DBMS Notes

Recoverability of Schedule

Sometimes a transaction may not execute completely due to some problems i.e. hardware failure, software issue, system crash etc. In that case, the failed transaction has to be rollback to its starting point of time. If rollback is done successfully then it will be recoverable schedule otherwise it will be a irrecoverable schedule.

Keep in Mind: Committed transaction can never be a rollback

1. Irrecoverable Schedule

When some values of failed transaction (say T1) is read by some other transaction (say T2) and T2 committed before T1 and T1 fail before to commit as given below. Then this case will be irrecoverable

 

Explanation

The above irrecoverable schedule shows two transactions (T1 and T2). T1 reads and writes the value of data “A”. T2 also read the same value of “A” which is written be T1. T2 commits but later on, T1 fails.

Due to the failure of T1, we have to rollback T1 and T2.

  • T1 is rollback due to its failure but T2 has to rollback because it read the data written by T1.

But T2 can’t be rollback because it is already committed. So this type of schedule is called irrecoverable schedule.

Note:  After rollback of T1 database should rollback to its original value 1500 but it is not possible because database updated with value 3000 by committing transaction T2. So it is a big problem in irrecoverable schedule.

2. Recoverable Schedule

The schedule will be recoverable if Tj (Say T2) reads the updated value of Ti (Say T1) and Ti committed before Tj commit.

Explanation 

The above table of recoverable schedule shows two transactions. T1 reads and writes the value of data “A”. T2 also read the same value of “A” which is written be T1. But later on, T1 fails. Due to this, we have to rollback T1. Transaction (T2) should also be rollback because T2 has read the same value of data “A” written by T1.

As, the transaction T2 is not committed before T1. So, we can rollback transaction T2 as well. So it is recoverable with cascade rollback.

Note: As nothing is committed by T1 and T2 so database value can be rollback to its original value 1500 successfully. It is called recoverable schedule.

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