# Undo Logging Rules. Solution of the exercises 1

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1 Undo Logging Rules Undo 1: If transaction T modifies the database element X that held value old Write T, X, old to the log Only when the log record appears on disk can we write the new value for X to disk. Undo 2: If transaction T commits, then Write all pages with modified database elements to disk Then, write COMMIT T to the log and disk, as soon as possible. Solution of the exercises 1

2 Redo Logging Rules Redo 1: If transaction T modifies the database element X setting its value to new Write T, X, new to the log Redo 2: If transaction T commits, then Write COMMIT T to the log, and flush the log to the disk. Only then, write the new value for X to disk. Hence, all log entries must be written to disk, before modifying any database element on disk. Solution of the exercises 2

3 Undo/Redo Logging Rules Undo/Redo 1: If transaction T modifies database element X that held the value old to the value new Write T, X, old, new to the log Log records must be flushed to disk before corresponding modified pages are written to disk. When the transaction commits, write COMMIT T to the log and flush the log. Modified database pages can be flushed before or after commit. Solution of the exercises 3

4 Task: Consider the following log: START T T, A, 10, 11 T, B, 20, 21 COMMIT T Tell all sequences of events that are legal for an Undo-, Redo-, and Undo/Redo-based recovery system, where the events are: Output(A), Output(B), Flush-Log(A), Flush-Log(B), Commit Note that for convenience of presentation, we only use Undo/Redo log events in these slides! Solution of the exercises 4

5 Solution (Undo) The constraints are: Flush-Log(A) < Output(A) Flush-Log(B) < Output(B) Flush-Log(A) < Flush-Log(B) < Commit Output(A) < Commit Output(B) < Commit Hence, the valid sequences are: Flush-Log(A), Output(A), Flush-Log(B), Output(B), Commit Flush-Log(A), Flush-Log(B), Output(A), Output(B), Commit Flush-Log(A), Flush-Log(B), Output(B), Output(A), Commit Solution of the exercises 5

6 Solution (Redo) The constraints are: Flush-Log(A) < Output(A) Flush-Log(B) < Output(B) Flush-Log(A) < Flush-Log(B) < Commit Commit < Output(A) Commit < Output(B) Hence, the valid sequences are: Flush-Log(A), Flush-Log(B), Commit, Output(A), Output(B) Flush-Log(A), Flush-Log(B), Commit, Output(B), Output(A) Solution of the exercises 6

7 Solution (Undo/Redo) The constraints are: Flush-Log(A) < Output(A) Flush-Log(B) < Output(B) Flush-Log(A) < Flush-Log(B) < Commit Hence, the valid sequences are: Flush-Log(A), Output(A), Flush-Log(B), Output(B), Commit Flush-Log(A), Flush-Log(B), Output(A), Output(B), Commit Flush-Log(A), Flush-Log(B), Output(B), Output(A), Commit Flush-Log(A), Flush-Log(B), Commit, Output(A), Output(B) Flush-Log(A), Flush-Log(B), Commit, Output(B), Output(A) Flush-Log(A), Output(A), Flush-Log(B), Commit, Output(B) Flush-Log(A), Flush-Log(B), Output(A), Commit, Output(B) Flush-Log(A), Flush-Log(B), Output(B), Commit, Output(A) Solution of the exercises 7

8 Task: Consider the following log, after a crash: START T T, A, 10, 11 START U What values might/must have been changed? How does the recovery manager get the database back to a consistent state? Discuss for Undo-, Redo-, and Undo/Redo-logging. Solution of the exercises 8

9 Solution (Undo) START T T, A, 10, 11 START U We first identify the transactions that we need to undo. They are T, and U. By reading the log we can conclude that: A might have had its value changed on disk. Solution of the exercises 9

10 Solution (Undo) START T T, A, 10, 11 START U Starting from the end of the log, we undo as follows: Append ABRT U to the log. Write value 10 for A. Append ABRT T to the log. Solution of the exercises 10

