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Recoverability

Recoverable Schedules:

Schedules in which transactions commit only after all transactions whose changes they read commit are called recoverable schedules. In other words, if some transaction Tj is reading value updated or written by some other transaction Ti, then the commit of Tj must occur after the commit of Ti.

S1: R1(x), W1(x), R2(x), R1(y), R2(y), W2(x), W1(y), C1, C2;

Given schedule follows order of Ti->Tj => C1->C2.

Transaction C1 follows C2 so there is no chance of conflict error

Consider the following schedule involving two transactions T1 and T2.

T1 T2

R(A)

W(A)

W(A)

R(A)

commit

commit

This is a recoverable schedule since T1 commits before T2, which makes the value read by T2 correct.

Irrecoverable Schedule:

The table below shows a schedule with two transactions, T1 reads and writes A and that value is read and written by T2. T2 commits. But later on, T1 fails. So we have to rollback T1. Since T2 has read the value written by T1, it should also be rollbacked. But we have already committed that. So this schedule is irrecoverable. When Tj is reading the value updated by Ti and Tj is committed before committing to Ti, the schedule will be irrecoverable.