Session 20.pptx - Session 20 2017-18 KL University The...

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Session 20 © 2017-18 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED
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Agenda: Serializability © 2017-18 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED
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5 Characterizing Schedules based on Serializability (1) Serial schedule : A schedule S is serial if, for every transaction T participating in the schedule, all the operations of T are executed consecutively in the schedule. Otherwise, the schedule is called nonserial schedule. Serializable schedule : A schedule S is serializable if it is equivalent to some serial schedule of the same n transactions. © 2017-18 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED
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Characterizing Schedules based on Serializability (2) Result equivalent : Two schedules are called result equivalent if they produce the same final state of the database. Conflict equivalent : Two schedules are said to be conflict equivalent if the order of any two conflicting operations is the same in both schedules. Conflict serializable : A schedule S is said to be conflict serializable if it is conflict equivalent to some serial schedule S’. © 2017-18 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED
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Characterizing Schedules based on Serializability (3) Being serializable is not the same as being serial Being serializable implies that the schedule is a correct schedule. It will leave the database in a consistent state. The interleaving is appropriate and will result in a state as if the transactions were serially executed, yet will achieve efficiency due to concurrent execution. © 2017-18 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED
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Characterizing Schedules based on Serializability (4) Serializability is hard to check. Interleaving of operations occurs in an operating system through some scheduler Difficult to determine beforehand how the operations in a schedule will be interleaved. © 2017-18 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED
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Characterizing Schedules based on Serializability (5) Practical approach: Come up with methods (protocols) to ensure serializability. It’s not possible to determine when a schedule begins and when it ends. Hence, we reduce the problem of checking the whole schedule to checking only a committed project of the schedule (i.e. operations from only the committed transactions.) Current approach used in most DBMSs: Use of locks with two phase locking © 2017-18 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED
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