TRANSACTION MANAGEMENT [CH 16]

Transcription

1 Sprig 2017 TRANSACTION MANAGEMENT [CH 16] 4/25/17 CS 564: Database Maagemet Systems; (c) Jigesh M. Patel,

2 Trasactio Maagemet Read (A); Check (A > $25); Pay ($25); A = A 25; Write (A); Yes You Read Balace: $100 Sufficiet fuds? Pay $25 New balace: $75 Bak Balace : $100 Bak Balace : $75! 4/25/17 CS 564: Database Maagemet Systems 2

3 Trasactio Maagemet Read (A); Check (A > $25); Pay ($25); A = A 25; Write (A); Yes Bak Balace : $100 You Your Sigificat Other Read Balace: $100 Read Balace: $100 Sufficiet fuds? Sufficiet fuds? Pay $25 Pay $25 Yes New balace: $75 New balace: $75 Icosistecy Iterleavig actios of differet user programs System crash/user abort/ Provide the users a illusio of a sigle-user system Could isist o admittig oly oe query ito the system at ay time lower utilizatio: CPU/IO overlap log ruig queries starve other queries Bak Balace : $75! 4/25/17 CS 564: Database Maagemet Systems 3

4 What is a Trasactio? Collectio of operatios that form a sigle logical uit A sequece of may actios cosidered to be oe atomic uit of work Logical uit: trasactio. (SQL) ed trasactio Operatios: Read (X), Write (X): Assume R/W o tuples (ca be relaxed) Special actios:,, abort Desirable Property: Must leave the DB i a cosistet state (DB is cosistet whe the trasactio s) Cosistecy: DBMS oly eforces ICs specified by the user DBMS does ot uderstad ay other sematics of the data 4/25/17 CS 564: Database Maagemet Systems 4

5 The ACID Properties TM Xact. Mgmt. (loggig) User CC Cocurrecy Ctrl. (lockig) Atomicity: All actios i the Xact happe, or oe happe. Cosistecy: Cosistet DB + cosistet Xact cosistet DB Isolatio: Executio of oe Xact is isolated from that of other Xacts. Begi Read (A); A = A 25; Write (A); Read (B); B = B + 25; Write (B); Commit RM Recovery Mgmt. (WAL, ) Durability: If a Xact s, its effects persist. 4/25/17 CS 564: Database Maagemet Systems 5

6 Schedules T1 T2 Schedule: A iterleavig of actios from a set of Xacts, where the actios of ay oe Xact are i the origial order. Actios of Xacts as see by the DB Complete schedule : each Xact eds i or abort Time R(C) W(C) R(B) W(B) Serial schedule : No iterleavig of actios from differet Xacts. abort Iitial State + Schedule Fial State 4/25/17 CS 564: Database Maagemet Systems 6

7 Acceptable Schedules Oe sesible isolated, cosistet schedule: Ru Xacts oe at a time (serial schedule) Serializable schedules: Fial state is what some complete serial schedule of ted trasactios would have produced. Ca differet serial schedules have differet fial states? Yes, all are OK! Aborted Xacts? igore them for a little while (made to disappear usig loggig) Other exteral actios (besides R/W to DB) e.g. prit a computed value, fire a missile, Assume (for this class) these values are writte to the DB, ad ca be udoe 4/25/17 CS 564: Database Maagemet Systems 7

8 Serializability (A,B) = (1000, 100) Ed (990, 210) T1 T2: (900, 200) (990, 220) T2 T1: (1100, 110) (1000, 210) W-R coflict: Dirty read Could lead to a oserializable executio Also R-W ad W-W coflicts T1: Trasfer $100 from A to B /A -= 100 Database Icosistet R(B) /B += 100 W(B) T2: Add 10% iterest to A & B /A *= 1.1 R(B) /B *= 1.1 W(B) 4/25/17 CS 564: Database Maagemet Systems 8

