COMPSCI/SOFTENG 351 & 751. Strategic Exercise 5 - Solutions. Transaction Processing, Crash Recovery and ER Diagrams. (May )

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COMPSCI/SOFTENG 351 & 751 Strategic Exercise 5 - Solutions Transaction Processing, Crash Recovery and ER Diagrams (May 23 2016) Exercise 1 : Multiple-Granularity Locking: Use the Database

1 COMPSCI/SOFTENG 351 & 751 Strategic Exercise 5 - Solutions Transaction Processing, Crash Recovery and ER Diagrams (May ) Exercise 1 : Multiple-Granularity Locking: Use the Database organization shown in the figure below and describe the locking scheme (what locks and at what granularity) you would apply for the following actions (your solution should allow the highest level of concurrency possible): I) Read record r a1 IS on D, IS on A 1, IS on F a, S on r a1 II) Modify record r b1 IX on D, IX on A 1, IX on F b, X on r b1 III) Delete some records in F c, the exact records to be modified can be determined only after reading. IX on D, IX on A 2 and S IX on F c Afterwards X on records to be modified

2 Exercise 2 : Consider the schedule S: r 1 (x), w 2 (x), w 1 (x), r 3 (x), c 1, c 2, c 3. Will this schedule be permitted under the a) 2PL Protocol? No. T2 cannot acquire an exclusive lock on x to write as long as T 1 holds a shared lock on it (as indicated by r 1 (x)). If T 1 releases the lock for T 2 to write, it cannot get a lock back on x if following 2PL. b) strict 2PL Protocol? No. T 3 reads x written by T 1 and T 2 without a commit. Exercise 3: For the schedule S: r 1 (x), w 2 (x), w 2 (y), w 3 (y),w 1 (y),c 1,c 2,c 3 For the following assume that the time stamp of transaction T i is i and a version of the protocol that allows recoverability is used. Will the schedules be permitted by the following protocols? a) Time-stamp based ordering protocol r 1 (x) permitted. R-timestamp(x) = 1 w 2 (x) TS(T 2 ) is 2 which is greater than R-timestamp(x) so permitted. W-timestamp(x) set to 2 w 2 (y) TS(T 2 ) is 2 so permitted. W-timestamp(y) is set to 2. w 3 (y) TS(T 3 ) is 3 which is greater W-timestamp(y) so permitted w 1 (y) TS(T 1 ) is 1 which is less than the W-timestamp(y) so T 1 is rolled back. Hence this schedule is not permitted under the Time-stamp Based ordering protocol. b) Time-stamp based ordering with Thomas Write Rule r 1 (x) permitted. R-timestamp(x) = 1 w 2 (x) TS(T 2 ) is 2 which is greater than R-timestamp(x) so permitted. W-timestamp(x) set to 2 w 2 (y) TS(T 2 ) is 2 so permitted. W-timestamp(y) is set to 2. w 3 (y) TS(T 3 ) is 3 which is greater W-timestamp(y) so permitted w 1 (y) TS(T 1 ) is 1 which is less than the W-timestamp(y) so under Thomas Write Rule this is ignored Hence this schedule is allowed with Time-stamp based Ordering with Thomas Write Rule. c) Validation Based Protocol T 1 has the earliest time stamp of 1 and does not need to be validated with other protocols. T 2 has the next time stamp and has to be validated with T 1.

3 T 1 writes object y which is written by T 2 fails condition so T 2 will be aborted and restarted Hence this schedule will not be permitted by the Validation-based protocol. d) Multiversion Concurrency Control r 1 (x) is permitted and RTS(x) is set to 1. w 2 (x) is permitted since TS(T 2 ) < RTS(x) and RTS(x) and WTS(x) is set to 2 and a new version created w 2 (y) is permitted and RTS(y) is set to 2 and a new version created w 3 (y) is permitted since TS(T 3 ) > and RTS(y) and a new version created. RTS(y) is set to 3. w 1 (y) is not permitted since TS(T 1 ) < RTS(y) Hence this schedule is not permitted under Multiversion Concurrency Control. Exercise 4 : Consider schedule S and answer the following questions based on your analysis of S. S: r 1 (x), w 2 (x), w 1 (x), r 3 (x), c 1, c 2, c 3 a) Is S conflict serializable? Explain. The precedence graph for this schedule is as shown: T1 T2 T3 There is a cycle in the graph (involving T 1 and T 2 ). Hence S is not conflict serializable. b) Does S avoid cascading rollback? Explain. No. S does not avoid cascading rollback. If T 2 aborted instead of committing, then T 3 would have read a value (object x) that will change when T 2 is rolled back. So T 3 must also be rolled back/aborted. Hence there is cascade. (similarly with T 1 and T 3 ). c) Is S recoverable? Explain. Yes. T 3 reads a value (object x) written by T 1 and T 2. T 3 commits after T 1 and T 2. Hence if T 1 or T 2 abort, T 3 can also cascade rollback and abort, so that no anomalies will exist. d) Is S strict? Explain.

