1. Review Lecture 30

2. Administrivia Office hours 1:30 – 2:15 today Final Exam May 16 8-11 a.m. Location: 22 Warren Topics since Midterm 2 –Transactions, concurrency control, locking, recovery –Logical design, ER Modeling, Functional Dependencies, Normalization, Data Mining –Guest lectures Cumulative questions from semester Review today Review Tuesday

3. Concurrent users introduce anomalies –Dirty reads (WR): T2 reads a value A that T1 wrote but didn’t commit –Unrepeatable Reads (RW): T1 reads a value A that is then written by T2 –Lost Updates (WW): T2 overwrites a write by T1 Serializable schedules: –A schedule that is equivalent to some serial execution of the transactions. Definition: Two operations conflict if: –They are by different transactions, –they are on the same object, –and at least one of them is a write. Concurrency R(A) R(B) W(A) W(B) R(A)R(B)W(B) T1: T2: W(A)

4. Conflict Serializability – Intuition A schedule S is conflict serializable if: –You are able to transform S into a serial schedule by swapping consecutive non-conflicting operations of different transactions. Example: R(A)R(B) W(A) W(B) R(A) W(A) R(B) W(B) W(A) R(B) R(A) W(B) W(A) W(B) R(A) R(B) W(A) W(B) R(A) W(A) R(B) W(B) T1: T2: T1: T2:

5. Dependency Graph Dependency graph: –One node per Xact –Edge from Ti to Tj if: An operation Oi of Ti conflicts with an operation Oj of Tj and Oi appears earlier in the schedule than Oj. Theorem: Schedule is conflict serializable if and only if its dependency graph is acyclic. TiTj

6. A schedule that is not conflict serializable: The cycle in the graph reveals the problem. The output of T2 depends on T1’s value of A, and the output of T1 depends on T2’s value of B. Another Example T1T2 A B Dependency graph T1: R(A), W(A), R(B), W(B) T2: T1: R(A), W(A), R(B), W(B) T2: R(A), W(A), R(B), W(B) T1: R(A), W(A), R(B), W(B) T2: R(A), W(A), R(B), W(B) T1: R(A), W(A), R(B), W(B) T2: R(A), W(A), R(B), W(B) T1: R(A), W(A), R(B), W(B) T2: R(A), W(A), R(B), W(B)

7. Review: Lock-Based Concurrency Control Two-phase Locking (2PL) Protocol: – Each Xact must obtain: a S (shared) lock on object before reading, and an X (exclusive) lock on object before writing. – If an Xact holds an X lock on an object, no other Xact can get a lock (S or X) on that object. – System can obtain these locks automatically – Two phases: acquiring locks, and releasing them No lock is ever acquired after one has been released “Growing phase” followed by “shrinking phase”. Ensures acyclic dependency graphs Allows only conflict serializable schedules

8. Review: Strict 2 Phase Locking Advantage: no other transaction reads anything you write until you commit. –e.g a transaction will only read committed data. Disadvantage: transactions end up waiting. Strict Two-phase Locking (Strict 2PL) Protocol: – Same as 2PL, except All locks held are released only when the transaction completes Ensures acyclic dependency graphs Allows only conflict serializable schedules Allows only strict schedules  No values written by an Xact T can be read or overwritten until T commits or aborts.

9. Deadlocks Deadlock: Cycle of transactions waiting for locks to be released by each other. Two ways of dealing with deadlocks: – Deadlock prevention Wait-die: new transactions aren’t allowed to wait Wound-wait: old transactions don’t have to wait Deadlock detection –Create a waits-for graph: – There is an edge from Ti to Tj if Ti is waiting for Tj to release a lock Periodically check for cycles in the waits-for graph

10. Deadlock Detection (Continued) Example: T1: S(A), S(D), S(B) T2: X(B) X(C) T3: S(D), S(C), X(A) T4: X(B) T1T2 T4T3 S(A) S(D) X(B) S(B) S(D) S(C) X(C) X(B) X(A)

11. Multi-granularity locking –Use database containment hierarchy to vary granularity of locks Full table insert: lock table vs read 1 row: lock record Locking in indexes –don’t want to lock a B-tree root for a whole transaction! –actually do non-2PL “latches” in B-trees CC w/out locking –“optimistic” concurrency control Lock Management Tuples Tables Pages Database contains

12. Multiple Granularity Lock Protocol Each Xact starts from the root of the hierarchy. Special SIX lock used when reading many records, and updating a few. –SIX lock conflicts are all S and IX conflicts (e.g. only compatible with IS locks). To get S or IS lock on a node, must hold IS or IX on parent node. To get X or IX or SIX on a node, must hold IX or SIX on parent node. Must release locks in bottom-up order.

