Failure Recovery Himanshu Gupta CSE 532-Recovery-1

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1 Failure Recovery CSE 532-Recovery-1

2 Data Integrity Protect data from system failures Key Idea: Logs recording change history. Today. Chapter 17. Maintain data integrity, when several queries/modifications are run together. Key Idea: Locks granting controlled access. Next week(s). Chapter 18 and 19. CSE 532-Recovery-2

3 DB State DB state: Value for each db element. Consistent DB state: That satisfies all database constraints (key, value constraints etc.) CSE 532-Recovery-3

4 Transactions Transactions: Processes that query and modify database. Executes a number of steps in sequence. State: gives what has been done so far. Desirable properties: ACID Atomic = Either all work is done, or none of it. Consistent = Database constraints are preserved. Isolated = Appear as if one process executes at a time. Often called serializable behavior. Durable = Effects are permanent irrespective of system crashes. Commit/Abort actions (next slide) Consistency is assumed. CSE 532-Recovery-4

5 Commit/Abort Decision Each transaction ends with either: Commit = Causes the transaction to complete. It s database modifications are now permanent in the database; previously its changes may be invisible to other transactions. Abort = no changes by the transaction appear in the database; it is as if the transaction never occurred. ROLLBACK is the term used in SQL and Oracle. Failures like division by 0 can also cause abortion/rollback, even if the user doesn t request it. CSE 532-Recovery-5

6 Consistency Property (Assumption) If T starts with consistent state AND T executes in isolation T leaves consistent state Consistent DB T Consistent DB CSE 532-Recovery-6

7 Failure Recovery Motivation Transactions should be atomic Ø Whole execution or nothing at all Challenge: Concurrent transactions may cause problems unless controlled. CSE 532-Recovery-7

8 Failure Possibilities Transaction bug DBMS bug Hardware failure e.g., disk crash alters balance of account Data sharing e.g.: T1: give 10% raise to programmers T2: change programmers systems analysts CSE 532-Recovery-8

9 Model CPU processor memory M D disk x x Memory Disk CSE 532-Recovery-9

10 Input (x): Output (x): Read (x,t): Operations Let Block(x) be the block containing x Block(x) memory Block(x) disk do input(x), if necessary t x Write (x,t): do input(x), if necessary x t CSE 532-Recovery-10

11 Key problem Unfinished transaction Example Constraint: A=B T1: A A 2 B B 2 CSE 532-Recovery-11

12 T1: Read (A,t); t t 2 Write (A,t); Read (B,t); t t 2 Write (B,t); Output (A); Output (B); failure! A: 8 B: A: 8 B: 8 16 memory disk How to ensure atomicity? CSE 532-Recovery-12

13 Solution: Log Old Values Before Disk-Change T1: Read (A,t); t t 2 A=B Write (A,t); Read (B,t); t t 2 Write (B,t); Output (A); Output (B); A:8 B: A:8 B:8 <T1, start> <T1, A, 8> 16 <T1, B, 8> <T1, commit> 16 memory disk log CSE 532-Recovery-13

14 First complication When should be log be written onto disk? memory A: 8 16 B: 8 16 Log: <T1,start> <T1, A, 8> <T1, B, 8> A: 8 B: 8 16 DB Log BAD STATE # 1 CSE 532-Recovery-14

15 Second complication What if Write(x) is not written to disk before commit log? memory A: 8 16 B: 8 16 Log: <T1,start> <T1, A, 8> <T1, B, 8> <T1, commit> A: 8 B: <T1, B, 8> <T1, commit> DB Log BAD STATE # 2 CSE 532-Recovery-15

16 Solution: Undo Logging Rules For a Write(x,V) in Ti 1. Generate log(x) = <Ti, x, oldvalue(x)> 2. Output(log(x)) before Output(x) [WAL: Write Ahead Logging] Output(x) before Output(<Ti,Commit>) CSE 532-Recovery-16

17 Recovery in Undo logging Basic Idea: For every transaction that hasn t been committed in the log, abort it. Commit check: <Ti, Commit> in log. Abort: Write back old values on disk using the logs, and write <Ti, Abort> in log. CSE 532-Recovery-17