4 No. T 3 reads x after uncommitted writes by T 1 and T 2 so S is not strict. Exercise 5: Write down the steps involved in the crash recovery for the following log record (assume that the T-table and DP-table are empty if not given with the checkpoint record, also assume pagelsn values have lower LSN values from previous committed transactions if not indicated in the current log). Show what the new log will look like: 0 Begin Checkpoint 5 End Checkpoint 10 T1: Update P1 (Old: YYY New:ZZZ) 15 T1: Update P2 (Old: WWW New: XXX) 20 T2: Update P3 (Old: UUU New:VVV) 25 T1: Commit 30 T2: Update P1 (Old:ZZZ New:TTT) Assume Crash occurred here Analysis Phase: Start at Begin Checkpoint Log record (LSN 0) 0-5: Initialize T-table and DP-table to Empty 10: Add (T1, lastlsn 10) to T-table. Add (P1, reclsn 10) to DP-table. 15: Change lastlsn of T1 to 15 in T-table. Add (P2, reclsn 15) to DP-table. 20: Add (T2, lastlsn 20) to T-table. Add (P3, reclsn 20) to DP-table. 25: Remove T1 from T-table. 30: Change lastlsn of T2 to 30 in T-table. Redo Phase: Start with smallest reclsn in Dp-table (10) 10: Fetch P1, check that pagelsn < 10, set P1 to ZZZ and set P1 pagelsn to : Fetch P2, check that pagelsn < 15, set P2 to XXX and set P2 pagelsn to : Fetch P3, check that pagelsn < 20, set P2 to VVV and set P3 pagelsn to : Write end record for T1 30: Fetch P1 if flushed, pagelsn should be 10 which is less than 30, set P1 to TTT, set pagelsn of P1 to 30.

5 Undo Phase: Loser transaction set is {T2} ToUndoSet is {30} 30: Undo LSN 30, set P1 to ZZZ, write CLR for this undo at LSN 40. The undonextlsn for CLR at 40 is 20. ToUndoSet is now {20} 20: Undo LSN 20, set P3 to UUU, write CLR for this undo at LSN 50. The undonextlsn for CLR at 50 is NULL. ToUndoSet is empty. Write End record for T2. New Log: 0 Begin Checkpoint 5 End Checkpoint 10 T1: Update P1 (Old: YYY New:ZZZ) 15 T1: Update P2 (Old: WWW New: XXX) 20 T2: Update P3 (Old: UUU New:VVV) 25,27 T1: Commit,End Record for T1 30 T2: Update P1 (Old:ZZZ New:TTT) 40 CLR: Undo LSN CLR: Undo LSN End Record for T2 (aborted)

6 Exercise 6: What are the schemas associated with the ER diagram below? The two relations from the above diagram are: Studios(name,addr) Crews(number,studioname,crewChief) Note that Unit-of(number,studioname,name) is not created. Exercise 7: Draw the ER diagram for the following description of movies and studios: Studios, Movies and Presidents are entity sets. Movies are owned by studios and every movie must be owned by one studio present in the Studios entity set. Every president in the Presidents entity set runs a studio. A studio can exist without a president. However, if a studio ceases to exist, the president can no longer be called a president.

Comp 5311 Database Management Systems. 14. Timestamp-based Protocols

Comp 5311 Database Management Systems 14. Timestamp-based Protocols 1 Timestamps Each transaction is issued a timestamp when it enters the system. If an old transaction T i has time-stamp TS(T i ), a new