Serializability By Amit Gupta
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Data Base Management System
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
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Anomalies due to Interleaved Execution There are three main ways in which a schedule involving two consistency preserving, committed transactions could run against a consistent database and leave it in an inconsistent state. l Write – Read (WR) conflict l Read – Write (RW) conflict l Write – Write (WW) conflict
Ø Reading Uncommitted Data (WR Conflicts) Here, a transaction T2 could read a database object A that has been modified by another transaction T1, which has not yet committed. Such a read is called Dirty Read. Exammple:
T1
T2
R(A) W(A) R(A) W(A) R(B) W(B) Commit R(B) W(B) Commit
Ø
Unrepeatable Reads (RW Conflicts) Here, a transaction T2 could change the value of an object A that has been read by a transaction T1, while T1 is still in progress. If T1 tries to read the value of A again, it will get a different result, even though it has not modified A in the meantime. This situation could not arise in a serial execution of two transactions. This type of read is called an Unrepeatable Read.
Ø
Overwriting Uncommitted Data (WW Conflicts) Here, a transaction T2 could overwrite the value of an object A, which has already been modified by a transaction T1, while T1 is still in progress. Here, the problem is that we have a lost update. This type of write is called a blind write.
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
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Data Base Management System
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View Serializability Two schedules S1 and S2 over the same set of transactions, are view equivalent if 1. If Ti reads the initial value of object A in S1, it must also read the initial value of A in S2. 2. If Ti reads a value of A written by Tj in S1, it must also read the value of A written by Tj in S2. 3. For each data object A, the transaction that performs the final write on A in S1 must also perform the final write on A in S2. * A schedule is view serializable if it is view equivalent to some serial schedule. * Every conflict serializable schedule is view serializable; but the converse is not true. * Any view serializable schedule that is not conflict variable contains a blind write.
Recoverability: If a transaction Ti fails, for whatever reason, we need to undo the effect of this transaction to ensure the atomicity property of the transaction. In a system that allows concurrent execution, it is necessary also to ensure that any transaction Tj that is dependent on Ti (that is, Tj has read data written by Ti) is also aborted. To achieve this surety, we need to place restrictions on the type of schedules permitted in the system. A recoverable schedule is one where, 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 commit operation of Tj .
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
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Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
Page 1 of 8
1
Data Base Management System
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
Page 2 of 8
2
Data Base Management System
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
Page 3 of 8
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Data Base Management System
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
Page 4 of 8
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Data Base Management System
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
Page 5 of 8
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Data Base Management System
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
Page 6 of 8
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Data Base Management System
Page 7 of 8
Anomalies due to Interleaved Execution There are three main ways in which a schedule involving two consistency preserving, committed transactions could run against a consistent database and leave it in an inconsistent state. l Write – Read (WR) conflict l Read – Write (RW) conflict l Write – Write (WW) conflict
Ø Reading Uncommitted Data (WR Conflicts) Here, a transaction T2 could read a database object A that has been modified by another transaction T1, which has not yet committed. Such a read is called Dirty Read. Exammple:
T1
T2
R(A) W(A) R(A) W(A) R(B) W(B) Commit R(B) W(B) Commit
Ø
Unrepeatable Reads (RW Conflicts) Here, a transaction T2 could change the value of an object A that has been read by a transaction T1, while T1 is still in progress. If T1 tries to read the value of A again, it will get a different result, even though it has not modified A in the meantime. This situation could not arise in a serial execution of two transactions. This type of read is called an Unrepeatable Read.
Ø
Overwriting Uncommitted Data (WW Conflicts) Here, a transaction T2 could overwrite the value of an object A, which has already been modified by a transaction T1, while T1 is still in progress. Here, the problem is that we have a lost update. This type of write is called a blind write.
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
7
Data Base Management System
Page 8 of 8
View Serializability Two schedules S1 and S2 over the same set of transactions, are view equivalent if 1. If Ti reads the initial value of object A in S1, it must also read the initial value of A in S2. 2. If Ti reads a value of A written by Tj in S1, it must also read the value of A written by Tj in S2. 3. For each data object A, the transaction that performs the final write on A in S1 must also perform the final write on A in S2. * A schedule is view serializable if it is view equivalent to some serial schedule. * Every conflict serializable schedule is view serializable; but the converse is not true. * Any view serializable schedule that is not conflict variable contains a blind write.
Recoverability: If a transaction Ti fails, for whatever reason, we need to undo the effect of this transaction to ensure the atomicity property of the transaction. In a system that allows concurrent execution, it is necessary also to ensure that any transaction Tj that is dependent on Ti (that is, Tj has read data written by Ti) is also aborted. To achieve this surety, we need to place restrictions on the type of schedules permitted in the system. A recoverable schedule is one where, 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 commit operation of Tj .
Amit Kr.Gupta B.E. 6th (I.T.) BIT,Durg
8
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