1. CS 277 – Spring 2002Notes 091 CS 277: Database System Implementation Notes 09: Concurrency Control Arthur Keller

2. CS 277 – Spring 2002Notes 092 Chapter 18Concurrency Control T1T2…Tn DB (consistency constraints)

3. CS 277 – Spring 2002Notes 093 Example: T1:Read(A)T2:Read(A) A  A+100A  A  2Write(A)Read(B) B  B+100B  B  2Write(B) Constraint: A=B

4. CS 277 – Spring 2002Notes 094 Schedule A T1T2 Read(A); A  A+100 Write(A); Read(B); B  B+100; Write(B); Read(A);A  A  2; Write(A); Read(B);B  B  2; Write(B); AB25 125 250 250

5. CS 277 – Spring 2002Notes 095 Schedule B T1T2 Read(A);A  A  2; Write(A); Read(B);B  B  2; Write(B); Read(A); A  A+100 Write(A); Read(B); B  B+100; Write(B); AB25 50 150 150

6. CS 277 – Spring 2002Notes 096 Schedule C T1T2 Read(A); A  A+100 Write(A); Read(A);A  A  2; Write(A); Read(B); B  B+100; Write(B); Read(B);B  B  2; Write(B); AB25 125 250 125 250250

7. CS 277 – Spring 2002Notes 097 Schedule D T1T2 Read(A); A  A+100 Write(A); Read(A);A  A  2; Write(A); Read(B);B  B  2; Write(B); Read(B); B  B+100; Write(B); AB25 125 250 50 150 250150

8. CS 277 – Spring 2002Notes 098 Schedule E T1T2’ Read(A); A  A+100 Write(A); Read(A);A  A  1; Write(A); Read(B);B  B  1; Write(B); Read(B); B  B+100; Write(B); AB25 125 25 125125 Same as Schedule D but with new T2’

9. CS 277 – Spring 2002Notes 099 Want schedules that are “good”, regardless of –initial state and –transaction semantics Only look at order of read and writes Example: Sc=r 1 (A)w 1 (A)r 2 (A)w 2 (A)r 1 (B)w 1 (B)r 2 (B)w 2 (B)

10. CS 277 – Spring 2002Notes 0910 Sc’=r 1 (A)w 1 (A) r 1 (B)w 1 (B)r 2 (A)w 2 (A)r 2 (B)w 2 (B) T 1 T 2 Example: Sc=r 1 (A)w 1 (A)r 2 (A)w 2 (A)r 1 (B)w 1 (B)r 2 (B)w 2 (B)

11. CS 277 – Spring 2002Notes 0911 However, for Sd: Sd=r 1 (A)w 1 (A)r 2 (A)w 2 (A) r 2 (B)w 2 (B)r 1 (B)w 1 (B) as a matter of fact, T 2 must precede T 1 in any equivalent schedule, i.e., T 2  T 1

12. CS 277 – Spring 2002Notes 0912 T 1 T 2 Sd cannot be rearranged into a serial schedule Sd is not “equivalent” to any serial schedule Sd is “bad” T 2  T 1 Also, T 1  T 2

13. CS 277 – Spring 2002Notes 0913 Returning to Sc Sc=r 1 (A)w 1 (A)r 2 (A)w 2 (A)r 1 (B)w 1 (B)r 2 (B)w 2 (B) T 1  T 2 T 1  T 2  no cycles  Sc is “equivalent” to a serial schedule (in this case T 1,T 2 )

14. CS 277 – Spring 2002Notes 0914 Concepts Transaction: sequence of r i (x), w i (x) actions Conflicting actions: r 1(A) w 2(A) w 1(A) w 2(A) r 1(A) w 2(A) Schedule: represents chronological order in which actions are executed Serial schedule: no interleaving of actions or transactions

15. CS 277 – Spring 2002Notes 0915 What about concurrent actions? Ti issuesSystemInput(X) t  x read(x,t)issuescompletes input(x) time T2 issues write(B,S) System issues input(B) completes B  S System issues output(B) completes

16. CS 277 – Spring 2002Notes 0916 So net effect is either S=…r 1 (x)…w 2 (b)… or S=…w 2 (B)…r 1 (x)…

17. CS 277 – Spring 2002Notes 0917 Assume equivalent to either r 1 (A) w 2 (A) orw 2 (A) r 1 (A)  low level synchronization mechanism Assumption called “atomic actions” What about conflicting, concurrent actions on same object? start r 1 (A)end r 1 (A) start w 2 (A) end w 2 (A) time

18. CS 277 – Spring 2002Notes 0918 Definition S 1, S 2 are conflict equivalent schedules if S 1 can be transformed into S 2 by a series of swaps on non-conflicting actions.