11 Solution (Redo) START T T, A, 10, 11 START U We first identify the transactions that we need to redo. No transaction has committed, so we do not need to redo any transaction. By reading the log we can conclude that: A cannot have had its value changed on disk. Solution of the exercises 11

12 Solution (Redo) START T T, A, 10, 11 START U We do not need to redo any transaction. Append ABRT U to the log. Append ABRT T to the log. Solution of the exercises 12

13 Solution (Undo/Redo) START T T, A, 10, 11 START U We first identify the transactions that we need to undo or redo. No transaction has committed, so we do not need to redo any transaction. We need to undo transactions T and U. By reading the log we can conclude that: A might have had its value changed on disk. We recover from the crash as with Undo. Solution of the exercises 13

14 Solution (Undo/Redo) START T T, A, 10, 11 START U We do not need to redo any transaction. Starting from the end of the log, we undo as follows: Append ABRT U to the log. Write value 10 for A. Append ABRT T to the log. Solution of the exercises 14

15 Task: Consider the following log, after a crash: START T T, A, 10, 11 START U U, B, 20, 21 T, C, 30, 31 U, D, 40, 41 COMMIT U What values might/must have been changed? How does the recovery manager get the database back to a consistent state? Discuss for Undo-, Redo-, and Undo/Redo-logging. Solution of the exercises 15

16 Solution (Undo) START T T, A, 10, 11 START U U, B, 20, 21 T, C, 30, 31 U, D, 40, 41 COMMIT U We first identify the transactions that we need to undo. Only transaction T must be undone. By reading the log we can conclude that: A might have had its value changed on disk. B must have had its value changed on disk. C might have had its value changed on disk. D must have had its value changed on disk. Solution of the exercises 16

17 Solution (Undo) START T T, A, 10, 11 START U U, B, 20, 21 T, C, 30, 31 U, D, 40, 41 COMMIT U Starting from the end of the log: Ignore changes of transaction U altogether. Write value 30 for C. Write value 10 for A. Append ABRT T to the log. Solution of the exercises 17

18 Solution (Redo) START T T, A, 10, 11 START U U, B, 20, 21 T, C, 30, 31 U, D, 40, 41 COMMIT U We first identify the transactions that we need to redo. Only transaction U must be redone. By reading the log we can conclude that: A cannot have had its value changed on disk. B might have had its value changed on disk. C cannot have had its value changed on disk. D might have had its value changed on disk. Solution of the exercises 18

19 Solution (Redo) START T T, A, 10, 11 START U U, B, 20, 21 T, C, 30, 31 U, D, 40, 41 COMMIT U Starting from the beginning of the log: Ignore changes of transaction T altogether. Write value 21 for B. Write value 41 for D. Append ABRT T to the log. Solution of the exercises 19

20 Solution (Undo/Redo) START T T, A, 10, 11 START U U, B, 20, 21 T, C, 30, 31 U, D, 40, 41 COMMIT U We first identify the transactions that we need to redo and those that we need to undo. U must be redone, while T must be undone. By reading the log we can conclude that: A might have had its value changed on disk. B might have had its value changed on disk. C might have had its value changed on disk. D might have had its value changed on disk. Solution of the exercises 20

21 Solution (Undo/Redo) START T T, A, 10, 11 START U U, B, 20, 21 T, C, 30, 31 U, D, 40, 41 COMMIT U Starting from the end of the log (undo): Ignore changes of transaction U altogether. Write value 30 for C. Write value 10 for A. Append ABRT T to the log. Then, starting from the beginning of the log (redo): Ignore changes of transaction T altogether. Write value 21 for B. Write value 41 for D. Solution of the exercises 21

