Advanced Database Management System - Tutorials and Notes: Is the database schedule is recoverable?

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Thursday, 2 April 2020

Is the database schedule is recoverable?

Is the database schedule is recoverable? Recoverable schedule example problem, Practice problem based on recoverable schedules.


Recoverable schedule - solved exercise




Question:
Is the folloiwng schedule S is a recoverable schedule?
Sl.No.
T1
T2
1

W(A)
2
W(B)

3
W(A)

4

R(B)
5
Commit

6

Commit


A recoverable schedule:

A schedule is said to be a recoverable schedule if and only if each transaction commits only after all transactions from which it has read has committed.

  • Example 1: Let us consider a schedule S with three transactions T1, T2, and T3 which are working on two data items X and Y. Assume that, T1 has written a new value for X and T2 has written a new value for Y. If suppose, T3 consumes the values of X and Y that are produced by T1 and T2, then T3 must execute a commit only after T1 and T2 executed their commits. Now, the schedule is recoverable.

  • Example 2: Consider the schedule given in example 1. Assume that, T1 writes a new X first and T2 consumes and writes a new X. T3 works on data items X and Y and consumed X that was written by T2. In this case, T1 has to commit first, then T2, and finally T3. Now, the schedule is a recoverable schedule.

  • Example 3: Consider the schedule S given in example 1. If we assume that, T3 commits before either T1 or T2, then the schedule is not recoverable.


Solution:

 
In the given schedule, transaction T2 (refer serial number 4) reads the value of data item B that was written by transaction T1 (refer serial number 2). But, T1 (refer serial number 5) has committed before T2 (refer serial number 6). Hence, this schedule is recoverable schedule.

Is the schedule a serializable? Please work on this.


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Recoverable schedules in concurrent schedules
what is recoverable schedule
define recoverable schedules
recoverable schedules example
how to identify a recoverable schedule
check for recoverability in concurrent schedules



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