13. Multi-Granularity Example Rules –Each Xact starts from the root of the hierarchy. –To get S or IS lock, must hold IS or IX on parent. –To get X or IX or SIX, must hold IX or SIX on parent. –Must release locks in bottom-up order. Tuple 1 Sailor Table Page 1 Database Page 2 Tuple 2Tuple 4Tuple 3 T1 wants to read & change tuple 2 T2 wants to read all of Page 1 T1 gets IX lock on DBMS, Sailor, Page 1 T1 gets X lock on Tuple 2 & changes it T2 gets IS lock on DBMS, Sailor T2 tries to get S lock on Page 1, but S conflicts with IX lock. T2 blocks. What if T2 had started first? IS IX SIX IS IX SIX      SX  S X   T1:IX T1:X T2:IS T2:wait

14. Multi-Granularity Example Rules –Each Xact starts from the root of the hierarchy. –To get S or IS lock, must hold IS or IX on parent. –To get X or IX or SIX, must hold IX or SIX on parent. –Must release locks in bottom-up order. Tuple 1 Sailor Table Page 1 Database Page 2 Tuple 2Tuple 4Tuple 3 T1 wants to read & change tuple 2 T2 wants to read all of Page 1 T2 gets IS lock on DBMS, Sailor T2 gets S lock on Page 1 T1 gets IX lock on DBMS, Sailor T1 tries to get IX lock on Page 1, waits IS IX SIX IS IX SIX      SX  S X   T1:IX T1:waits T2:IS T2:S

15. Locking in B+ Trees 24 30 7 9 1415 13 20 1.Higher levels of the tree only direct searches for leaf pages. 2.For inserts: – a node must be X locked only if a split can propagate up to it from the modified leaf. –Example: insert 9 vs insert 15 16 15 16 We can exploit these observations to design efficient locking protocols that guarantee serializability even though they violate 2PL.

16. Simple Locking in B+ Trees Search: Start at root and go down; –S lock node. –Unlock its parent. Insert/Delete: Start at root and go down, –X lock node. –If node is safe, release all locks on ancestors. Safe node: Node such that changes will not propagate up beyond this node. –Inserts: Node is not full. –Deletes: Node is not half-empty.

17. Example ROOT A B C DE F G H I 20 35 20* 3844 22*23*24*35*36*38*41*44* T1: Search 38 T2: Insert 45 T3: Insert 25 23 T1:S T2:X Search: –S lock node. –Unlock its parent. Insert/Delete: –X lock node. –If node is safe, release all locks on ancestors. T3:X

18. Optimistic CC (Kung-Robinson) Locking is a conservative approach in which conflicts are prevented. Disadvantages: –Lock management overhead. –Deadlock detection/resolution. –Lock contention for heavily used objects. If conflicts are rare, we might be able to gain concurrency by not locking, and instead checking for conflicts before Xacts commit.

19. Kung-Robinson Model Xacts have three phases: –READ: Xacts read from the database, but make changes to private copies of objects. –VALIDATE: Check for conflicts. –WRITE: Make local copies of changes public. Tj RV W 14 Buffer Pool 2327 1423 Tj private copies Reads from Writes to Writes back Validation, and Write phase are done inside a critical section! –i.e., Nothing else goes on concurrently.

20. Validation Phase Tests conditions that are sufficient to ensure that no conflict occurred. –If conflict did occur, restart transaction. Each Xact is assigned a timestamp at end of READ phase, just before validation begins. –Also keep track of xact phase begin & end times Compute –ReadSet(Tj): Set of objects read by Xact Tj. –WriteSet(Tj): Set of objects modified by Tj.