18 Recovery rules: Undo logging (1) Let S = set of transactions with <Ti, start> in log, but no <Ti, commit> (or <Ti, abort>) record in log (2) For each <Ti, X, v> in log, in reverse order (latest earliest) do: - if Ti S then - write (X, v) - output (X) (3) For each Ti S do - write <Ti, abort> to log CSE 532-Recovery-18

19 What if failure during recovery? No problem! Undo is idempotent CSE 532-Recovery-19

20 To discuss: Redo logging Undo/redo logging, why both? Checkpoints CSE 532-Recovery-20

21 Redo logging (deferred modification) T1: Read(A,t); t t 2; write (A,t); Read(B,t); t t 2; write (B,t); Output(A); Output(B) A: 8 B: output A: 8 B: 8 16 <T1, start> <T1, A, 16> <T1, B, 16> <T1, commit> memory DB LOG CSE 532-Recovery-21

22 Redo Logging Rules For a Write(x,V) in Ti 1. Generate log(x) = <Ti, x, newvalue(x)> To commit: Flush log including <Ti, commit> Output(x) for all x in Ti. CSE 532-Recovery-22

23 Recovery rules: Redo logging (1) Let S = set of transactions with <Ti, commit> in log (2) For each <Ti, X, v> in log, in forward order (earliest latest) do: - if Ti S then Write(X, v) Output(X) optional CSE 532-Recovery-23

24 Undo vs. Redo Logging Order of writes Output(<Ti, x, old/new>) (old for undo) Output(<Ti, commit>) Undo: Output(x). Redo: Output(x). Undo: <Ti, commit> on disk implies Ti completed. Redo: No <Ti,commit> implies Ti did nothing. CSE 532-Recovery-24

25 Redo Recovery is very, very SLOW! Redo log: First T1 wrote A,B Last Record Committed a year ago Record (1 year ago) --> STILL, Need to redo after crash!! Crash CSE 532-Recovery-25

26 Solution: Checkpoint (simple version) Periodically: (1) Do not accept new transactions (2) Wait until all transactions finish (3) Flush all log records to disk (log) (4) Flush all buffers to disk (DB) (do not discard buffers) (5) Write checkpoint record on disk (log) (6) Resume transaction processing CSE 532-Recovery-26

27 Example: what to do at recovery? Redo log (disk): <T1,A,16> <T1,commit> Checkpoint <T2,B,17> <T2,commit> <T3,C,21> Crash CSE 532-Recovery-27

28 Key drawbacks: Undo logging: need to flush all Write(x) s before commit. Redo logging: can t flush Write(x) s until commit CSE 532-Recovery-28

29 Solution: Undo/Redo Logging Update <Ti, Xid, New X val, Old X val> ONLY Rule: Output(log(x)) before Output(x) Recovery: Redo all committed TRs (earliest first) Undo all incomplete TRs (latest first) CSE 532-Recovery-29

30 Nonquiescent checkpoint L O G... Start-ckpt active TR: Ti,T2, End-Ckpt... Flush all dirty buffers Log chains of active transactions. Needed for Undo CSE 532-Recovery-30

31 Examples what to do at recovery time? L O G... T1,- a... Start-Ckt T1... Ckpt end... no T1 commit T1- b Undo T1 (undo a,b) CSE 532-Recovery-31

32 Example L O G... T1 a ckpt-s T1 T1 b ckpt T1 end c... T1 cmt... Redo T1: (redo b,c). No need to (redo a). CSE 532-Recovery-32

33 Recovery process: Backwards pass (end of log latest checkpoint start) construct set S of committed transactions undo actions of transactions not in S Undo pending transactions follow undo chains for transactions in (checkpoint active list) - S Forward pass (latest checkpoint start end of log) redo actions of S transactions start checkpoint backward pass forward pass CSE 532-Recovery-33

34 Summary Consistency of data One source of problems: failures - Logging - Redundancy Another source of problems: Data Sharing... next CSE 532-Recovery-34

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