9 More Coflicts RW Coflicts (Urepeatable Read) R T2 (X) W T1 (X), T1 overwrites what T2 read. R T2 (X) W T1 (X) R T2 (X). T2 sees a differet X value! WW Coflicts (Overwritig Ued Data) T2 overwrites what T1 wrote. E.g. : Studets i the same group get the same project grade. T P : W (X=A), W (Y=A) T TA : W (X=B), W (Y=B) W P (X=A) W TA (X=B) W TA (Y=B) W P (Y=A) [Note: o reads] Usually occurs i cojuctio with other aomalies. Uless you have blid writes. 4/25/17 CS 564: Database Maagemet Systems 9

10 Now, Aborted Trasactios Serializable schedule: Equivalet to a serial schedule of ted Xacts. as if aborted Xacts ever happeed. Two Issues: How does oe udo the effects of a Xact? We ll cover this i loggig/recovery What if aother Xact sees these effects?? Must udo that Xact as well! 4/25/17 CS 564: Database Maagemet Systems 10

11 Cascadig Aborts Abort of T1 requires abort of T2! Cascadig Abort T1 abort abort T2 4/25/17 CS 564: Database Maagemet Systems 11

12 Cascadig Aborts Abort of T1 requires abort of T2! Cascadig Abort Cosider of T2 Ca we udo T2? Recoverable schedule: Commit oly after all xacts that supply dirty data have ted. T1 abort commi abort t T2 4/25/17 CS 564: Database Maagemet Systems 12

13 Cascadig Aborts T1 T2 ACA (avoids cascadig abort) schedule Trasactio oly reads ted data Oe i which cascadig abort caot arise. Schedule is also recoverable T1 T2 commi abort abort t Commit 4/25/17 CS 564: Database Maagemet Systems 13

14 Lockig: A Techique for C. C. Cocurrecy cotrol usually doe via lockig. Lock ifo maitaied by a lock maager : Stores (XID, RID, Mode) triples. This is a simplistic view; suffices for ow. Mode Î {S,X} Lock compatibility table: If a Xact ca t get a lock Suspeded o a wait queue Whe are locks acquired? Buffer maager call! -- S X -- S X 4/25/17 CS 564: Database Maagemet Systems 14

15 2PL: Two-Phase Lockig (2PL) If T wats to read (modify) a object, first obtais a S (X) lock If T releases ay lock, it ca acquire o ew locks! Guratees serializability! Why? Strict 2PL: Hold all locks util ed of Xact Guaratees serializability, ad ACA too! Note ACA schedules are always recoverable growig phase # locks Time lock poit Shrikig phase 4/25/17 CS 564: Database Maagemet Systems 15

16 Schedule with Locks T1: Trasfer $100 from A to B /A -= 100 R(B) /B += 100 W(B) T2: Add 10% iterest to A & B /A *= 1.1 R(B) /B *= 1.1 W(B) T1 X(A) X(B) R(B) W(B) U x (A), U x (B)/ T2 X(A) Wait! 4/25/17 CS 564: Database Maagemet Systems 16

17 Deadlocks A X T1 (B), X T2 (A), S T1 (A), S T2 (B) T1 B T2 Deadlocks ca cause the system to wait forever. Need to detect deadlock ad break, or prevet deadlocks Simple mechaism: timeout ad abort More sophisticated methods exist 4/25/17 CS 564: Database Maagemet Systems 17

18 Precedece Graph Precedece (or Serializability) graph: Nodes = Committed Xacts Coflicts = Arcs T1 T2 Coflict equivalet: Same sets of actios Coflictig actios i the same order Coflict serializable: Coflict equivalet to a serial schedule T1: Xfer. $100 from A to B R(B) W(B) T2: Add 10% iterest R(B) W(B) 4/25/17 CS 564: Database Maagemet Systems 18

19 Schedule with Locks T1: Trasfer $100 from A to B /A -= 100 T1 R(B) /B += 100 W(B) T2 T2: Add 10% iterest to A & B /A *= 1.1 R(B) /B *= 1.1 W(B) T1 T2 X(A) X(A) Wait! X(B) R(B) W(B) T1 T2 U x (A), U x (B)/ 4/25/17 CS 564: Database Maagemet Systems 19

