Elm04_19 - Chapter 19: Database Recovery Techniques 1...

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Chapter 19: Database Recovery Techniques CHAPTER 19: DATABASE RECOVERY TECHNIQUES Answers to Selected Exercises 19.21 Suppose that the system crashes before the [read_item,T3,A] entry is written to the log in Figure 19.1(b); will that make any difference in the recovery process? Answer: There will be no difference in the recovery process, because read_item operations are needed only for determining if cascading rollback of additional transactions is necessary. 19.22 Suppose that the system crashes before the [write_item,T2,D,25,26] entry is written to the log in Figure 19.1(b); will that make any difference in the recovery process? Answer: Since both transactions T2 and T3 are not yet committed, they have to be rolled back during the recovery process. 19.23 Figure 19.7 shows the log corresponding to a particular schedule at the point of a system crash for the four transactions T1, T2, T3, and T4 of Figure 19.4. Suppose that we use the immediate update protocol with checkpointing. Describe the recovery process from the system crash. Specify which transactions are rolled back, which operations in the log are redone and which (if any) are undone, and whether any cascading rollback takes place. Answer: First, we note that this schedule is not recoverable, since transaction T4 has read the value of B written by T2, and then T4 committed before T2 committed. Similarly, transaction T4 has read the value of A written by T3, and then T4 committed before T3 committed. The [commit, T4] should not be allowed in the schedule if a recoverable protocol is used, but should be postponed till after T2 and T3 commit. For this problem, let us assume that we can roll back a committed transaction in a non-recoverable schedule, such as the one shown in Figure 21.7. By using the procedure RIU_M (recovery using immediate updates for a multiuser
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Elm04_19 - Chapter 19: Database Recovery Techniques 1...

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