22 Task: Consider the following log, where a checkpoint start has been added: START S S, A, 60 COMMIT S START T T, A, 10 CKPT START START U U, B, 20 T, C, 30 START V U, D, 40 V, F, 70 COMMIT U T, E, 50 COMMIT T V, B, 80 COMMIT V When is CKPT END written? What happens if a crash occurs? (for each possible point at which a crash can occur) Discuss for Undo-, Redo-, and Undo/Redo-logging. Solution of the exercises 22

23 Solution (Undo) START S S, A, 60 COMMIT S START T T, A, 10 CKPT START START U U, B, 20 T, C, 30 START V U, D, 40 V, F, 70 COMMIT U T, E, 50 COMMIT T V, B, 80 COMMIT V The checkpoint entry identifies the transactions that are currently active. Hence, the checkpoint entry is CKPT START (T ). Every one of these active transactions must commit before writing CKPT END. We can write CKPT END right after COMMIT T. Solution of the exercises 23

24 Solution (Undo) START S S, A, 60 COMMIT S START T T, A, 10 CKPT START START U U, B, 20 T, C, 30 START V U, D, 40 V, F, 70 COMMIT U T, E, 50 COMMIT T V, B, 80 COMMIT V The recovery depends on whether we first meet CKPT END or CKPT START : if CKPT END was written last, we only need to consider the log up to CKPT START (T ). if CKPT START (T ) was written last, we need to consider the log up to START T, as it was the only active transaction. Solution of the exercises 24

25 Solution (Redo) START S S, A, 60 COMMIT S START T T, A, 10 CKPT START START U U, B, 20 T, C, 30 START V U, D, 40 V, F, 70 COMMIT U T, E, 50 COMMIT T V, B, 80 COMMIT V The checkpoint entry identifies the transactions that are currently active. Hence, the checkpoint entry is CKPT START (T ). We first write blocks from transaction that were commited at CKPT START to the disk. We only write CKPT END after these blocks. We cannot predict when the dirty blocks will be written on disk: CKPT END can occur anywhere after CKPT START. Solution of the exercises 25

26 Solution (Redo) START S S, A, 60 COMMIT S START T T, A, 10 CKPT START START U U, B, 20 T, C, 30 START V U, D, 40 V, F, 70 COMMIT U T, E, 50 COMMIT T V, B, 80 COMMIT V The recovery depends on whether we the last checkpoint entry was CKPT END or CKPT START (T ) : if CKPT END was written last, we know that transaction S was fully written. The transactions that were active at CKPT START (T ) or started later and that have committed must be redone (that is, T, U, and V, depending on the place of the crash). if CKPT START (T ) was written last, the checkpoint does not help. We need to go back to the previous CKPT END, or to the beginning of the log. Solution of the exercises 26

27 Solution (Undo/Redo) START S S, A, 60 COMMIT S START T T, A, 10 CKPT START START U U, B, 20 T, C, 30 START V U, D, 40 V, F, 70 COMMIT U T, E, 50 COMMIT T V, B, 80 COMMIT V The checkpoint entry identifies the transactions that are currently active. Hence, the checkpoint entry is CKPT START (T ). All dirty blocks are written to disk first. We only write CKPT END after these blocks. We cannot predict when the dirty blocks will be written on disk: CKPT END can occur anywhere after CKPT START. Solution of the exercises 27

28 Solution (Undo/Redo) START S S, A, 60 COMMIT S START T T, A, 10 CKPT START START U U, B, 20 T, C, 30 START V U, D, 40 V, F, 70 COMMIT U T, E, 50 COMMIT T V, B, 80 COMMIT V The recovery depends on whether we the last checkpoint entry was CKPT END or CKPT START (T ) : if CKPT END was written last, we know that all the dirty buffers were written to disk. We only need to redo transactions from CKPT START (T ), but we also need to undo transactions that were active at CKPT START (T ). Hence, we may need to go back to START T when undoing. if CKPT START (T ) was written last, the checkpoint does not help. We need to go back to the previous CKPT END, or to the beginning of the log. Solution of the exercises 28

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