19. CS 277 – Spring 2002Notes 0919 Definition A schedule is conflict serializable if it is conflict equivalent to some serial schedule.

20. CS 277 – Spring 2002Notes 0920 Nodes: transactions in S Arcs: Ti  Tj whenever - p i (A), q j (A) are actions in S - p i (A) < S q j (A) - at least one of p i, q j is a write Precedence graph P(S) (S is schedule )

21. CS 277 – Spring 2002Notes 0921 Exercise: What is P(S) for S = w 3 (A) w 2 (C) r 1 (A) w 1 (B) r 1 (C) w 2 (A) r 4 (A) w 4 (D) Is S serializable?

22. CS 277 – Spring 2002Notes 0922 Lemma S 1, S 2 conflict equivalent  P(S 1 )=P(S 2 ) Proof: Assume P(S 1 )  P(S 2 )   T i : T i  T j in S 1 and not in S 2  S 1 = …p i (A)... q j (A)… p i, q j S 2 = …q j (A)…p i (A)... conflict  S 1, S 2 not conflict equivalent

23. CS 277 – Spring 2002Notes 0923 Note: P(S 1 )=P(S 2 )  S 1, S 2 conflict equivalent Counter example: S 1 =w 1 (A) r 2 (A) w 2 (B) r 1 (B) S 2 =r 2 (A) w 1 (A) r 1 (B) w 2 (B)

24. CS 277 – Spring 2002Notes 0924 Theorem P(S 1 ) acyclic  S 1 conflict serializable (  ) Assume S 1 is conflict serializable   S s : S s, S 1 conflict equivalent  P(S s ) = P(S 1 )  P(S 1 ) acyclic since P(S s ) is acyclic

25. CS 277 – Spring 2002Notes 0925 (  ) Assume P(S 1 ) is acyclic Transform S 1 as follows: (1) Take T 1 to be transaction with no incident arcs (2) Move all T 1 actions to the front S 1 = ……. q j (A)……. p 1 (A)….. (3) we now have S 1 = (4) repeat above steps to serialize rest! T 1 T 2 T 3 T 4 Theorem P(S 1 ) acyclic  S 1 conflict serializable

26. CS 277 – Spring 2002Notes 0926 How to enforce serializable schedules? Option 1: run system, recording P(S); at end of day, check for P(S) cycles and declare if execution was good

27. CS 277 – Spring 2002Notes 0927 Option 2: prevent P(S) cycles from occurring T 1 T 2 …..T n Scheduler DB How to enforce serializable schedules?

28. CS 277 – Spring 2002Notes 0928 A locking protocol Two new actions: lock (exclusive):l i (A) unlock:u i (A) scheduler T 1 T 2 lock table

29. CS 277 – Spring 2002Notes 0929 Rule #1: Well-formed transactions T i : … l i (A) … p i (A) … u i (A)...

30. CS 277 – Spring 2002Notes 0930 Rule #2 Legal scheduler S = …….. l i (A) ………... u i (A) ……... no l j (A)

31. CS 277 – Spring 2002Notes 0931 What schedules are legal? What transactions are well-formed? S1 = l 1 (A)l 1 (B)r 1 (A)w 1 (B)l 2 (B)u 1 (A)u 1 (B) r 2 (B)w 2 (B)u 2 (B)l 3 (B)r 3 (B)u 3 (B) S2 = l 1 (A)r 1 (A)w 1 (B)u 1 (A)u 1 (B) l 2 (B)r 2 (B)w 2 (B)l 3 (B)r 3 (B)u 3 (B) S3 = l 1 (A)r 1 (A)u 1 (A)l 1 (B)w 1 (B)u 1 (B) l 2 (B)r 2 (B)w 2 (B)u 2 (B)l 3 (B)r 3 (B)u 3 (B) Exercise:

32. CS 277 – Spring 2002Notes 0932 What schedules are legal? What transactions are well-formed? S1 = l 1 (A)l 1 (B)r 1 (A)w 1 (B)l 2 (B)u 1 (A)u 1 (B) r 2 (B)w 2 (B)u 2 (B)l 3 (B)r 3 (B)u 3 (B) S2 = l 1 (A)r 1 (A)w 1 (B)u 1 (A)u 1 (B) l 2 (B)r 2 (B)w 2 (B)l 3 (B)r 3 (B)u 3 (B) S3 = l 1 (A)r 1 (A)u 1 (A)l 1 (B)w 1 (B)u 1 (B) l 2 (B)r 2 (B)w 2 (B)u 2 (B)l 3 (B)r 3 (B)u 3 (B) Exercise:

