what is the actual meaning of rollback in a transaction ?

Question

we say that when any transaction is rolled-back that means it would be re-started again , so lets say if T2 reads a data item from T1 , perform some computation on it and then writes it back onto disk and after some T1 fails , so its a dirty read problem and we say that this schedule is not recoverable , so then if T1 is rolled back i.e. it is restarted again so then will T1 execute in a serial fashion from start till end or whether there would be still some concurrent execution between T1 and T2 .

 

After roll-back will the value of data item be set to its initial value or since T1 is restarted again it would be set to the value as computed by T1 again ?

Answer 1

rollback is just associated with one T or better to say corresponding transaction that needs rollback.

. others will continue as they were or wait or nythng,tas y we wish recoverability and cascadeless schedules else just for one transaction's lie others would be blamed just coz they trusted the one who rolled back from his words.

Answer 2

It is a signal means that the transaction has ended unsuccessfully, so that any changes or effects that the transaction may have applied to the database must be undone.