sudarshan,Bo Zhou15.9Database System ConceptsExample SchedulesnLet T1 transfer$50 from A to B,and T2 transfer 10%of the balance from A to B.The following is a serial schedule(Schedule 1 in the text),in which T1 is followed by T2.Silberschatz,Korth and Sudarshan,Bo Zhou15.10Database System ConceptsExample Schedule(Cont.)nLet T1 and T2 be the transactions defined previously.The following schedule(Schedule 3 in the text)is not a serial schedule,but it is equivalent to Schedule 1.In both Schedule 1 and 3,the sum A+B is preserved.Silberschatz,Korth and Sudarshan,Bo Zhou15.11Database System ConceptsExample Schedules(Cont.)nThe following concurrent schedule(Schedule 4 in the text)does not preserve the value of the the sum A+B.Silberschatz,Korth and Sudarshan,Bo Zhou15.12Database System ConceptsSerializabilitynBasic Assumption Each transaction preserves database consistency.We ignore operations other than read and write instructions,and we assume that transactions may perform arbitrary computations on data in local buffers in between reads and writes.Our simplified schedules consist of only read and write instructions.nThus serial execution of a set of transactions preserves database consistency.nA(possibly concurrent)schedule is serializable if it is equivalent to a serial schedule.nHow to determine a schedule is equivalent to a serial schedule?Different forms of schedule equivalence give rise to the notions of:1.conflict serializability2.view serializabilitySilberschatz,Korth and Sudarshan,Bo Zhou15.13Database System ConceptsConflict SerializabilitynInstructions li and lj of transactions Ti and Tj respectively,conflict if and only if there exists some item Q accessed by both li and lj,and at least one of these instructions wrote Q.1.li=read(Q),lj=read(Q).li and lj dont conflict.2.li=read(Q),lj=write(Q).They conflict.3.li=write(Q),lj=read(Q).They conflict4.li=write(Q),lj=write(Q).They conflictnIntuitively,a conflict between li and lj forces a(logical)temporal order between them.If li and lj are consecutive in a schedule and they do not conflict,their results would remain the same even if they had been interchanged in the schedule.Silberschatz,Korth and Sudarshan,Bo Zhou15.14Database System ConceptsConflict Serializability(Cont.)nIf a schedule S can be transformed into a schedule S by a series of swaps of non-conflicting instructions,we say that S and S are conflict equivalent.nWe say that a schedule S is conflict serializable if it is conflict equivalent to a serial schedulenExample of a schedule that is not conflict serializable:T3T4read(Q)write(Q)write(Q)We are unable to swap instructions in the above schedule to obtain either the serial schedule,or the serial schedule.Silberschatz,Korth and Sudarshan,Bo Zhou15.15Database System ConceptsConflict Serializability(Cont.)nSchedule 3 below can be transformed into Schedule 1,a serial schedule where T2 follows T1,by series of swaps of non-conflicting instructions.Therefore Schedule 3 is conflict serializable.Silberschatz,Korth and Sudarshan,Bo Zhou15.16Database System ConceptsView SerializabilitynLet S and S be two schedules with the same set of transactions.S and S are view equivalent if the following three conditions are met:1.For each data item Q,if transaction Ti reads the initial value of Q in schedule S,then transaction Ti must,in schedule S,also read the initial value of Q.2.For each data item Q if transaction Ti executes read(Q)in schedule S,and that value was produced by transaction Tj (if any),then transaction Ti must in schedule S also read the value of Q that was produced by transaction Tj.3.For each data item Q,the transaction(if any)that performs the final write(Q)operation in schedule S must perform the final write(Q)operation in schedule S.As can be seen,view equivalence is also based purely on readsand writes alone.Silberschatz,Korth and Sudarshan,Bo Zhou15.17Database System ConceptsView Serializability(Cont.)nA schedule S is view serializable it is view equivalent to a serial schedule.nEvery conflict serializable schedule is also view serializable.nSchedule 9(from text)a schedule which is view-serializable but not conflict serializable.nEvery view serializable schedule that is not conflict serializable has blind writes(write(Q)without having performed a read(Q)operation).Silberschatz,Korth and Sudarshan,Bo Zhou15.18Database System ConceptsOther Notions of SerializabilitynSchedule 8(from text)given below produces same outcome as the serial schedule,yet is not conflict equivalent or view equivalent to it.nDetermining such equivalence requires analysis of operations other than read and write.Silberschatz,Korth and Sudarshan,Bo Zhou15.19Database System ConceptsTesting for Conflict SerializabilitynConsider some schedule of a set of transactions T1,T2,.,TnnPrecedence graph a direct graph where the vertices are the transactions(names).nWe draw an arc from Ti to Tj if the two transaction conflict,and Ti accessed the