33. CS 277 – Spring 2002Notes 0933 Schedule F T1 T2 l 1 (A);Read(A) A A+100;Write(A);u 1 (A) l 2 (A);Read(A) A Ax2;Write(A);u 2 (A) l 2 (B);Read(B) B Bx2;Write(B);u 2 (B) l 1 (B);Read(B) B B+100;Write(B);u 1 (B)

34. CS 277 – Spring 2002Notes 0934 Schedule F T1 T2 25 25 l 1 (A);Read(A) A A+100;Write(A);u 1 (A) 125 l 2 (A);Read(A) A Ax2;Write(A);u 2 (A) 250 l 2 (B);Read(B) B Bx2;Write(B);u 2 (B) 50 l 1 (B);Read(B) B B+100;Write(B);u 1 (B) 150 250 150 A B

35. CS 277 – Spring 2002Notes 0935 Rule #3 Two phase locking (2PL) for transactions T i = ……. l i (A) ………... u i (A) ……... no unlocks no locks

36. CS 277 – Spring 2002Notes 0936 # locks held by Ti Time Growing Shrinking Phase Phase

37. CS 277 – Spring 2002Notes 0937 Schedule G delayed

38. CS 277 – Spring 2002Notes 0938 Schedule G delayed

39. CS 277 – Spring 2002Notes 0939 Schedule G delayed

40. CS 277 – Spring 2002Notes 0940 Schedule H (T 2 reversed) delayed

41. CS 277 – Spring 2002Notes 0941 Assume deadlocked transactions are rolled back –They have no effect –They do not appear in schedule E.g., Schedule H = This space intentionally left blank!

42. CS 277 – Spring 2002Notes 0942 Next step: Show that rules #1,2,3  conflict- serializable schedules

43. CS 277 – Spring 2002Notes 0943 Conflict rules for l i (A), u i (A): l i (A), l j (A) conflict l i (A), u j (A) conflict Note: no conflict,,...

44. CS 277 – Spring 2002Notes 0944 Theorem Rules #1,2,3  conflict (2PL) serializable schedule To help in proof: Definition Shrink(Ti) = SH(Ti) = first unlock action of Ti

45. CS 277 – Spring 2002Notes 0945 Lemma Ti  Tj in S  SH(Ti) < S SH(Tj) Proof of lemma: Ti  Tj means that S = … p i (A) … q j (A) …; p,q conflict By rules 1,2: S = … p i (A) … u i (A) … l j (A)... q j (A) … By rule 3: SH(Ti) SH(Tj) So, SH(Ti) < S SH(Tj)

46. CS 277 – Spring 2002Notes 0946 Proof: (1) Assume P(S) has cycle T 1  T 2  …. T n  T 1 (2) By lemma: SH(T 1 ) < SH(T 2 ) <... < SH(T 1 ) (3) Impossible, so P(S) acyclic (4)  S is conflict serializable Theorem Rules #1,2,3  conflict (2PL) serializable schedule

47. CS 277 – Spring 2002Notes 0947 Beyond this simple 2PL protocol, it is all a matter of improving performance and allowing more concurrency…. –Shared locks –Multiple granularity –Inserts, deletes and phantoms –Other types of C.C. mechanisms

48. CS 277 – Spring 2002Notes 0948 Shared locks So far: S =...l 1 (A) r 1 (A) u 1 (A) … l 2 (A) r 2 (A) u 2 (A) … Do not conflict Instead: S=... ls 1 (A) r 1 (A) ls 2 (A) r 2 (A) …. us 1 (A) us 2 (A)

49. CS 277 – Spring 2002Notes 0949 Lock actions l-t i (A): lock A in t mode (t is S or X) u-t i (A): unlock t mode (t is S or X) Shorthand: u i (A): unlock whatever modes T i has locked A

50. CS 277 – Spring 2002Notes 0950 Rule #1 Well formed transactions T i =... l-S 1 (A) … r 1 (A) … u 1 (A) … T i =... l-X 1 (A) … w 1 (A) … u 1 (A) …

51. CS 277 – Spring 2002Notes 0951 What about transactions that read and write same object? Option 1: Request exclusive lock T i =...l-X 1 (A) … r 1 (A)... w 1 (A)... u(A) …

52. CS 277 – Spring 2002Notes 0952 Option 2: Upgrade (E.g., need to read, but don’t know if will write…) T i =... l-S 1 (A) … r 1 (A)... l-X 1 (A) …w 1 (A)...u(A)… Think of - Get 2nd lock on A, or - Drop S, get X lock What about transactions that read and write same object?

