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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