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this quesion is related to Database Management

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Consider the following four transactions . Transaction T1: read item(A); read item(D); write item(D); . Transaction T2: read item(B); write item(B); read item(D); write item(D) . Transaction T3: read item(C); write item(C): . Transaction T4: read item(D); write_item(D); read item(A); write_item(A); Following is a log corresponding to a particular schedule at the point of a system crash. [start_transaction, T1] read item, T1, A] [read item, T1, D] write item, T1, D, 20,25 commit, T1 [checkpoint start_transaction, T2] [read item, T2, B] write item, T2, B, 12, 18] start transaction, T4] [read item, T4, D write item, T4, D, 15,25] start transaction, T3 [write_item, T3, C, 30, 40] [read_item, T4, A] write_item, T4, A, 30, 20] commit, T4 read item, T2, D] write_item, T2, D, 15, 251 System Crash

Explanation / Answer

1. There are transctions which are not committed yet, therefore they cannot be recovered in recovery process, T2 and T3 has not been committed yet.

2.The transactions which are not committed yet will be rolled back. Again, T2 and T3 are roled back and hence they were not the part of recovery process.

3.In case of recovery after system crash, a transaction needs to be redone if and only if log has both <Ti start> and <Ti commit>. So, T1 and T4 will be redone.

4.All transactions are undone if there is no <Ti commit>, operations are undone in the backward order i.e.

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