21. Validation Test 1 for Tj: no overlap For all i and j such that Ti < Tj, check that Ti completes write phase before Tj begins read phase. Ti Tj RVW RVW  Implies a serial order for Ti and Tj; Ti came first.

22. Validation Test 2 for Tj: Overlapping read phase For all i and j such that Ti < Tj, check that: –Ti completes before Tj begins its Write phase + –WriteSet(Ti) ReadSet(Tj) is empty. Ti Tj RVW RVW  Ensures Tj does not read any object written by Ti.  Implies a serial order; Tj might write same set of objects, but writes are in a serial order; Ti’s writes come first.

23. Validation Test 3 for Tj: Overlapping write phase For all i and j such that Ti < Tj, check that: –Ti completes Read phase before Tj does + –WriteSet(Ti) ReadSet(Tj) is empty + –WriteSet(Ti) WriteSet(Tj) is empty. Ti Tj RVW RVW  Ensures Tj does not read or write any object written by Ti.  Implies a serial order; Tj reads and writes are to different objects than those written by Ti.

24. Optimistic CC Overhead Must record read/write activity in ReadSet and WriteSet per Xact. –Must create and destroy these sets as needed. Must check for conflicts during validation, and must make validated writes ``global’’. –Critical section can reduce concurrency. –Scheme for making writes global can reduce clustering of objects. Optimistic CC restarts Xacts that fail validation. –Work done so far is wasted; requires clean-up.

25. Write-Ahead Logging (WAL) The Write-Ahead Logging Protocol:  Must force the log record for an update before the corresponding data page gets to disk.  Must force all log records for a Xact before commit. (or, a transaction is not committed until all of its log records including its “commit” record are on the stable log.) #1 (with UNDO info) helps guarantee Atomicity. #2 (with REDO info) helps guarantee Durability. This allows us to implement Steal/No-Force buffer management policy

26. Buffer Management summary Force No Force No Steal Steal No REDO No UNDO UNDO No REDO UNDO REDO No UNDO REDO Force No Force No Steal Steal Slowest Fastest Performance Implications Logging/Recovery Implications

27. WAL & the Log Each log record has a unique Log Sequence Number (LSN). –LSNs always increasing. Each data page contains a pageLSN. –The LSN of the most recent log record for an update to that page. System keeps track of flushedLSN. –The max LSN flushed so far. WAL: Before page i is written to DB log must satisfy: pageLSN i  flushedLSN LSNspageLSNs RAM flushedLSN pageLSN Log records flushed to disk “Log tail” in RAM flushedLSN DB

28. Log Records prevLSN is the LSN of the previous log record written by this Xact (so records of an Xact form a linked list backwards in time) Possible log record types: Update, Commit, Abort Checkpoint (for log maintenance) Compensation Log Records (CLRs) –for UNDO actions End (end of commit or abort) LSN prevLSN XID type length pageID offset before-image after-image LogRecord fields: update records only

29. Other Log-Related State Two in-memory tables: Transaction Table –One entry per currently active Xact. entry removed when Xact commits or aborts –Contains XID, status (running/committing/aborting), and lastLSN (most recent LSN written by Xact). Dirty Page Table: –One entry per dirty page currently in buffer pool. –Contains recLSN -- the LSN of the log record which first caused the page to be dirty.

30. The Big Picture: What’s Stored Where DB Data pages each with a pageLSN Xact Table lastLSN status Dirty Page Table recLSN flushedLSN RAM LSN prevLSN XID type length pageID offset before-image after-image LogRecords LOG Master record

31. Example GDE Page 1 LSN:2 ABC Page 2 LSN:4 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 1.T1 update 2 (DEF) (assume written to disk) 2.T2 update 3 (KLM) 3.T2 update 1 (QRS) 4.T1 update 2 (WXY) 5.T2 commit 6.T1 update 4 (RST) 7.SYSTEM CRASH LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM ABC Page 2 LSN:4 DEF Page 2 LSN:11 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end 16 14T1 update 4 OPQ RST SYSTEM CRASH KLM Page 3 LSN:12 WXY Page 2 LSN:14 To disk GDE Page 1 LSN:2 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 QRS Page 1 LSN:13 OPQ Page 4 LSN:8 RST Page 4 LSN:16 BEGIN_CHKPT END_CHKPT DEF Page 2 LSN:11