data item on which the conflict arose earlier.nWe may label the arc by the item that was accessed.nExample 1xySilberschatz,Korth and Sudarshan,Bo Zhou15.20Database System ConceptsExample Schedule(Schedule A)T1 T2 T3 T4 T5read(X)read(Y)read(Z)read(V)read(W)read(W)read(Y)write(Y)write(Z)read(U)read(Y)write(Y)read(Z)write(Z)read(U)write(U)Silberschatz,Korth and Sudarshan,Bo Zhou15.21Database System ConceptsPrecedence Graph for Schedule AT3T4T1T2T5Silberschatz,Korth and Sudarshan,Bo Zhou15.22Database System ConceptsTest for Conflict SerializabilitynA schedule is conflict serializable if and only if its precedence graph is acyclic.nIf precedence graph is acyclic,the serializability order can be obtained by a topological sorting of the graph.This is a linear order consistent with the partial order of the graph.For example,a serializability order for Schedule A would beT5 T1 T3 T2 T4 orSilberschatz,Korth and Sudarshan,Bo Zhou15.23Database System ConceptsConcurrency Control vs.Serializability TestsConcurrency Control vs.Serializability TestsnTesting a schedule for serializability after it has executed is a little too late!nGoal to develop concurrency control protocols that will assure serializability.They will generally not examine the precedence graph as it is being created;instead a protocol will impose a discipline that avoids nonseralizable schedules.Will study such protocols in Chapter 16.nDifferent concurrency control protocols provide different tradeoffs between the amount of concurrency they allow and the amount of overhead that they incur.nTests for serializability help understand why a concurrency control protocol is correct.Silberschatz,Korth and Sudarshan,Bo Zhou15.24Database System ConceptsRecoverabilitynRecoverable schedule if a transaction Tj reads a data items previously written by a transaction Ti,the commit operation of Ti appears before the commit operation of Tj.nThe following schedule(Schedule 11)is not recoverable if T9 commits immediately after the readnIf T8 should abort,T9 would have read(and possibly shown to the user)an inconsistent database state.Hence database must ensure that schedules are recoverable.Need to address the effect of transaction failures on concurrently running transactions.Silberschatz,Korth and Sudarshan,Bo Zhou15.25Database System ConceptsRecoverability(Cont.)nCascading rollback a single transaction failure leads to a series of transaction rollbacks.Consider the following schedule where none of the transactions has yet committed(so the schedule is recoverable)If T10 fails,T11 and T12 must also be rolled back.nCan lead to the undoing of a significant amount of worknTheoretically,it is not mandatory to avoid cascading rollback.Silberschatz,Korth and Sudarshan,Bo Zhou15.26Database System ConceptsRecoverability(Cont.)nCascadeless schedules cascading rollbacks cannot occur;for each pair of transactions Ti and Tj such that Tj reads a data item previously written by Ti,the commit operation of Ti appears before the read operation of Tj.nEvery cascadeless schedule is recoverablenIt is desirable to restrict the schedules to those that are cascadelessSilberschatz,Korth and Sudarshan,Bo Zhou15.27Database System ConceptsTransaction Definition in SQLnData manipulation language must include a construct for specifying the set of actions that comprise a transaction.nIn SQL,a transaction begins implicitly.Some commercial DBMS support BEGIN TRANSACTION;Auto Commit propertynA transaction in SQL ends by:Commit work commits current transaction and begins a new one.Rollback work causes current transaction to abort.nLevels of consistency specified by SQL-92:Serializable defaultRepeatable readRead committedRead uncommittedSilberschatz,Korth and Sudarshan,Bo Zhou15.28Database System ConceptsLevels of Consistency in SQL-92nSerializable defaultnRepeatable read only committed records to be read,repeated reads of same record must return same value.However,a transaction may not be serializable it may find some records inserted by a transaction but not find others.The phantom PhenomenonnRead committed only committed records can be read,but successive reads of record may return different(but committed)values.nRead uncommitted even uncommitted records may be read.Lower degrees of consistency useful for gathering approximateinformation about the database,e.g.,statistics for query optimizer.Silberschatz,Korth and Sudarshan,Bo Zhou15.29Database System ConceptsHow to use different level of ConsistencyHow to use different level of ConsistencynDifferent level of consistency presents:Different level of correctness of data been retrieved.Different cost of query execution.nWhat the meaning of“data correctness”?nCan we have lower degree of transaction consistency level to improve the performance,and keep the database consistency?Need further understanding of the mechanism of concurrency control of DBMS.End of ChapterEND
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