20 Coflict Serializability & Graphs Theorem: A schedule is coflict serializable iff its precedece graph is acyclic Theorem: 2PL esures that the precedece graph will be acyclic Why Strict 2PL? Guaratees ACA read oly ted values How? Write locks util EOT No WW or WR => o abort replace origial value 4/25/17 CS 564: Database Maagemet Systems 20

21 Deadlocks X T1 (B), X T2 (A), S T1 (A), S T2 (B) Deadlocks ca cause the system to wait forever. Need to detect deadlock ad break, or prevet deadlocks Detect deadlock Draw a lock graph. Cycles implies a deadlock Alterative ways of dealig with deadlock Break Deadlock O each lock request update the lock graph. If a cycle is detected, abort oe of the trasactios. The aborted trasactio is restarted after waitig for a time-out iterval. Prevet deadlock Assig priorities to the trasactios. If a trasactio, T1, requests a lock that is beig held by aother trasactio, T2, with a lower priority, the T1 satches the lock from T2 by abortig T2 (which frees up the lock o the resource). T2 is the restarted agai after a time-out. T1 A B T2 4/25/17 CS 564: Database Maagemet Systems 21

22 Trasactio Support i SQL Trasactio boudary Begi implicitly, or ed by Commit work, Rollback work For log ruig trasactios: Savepoit Trasactio characteristics Diagostic size: # error messages Access mode: Read oly, Read Write Isolatio level Serializable: default Repeatable reads: Read oly ted records Betwee two reads by the same Xact, o updates by aother Xact Read ted Read oly ted records Read uted (ot i the official course syllabus) (log-term R/W locks o phatoms too) (log-term R/W locks o real objects) (log-term W locks/short-term R locks) (Read oly, o R locks!) 4/25/17 CS 564: Database Maagemet Systems 22

23 (ot i the official course syllabus) Phatom Problem T1: Sca Sailors for the oldest sailor for ratigs 1 ad 2 Assume that at the start the oldest sailor with ratig 1 has age 80, oldest sailor with ratig 2 has age 90, ad the secod oldest sailor with ratig 2 is 85 years old T1 idetifies pages with sailors havig a ratig 1, ad locks these pages. It computes the first tuple (ratig = 1, oldest-age = 80) T1 the gets ready to lock pages with sailor tuples with ratig 2. However, before it ca get started, T2 arrives T2: Iserts a tuple with ratig 1 ad age 99, ad deletes the oldest sailor with ratig 2 (whose age is 90) The ew tuple is iserted ito a page that does t have a sailor with ratig 1 or 2, ad is ot locked by T1 T2 s T1 ow resumes ad completes lookig at sailors with ratig 2. The fial aswer produced by T1 is (1,80) (2,85) does ot correspod to either of the two serial schedules: T1 -> T2 Aswer: (1, 80), (2, 90) T2 -> T1 Aswer: (1, 99), (2, 85) 4/25/17 CS 564: Database Maagemet Systems 23

24 (ot i the official course syllabus) Trasactio ad Costraits Create Table A (akey, bref, ) Create Table B (bkey, aref, ) Q: How to isert the first tuple, either i A or B? Solutio: Isert tuples i the same trasactio Defer the costrait checkig SQL costrait modes DEFERRED: Check at time. IMMEDIATE: Check immediately 4/25/17 CS 564: Database Maagemet Systems 24

25 The ACID Properties TM Xact. Mgmt. (loggig) User Atomicity: All actios i the Xact happe, or oe happe. Cosistecy: Cosistet DB + cosistet Xact cosistet DB CC Cocurrecy Ctrl. (lockig) RM Recovery Mgmt. (WAL, ) Isolatio: Executio of oe Xact is isolated from that of other Xacts. Durability: If a Xact s, its effects persist. 4/25/17 CS 564: Database Maagemet Systems 25