53. CS 277 – Spring 2002Notes 0953 Rule #2 Legal scheduler S =....l-S i (A) … … u i (A) … no l-X j (A) S =... l-X i (A) … … u i (A) … no l-X j (A) no l-S j (A)

54. CS 277 – Spring 2002Notes 0954 A way to summarize Rule #2 Compatibility matrix Comp S X S true false Xfalse false

55. CS 277 – Spring 2002Notes 0955 Rule # 3 2PL transactions No change except for upgrades: (I) If upgrade gets more locks (e.g., S  {S, X}) then no change! (II) If upgrade releases read (shared) lock (e.g., S  X) - can be allowed in growing phase

56. CS 277 – Spring 2002Notes 0956 Proof: similar to X locks case Detail: l-t i (A), l-r j (A) do not conflict if comp(t,r) l-t i (A), u-r j (A) do not conflict if comp(t,r) Theorem Rules 1,2,3  Conf.serializable for S/X locks schedules

57. CS 277 – Spring 2002Notes 0957 Lock types beyond S/X Examples: (1) increment lock (2) update lock

58. CS 277 – Spring 2002Notes 0958 Example (1): increment lock Atomic increment action: IN i (A) {Read(A); A  A+k; Write(A)} IN i (A), IN j (A) do not conflict! A=7 A=5A=17 A=15 IN i (A) +2 IN j (A) +10 IN j (A) +2 IN i (A)

59. CS 277 – Spring 2002Notes 0959 CompSXI S X I

60. CS 277 – Spring 2002Notes 0960 CompSXI STFF XFFF IFFT

61. CS 277 – Spring 2002Notes 0961 Update locks

62. CS 277 – Spring 2002Notes 0962 Solution If T i wants to read A and knows it may later want to write A, it requests update lock (not shared)

63. CS 277 – Spring 2002Notes 0963 CompSXU S X U New request Lock already held in

64. CS 277 – Spring 2002Notes 0964 CompSXU STFT XFFF U TorF FF -> symmetric table? New request Lock already held in

65. CS 277 – Spring 2002Notes 0965 Note: object A may be locked in different modes at the same time... S 1 =...l-S 1 (A)…l-S 2 (A)…l-U 3 (A)… l-S 4 (A)…? l- U 4 (A)…? To grant a lock in mode t, mode t must be compatible with all currently held locks on object

66. CS 277 – Spring 2002Notes 0966 How does locking work in practice? Every system is different (E.g., may not even provide CONFLICT-SERIALIZABLE schedules) But here is one (simplified) way...

67. CS 277 – Spring 2002Notes 0967 (1) Don’t trust transactions to request/release locks (2) Hold all locks until transaction commits # locks time Sample Locking System:

68. CS 277 – Spring 2002Notes 0968 Ti Read(A),Write(B) l(A),Read(A),l(B),Write(B)… Read(A),Write(B) Scheduler, part I Scheduler, part II DB lock table

69. CS 277 – Spring 2002Notes 0969 Lock table Conceptually A  B C ... Lock info for B Lock info for C If null, object is unlocked Every possible object

70. CS 277 – Spring 2002Notes 0970 But use hash table: A If object not found in hash table, it is unlocked Lock info for A A... H

71. CS 277 – Spring 2002Notes 0971 Lock info for A - example tran mode wait? Nxt T_link Object:A Group mode:U Waiting:yes List: T1 S no T2 U no T3X yes  To other T3 records

72. CS 277 – Spring 2002Notes 0972 What are the objects we lock? ? Relation A Relation B... Tuple A Tuple B Tuple C... Disk block A Disk block B... DB

73. CS 277 – Spring 2002Notes 0973 Locking works in any case, but should we choose small or large objects? If we lock large objects (e.g., Relations) –Need few locks –Low concurrency If we lock small objects (e.g., tuples,fields) –Need more locks –More concurrency

74. CS 277 – Spring 2002Notes 0974 We can have it both ways!! Ask any janitor to give you the solution... hall Stall 1Stall 2Stall 3Stall 4 restroom