32. Crash Recovery: Big Picture v Start from a checkpoint (found via master record). v Three phases. Need to do: –Analysis - Figure out which Xacts committed since checkpoint, which failed. –REDO all actions. (repeat history) –UNDO effects of failed Xacts. Oldest log rec. of Xact active at crash Smallest recLSN in dirty page table after Analysis Last chkpt CRASH A RU

33. End result – goal of recovery GDE Page 1 LSN:2 ABC Page 2 LSN:4 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 1.T1 update 2 (DEF) 2.T2 update 3 (KLM) 3.T2 update 1 (QRS) 4.T1 update 2 (WXY) 5.T2 commit 6.T1 update 4 (RST) 7.SYSTEM CRASH KLM Page 3 LSN:12 QRS Page 1 LSN:2 T1 aborts Roll back updates if they made it to disk. T2 commits Re-apply updates if needed

34. Recovery: The Analysis Phase Re-establish knowledge of state at checkpoint. –via transaction table and dirty page table stored in the checkpoint Scan log forward from checkpoint. –End record: Remove Xact from Xact table. –All Other records: Add Xact to Xact table, set lastLSN=LSN, change Xact status on commit. –also, for Update records: If page P not in Dirty Page Table, Add P to DPT, set its recLSN=LSN. At end of Analysis… –transaction table says which xacts were active at time of crash. –DPT says which dirty pages might not have made it to disk

35. Analysis GDE Page 1 LSN:2 DEF Page 2 LSN:11 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 Xact IDLast LSNState T1 11 U ABC Page 2 LSN:4 DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 GDE Page 1 LSN:2 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 QRS Page 1 LSN:13 OPQ Page 4 LSN:8 RST Page 4 LSN:16 LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end BEGIN_CHKPT END_CHKPT Xact Table T2 12 U T2 13 U T1 14 U T2 15 C 1. Create entries the Xact table with xacts active at time of crash.

36. Analysis GDE Page 1 LSN:2 DEF Page 2 LSN:11 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 Xact IDLast LSNState ABC Page 2 LSN:4 DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 GDE Page 1 LSN:2 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 QRS Page 1 LSN:13 OPQ Page 4 LSN:8 RST Page 4 LSN:16 Page IDrec LSN LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end BEGIN_CHKPT END_CHKPT Xact Table Dirty Page Table T1 14 U 2 11 3 12 1 13 2. Create entries in the Dirty Page table with pages that might not have made it to disk.

37. Phase 2: The REDO Phase We Repeat History to reconstruct state at crash: –Reapply all updates (even of aborted Xacts!), redo CLRs. Scan forward from log rec containing smallest recLSN in DPT. Q: why start here? For each update log record or CLR with a given LSN, REDO the action unless: –Affected page is not in the Dirty Page Table, or –Affected page is in D.P.T., but has recLSN > LSN, or –pageLSN (in DB)  LSN. (this last case requires I/O) To REDO an action: –Reapply logged action. –Set pageLSN to LSN. No additional logging, no forcing!

38. Redo GDE Page 1 LSN:2 DEF Page 2 LSN:11 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 Xact IDLast LSNState ABC Page 2 LSN:4 DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 GDE Page 1 LSN:2 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 QRS Page 1 LSN:13 OPQ Page 4 LSN:8 RST Page 4 LSN:16 Page IDrec LSN LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end BEGIN_CHKPT END_CHKPT Xact Table Dirty Page Table T1 14 U 2 11 3 12 1 13 Step 1. Find lowest rec LSN in Dirty Page Table.

39. Redo GDE Page 1 LSN:2 DEF Page 2 LSN:11 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 Xact IDLast LSNState ABC Page 2 LSN:4 DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 GDE Page 1 LSN:2 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 QRS Page 1 LSN:13 OPQ Page 4 LSN:8 RST Page 4 LSN:16 Page IDrec LSN LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end BEGIN_CHKPT END_CHKPT Xact Table Dirty Page Table T1 14 U 2 11 3 12 1 13 Step 2. Scan forward and redo all redoable log records.

