serial_update - SERIALIZABILITY P.MIRUNALINI AP,CSE SSNCE...

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SERIALIZABILITY P.MIRUNALINI AP,CSE SSNCE
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OVERVIEW s Schedules s Conflict Operations s Characterizing schedules s Recoverable and non-recoverable schedules s Cascading rollback and cascadeless s Serializability of Schedules s Serial and non-serial schedules s Conflict serializability
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Schedules s A schedule ( or history) S of n transactions T1, T2, . ..., Tn is an ordering of the operations of the transactions subject to the constraint that: s For each transaction Ti that participates in S, the operation of Ti in S must appear in the same order in which they occur in Ti. s Sometimes, the operations from other transactions Tj can be interleaved with the operations of Ti in S.
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Schedules s Sa: r1(X); r2(X); w1(X); r1(Y); w2(X); c2; w1(Y); c1;
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Schedules s Two operations in a schedule are said to conflict: s if they belong to different transactions s if they access the same item X s if one of the two operations is a write_item(X)
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Characterizing Schedules s Recoverable Schedule : s Once a transaction T is committed, it should never be necessary to roll back T. The schedules that meet this criteria are called recoverable schedules. s A schedule S is said to be recoverable if no transaction T in S commits until all transactions T' that have written an item that T reads have committed.
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Recovery – Schedule s A schedule to be recoverable is as follows: s If A sees any of B's updates, then A must not commit before B terminates. s w2(t); r1(t); w1(t); c1; a2 unrecoverable schedule
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Characterizing Schedules Consider two schedules: Sc: r1 (X); w1 (X); r2 (X); r1(Y); w2 (X); c2; a1; Sd: r1 (X); w1 (X); r2(X); r1(Y); w2 (X); w1(Y); c1; c2; s a) Sd is recoverable s b) Sc is not recoverable, because T2 reads item X from T1, and then T2 commits before T1 commits. If T1 aborts after the C2; then the value of X that T2 read is no longer valid and T2 must be aborted after it had already committed ---> schedule not recoverable.
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Characterizing Schedules s Cascadeless Schedule : An uncommitted transaction has to be rolled back because it read an item from a transaction that failed.
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serial_update - SERIALIZABILITY P.MIRUNALINI AP,CSE SSNCE...

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