75. CS 277 – Spring 2002Notes 0975 Example R1 t1t1 t2t2 t3t3 t4t4 T 1 (IS) T 1 (S), T 2 (S)

76. CS 277 – Spring 2002Notes 0976 Example R1 t1t1 t2t2 t3t3 t4t4 T 1 (IS) T 1 (S), T 2 (IX) T 2 (IX)

77. CS 277 – Spring 2002Notes 0977 Multiple granularity CompRequestor IS IX S SIX X IS Holder IX S SIX X

78. CS 277 – Spring 2002Notes 0978 Multiple granularity CompRequestor IS IX S SIX X IS Holder IX S SIX X TTTTF F F F FFFFF FFFT FTFT FFTT

79. CS 277 – Spring 2002Notes 0979 ParentChild can belocked in IS IX S SIX X P C

80. CS 277 – Spring 2002Notes 0980 ParentChild can belocked in IS IX S SIX X P C IS, S IS, S, IX, X, SIX [S, IS] not necessary X, IX, [SIX] none

81. CS 277 – Spring 2002Notes 0981 Rules (1) Follow multiple granularity comp function (2) Lock root of tree first, any mode (3) Node Q can be locked by Ti in S or IS only if parent(Q) locked by Ti in IX or IS (4) Node Q can be locked by Ti in X,SIX,IX only if parent(Q) locked by Ti in IX,SIX (5) Ti is two-phase (6) Ti can unlock node Q only if none of Q’s children are locked by Ti

82. CS 277 – Spring 2002Notes 0982 Exercise: Can T 2 access object f 2.2 in X mode? What locks will T 2 get? R1 t1t1 t2t2 t3t3 t4t4 T 1 (IX) f 2.1 f 2.2 f 3.1 f 3.2 T 1 (IX) T 1 (X)

83. CS 277 – Spring 2002Notes 0983 Exercise: Can T 2 access object f 2.2 in X mode? What locks will T 2 get? R1 t1t1 t2t2 t3t3 t4t4 T 1 (X) f 2.1 f 2.2 f 3.1 f 3.2 T 1 (IX)

84. CS 277 – Spring 2002Notes 0984 Exercise: Can T 2 access object f 3.1 in X mode? What locks will T 2 get? R1 t1t1 t2t2 t3t3 t4t4 T 1 (S) f 2.1 f 2.2 f 3.1 f 3.2 T 1 (IS)

85. CS 277 – Spring 2002Notes 0985 Exercise: Can T 2 access object f 2.2 in S mode? What locks will T 2 get? R1 t1t1 t2t2 t3t3 t4t4 T 1 (IX) f 2.1 f 2.2 f 3.1 f 3.2 T 1 (SIX) T 1 (X)

86. CS 277 – Spring 2002Notes 0986 Exercise: Can T 2 access object f 2.2 in X mode? What locks will T 2 get? R1 t1t1 t2t2 t3t3 t4t4 T 1 (IX) f 2.1 f 2.2 f 3.1 f 3.2 T 1 (SIX) T 1 (X)

87. CS 277 – Spring 2002Notes 0987 Insert + delete operations Insert A Z ...

88. CS 277 – Spring 2002Notes 0988 Modifications to locking rules: (1) Get exclusive lock on A before deleting A (2) At insert A operation by Ti, Ti is given exclusive lock on A

89. CS 277 – Spring 2002Notes 0989 Phantoms Still have a problem: Phantoms Example: relation R (E#,name,…) constraint: E# is key use tuple locking RE#Name…. o155Smith o275Jones

90. CS 277 – Spring 2002Notes 0990 T 1 : Insert into R T 2 : Insert into R T 1 T 2 S 1 (o 1 ) S 2 (o 1 ) S 1 (o 2 ) S 2 (o 2 ) Check Constraint Insert o 3 [99,Gore,..] Insert o 4 [99,Bush,..]...

91. CS 277 – Spring 2002Notes 0991 Solution Use multiple granularity tree Before insert of node Q, lock parent(Q) in X mode R1 t1t1 t2t2 t3t3

92. CS 277 – Spring 2002Notes 0992 Back to example T 1 : Insert T 2 : Insert T 1 T 2 X 1 (R) Check constraint Insert U(R) X 2 (R) Check constraint Oops! e# = 99 already in R! delayed

93. CS 277 – Spring 2002Notes 0993 Instead of using R, can use index on R: Example: R Index 0<E#<100 Index 100<E#<200 E#=2E#=5 E#=107 E#=109...