40. Redo GDE Page 1 LSN:2 DEF Page 2 LSN:11 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 Xact IDLast LSNState ABC Page 2 LSN:4 DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 GDE Page 1 LSN:2 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 QRS Page 1 LSN:13 OPQ Page 4 LSN:8 RST Page 4 LSN:16 Page IDrec LSN LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end BEGIN_CHKPT END_CHKPT Xact Table Dirty Page Table T1 14 U 2 11 3 12 1 13 1. Reapply LSN 11 T1 update 2 (DEF) DEF Page 2 LSN:11 KLM Page 3 LSN:12 2. Reapply LSN 12 T2 update 3 (KLM)

41. Redo GDE Page 1 LSN:2 DEF Page 2 LSN:11 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 Xact IDLast LSNState ABC Page 2 LSN:4 DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 GDE Page 1 LSN:2 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 QRS Page 1 LSN:13 OPQ Page 4 LSN:8 RST Page 4 LSN:16 Page IDrec LSN LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end BEGIN_CHKPT END_CHKPT Xact Table Dirty Page Table T1 14 U 2 11 3 12 1 13 3. Reapply LSN 13 T2 update 1 (QRS) DEF Page 2 LSN:11 KLM Page 3 LSN:12 4. Reapply LSN 14 T1 update 2 (WXY) GDE Page 1 LSN:2 QRS Page 1 LSN:13 WXY Page 2 LSN:14

42. Redo GDE Page 1 LSN:2 DEF Page 2 LSN:11 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 Xact IDLast LSNState ABC Page 2 LSN:4 DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 GDE Page 1 LSN:2 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 OPQ Page 4 LSN:8 RST Page 4 LSN:16 Page IDrec LSN LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end BEGIN_CHKPT END_CHKPT Xact Table Dirty Page Table T1 14 U 2 11 3 12 1 13 5. Reapply T2 commit (and we’ll write dirty pages to disk.) DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 QRS Page 1 LSN:13 QRS Page 1 LSN:13

43. Phase 3: The UNDO Phase We undo actions of all active but not committed xacts at the time of the crash. – May even need to undo some of what we did in REDO phase! ToUndo={lastLSNs of all Xacts in the Trans Table} a.k.a. “losers” Repeat: –Choose (and remove) largest LSN among ToUndo. –If this LSN is a CLR and undonextLSN==NULL Write an End record for this Xact. –If this LSN is a CLR, and undonextLSN != NULL Add undonextLSN to ToUndo –Else this LSN is an update. Undo the update, write a CLR, add prevLSN to ToUndo. Until ToUndo is empty. v CLRs will help us remember where we are in case of system crash during recovery.

44. Undo GDE Page 1 LSN:2 DEF Page 2 LSN:11 HIJ Page 3 LSN:6 OPQ Page 4 LSN:8 Buffer Frame 1Buffer Frame 2Buffer Frame 3 Xact IDLast LSNState ABC Page 2 LSN:4 DEF Page 2 LSN:11 KLM Page 3 LSN:12 WXY Page 2 LSN:14 HIJ Page 3 LSN:6 KLM Page 3 LSN:12 OPQ Page 4 LSN:8 RST Page 4 LSN:16 Last LSN LSNPrevXactIDTypepageIDBeforeAfter 11 nullT1 update 2 ABC DEF 12 nullT2 update 3 HIJ KLM 13 12T2 update 1 GDE QRS 14 11T1 update 2 DEF WXY 15 13T2 commit and end BEGIN_CHKPT END_CHKPT Xact Table ToUndo T1 14 U 14 11 1. Add last LSN for all transactions in Xact table QRS Page 1 LSN:13 2. Recursively Process each last LSN in To Undo table. DEF Page 2 LSN:11 16 undoNextLSN=nullT1 CLR 2 DEF ABC ABC Page 2 LSN:16 ABC Page 2 LSN:4