94. CS 277 – Spring 2002Notes 0994 This approach can be generalized to multiple indexes...

95. CS 277 – Spring 2002Notes 0995 Next: Tree-based concurrency control Validation concurrency control

96. CS 277 – Spring 2002Notes 0996 Example A B C D EF all objects accessed through root, following pointers T 1 lock  can we release A lock if we no longer need A??

97. CS 277 – Spring 2002Notes 0997 Idea: traverse like “Monkey Bars” A B C D EF T 1 lock

98. CS 277 – Spring 2002Notes 0998 Why does this work? Assume all T i start at root; exclusive lock T i  T j  T i locks root before T j Actually works if we don’t always start at root Root Q T i  T j

99. CS 277 – Spring 2002Notes 0999 Rules: tree protocol (exclusive locks) (1) First lock by T i may be on any item (2) After that, item Q can be locked by T i only if parent(Q) locked by T i (3) Items may be unlocked at any time (4) After T i unlocks Q, it cannot relock Q

100. CS 277 – Spring 2002Notes 09100 Tree-like protocols are used typically for B-tree concurrency control E.g., during insert, do not release parent lock, until you are certain child does not have to split Root

101. CS 277 – Spring 2002Notes 09101 Validation Transactions have 3 phases: (1) Read –all DB values read –writes to temporary storage –no locking (2) Validate –check if schedule so far is serializable (3) Write –if validate ok, write to DB

102. CS 277 – Spring 2002Notes 09102 Key idea Make validation atomic If T 1, T 2, T 3, … is validation order, then resulting schedule will be conflict equivalent to S s = T 1 T 2 T 3...

103. CS 277 – Spring 2002Notes 09103 To implement validation, system keeps two sets: FIN = transactions that have finished phase 3 (and are all done) VAL = transactions that have successfully finished phase 2 (validation)

104. CS 277 – Spring 2002Notes 09104 Example of what validation must prevent: RS(T 2 )={B} RS(T 3 )={A,B} WS(T 2 )={B,D} WS(T 3 )={C} time T 2 start T 2 validated T 3 validated T 3 start  = 

105. CS 277 – Spring 2002Notes 09105 T 2 finish phase 3 Example of what validation must prevent: RS(T 2 )={B} RS(T 3 )={A,B} WS(T 2 )={B,D} WS(T 3 )={C} time T 2 start T 2 validated T 3 validated T 3 start  =  allow T 3 start

106. CS 277 – Spring 2002Notes 09106 Another thing validation must prevent: RS(T 2 )={A} RS(T 3 )={A,B} WS(T 2 )={D,E} WS(T 3 )={C,D} time T 2 validated T 3 validated finish T 2 BAD: w 3 (D) w 2 (D)

107. CS 277 – Spring 2002Notes 09107 finish T 2 Another thing validation must prevent: RS(T 2 )={A} RS(T 3 )={A,B} WS(T 2 )={D,E} WS(T 3 )={C,D} time T 2 validated T 3 validated allow finish T 2

108. CS 277 – Spring 2002Notes 09108 Validation rules for T j : (1) When T j starts phase 1: ignore(T j )  FIN (2) at T j Validation: if check (T j ) then [ VAL  VAL U {T j }; do write phase; FIN  FIN U {T j } ]

109. CS 277 – Spring 2002Notes 09109 Check (T j ): For T i  VAL - IGNORE (T j ) DO IF [ WS(T i )  RS(T j )   OR T i  FIN ] THEN RETURN false; RETURN true; Is this check too restrictive ?

110. CS 277 – Spring 2002Notes 09110 Improving Check(T j ) For T i  VAL - IGNORE (T j ) DO IF [ WS(T i )  RS(T j )   OR ( T i  FIN AND WS(T i )  WS(T j )   )] THEN RETURN false; RETURN true;

111. CS 277 – Spring 2002Notes 09111 Exercise: T: RS(T)={A,B} WS(T)={A,C} V: RS(V)={B} WS(V)={D,E} U: RS(U)={B} WS(U)={D} W: RS(W)={A,D} WS(W)={A,C} start validate finish

112. CS 277 – Spring 2002Notes 09112 Validation (also called optimistic concurrency control) is useful in some cases: - Conflicts rare - System resources plentiful - Have real time constraints

113. CS 277 – Spring 2002Notes 09113 Summary Have studied C.C. mechanisms used in practice - 2 PL - Multiple granularity - Tree (index) protocols - Validation