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ICS 214B: Transaction Processing and Distributed Data Management

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ICS 214B: Transaction Processing and Distributed Data Management. Lecture 2: Enforcing Serializable Schedules Professor Chen Li. 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.

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ICS 214B: Transaction Processing and Distributed Data Management
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  1. ICS 214B: Transaction Processing and Distributed Data Management Lecture 2: Enforcing Serializable Schedules Professor Chen Li

  2. 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 Notes 02

  3. How to enforce serializable schedules? Option 2: prevent P(S) cycles from occurring T1 T2 ….. Tn Scheduler DB Notes 02

  4. A locking protocol Two new actions: lock (exclusive): li (A) unlock: ui (A) T1 T2 lock table scheduler Notes 02

  5. Rule #1: Well-formed transactions Ti: … li(A) … pi(A) … ui(A) ... 1) Lock an element before accessing it 2) Unlock the element after accessing it Notes 02

  6. Rule #2 Legal scheduler S = …….. li(A) ………... ui(A) ……... Once an element A is locked by one transaction Ti, the schedule will not grant any other lock requests on A no lj(A) Notes 02

  7. Exercise • What schedules are legal?What transactions are well-formed? S1 = l1(A)l1(B)r1(A)w1(B)l2(B)u1(A)u1(B) r2(B)w2(B)u2(B)l3(B)r3(B)u3(B) S2 = l1(A)r1(A)w1(B)u1(A)u1(B) l2(B)r2(B)w2(B)l3(B)r3(B)u3(B) S3 = l1(A)r1(A)u1(A)l1(B)w1(B)u1(B) l2(B)r2(B)w2(B)u2(B)l3(B)r3(B)u3(B) Notes 02

  8. Another problem: schedule F is still allowed, but it’s bad! A B T1 T2 25 25 l1(A);Read(A) A A+100;Write(A);u1(A) 125 l2(A);Read(A) A Ax2;Write(A);u2(A) 250 l2(B);Read(B) B Bx2;Write(B);u2(B) 50 l1(B);Read(B) B B+100;Write(B);u1(B) 150 250 150 Notes 02

  9. no unlocks no locks Rule #3 Two phase locking (2PL) Ti = ……. li(A) ………... ui(A) ……... A transaction once releases a lock, it cannot request more locks. Notes 02

  10. # locks held by Ti Time Growing Shrinking Phase Phase Notes 02

  11. Schedule G T1 T2 l1(A);Read(A) A A+100;Write(A) l1(B); u1(A) l2(A);Read(A) A Ax2;Write(A);l2(B) request delayed Notes 02

  12. Schedule G T1 T2 l1(A);Read(A) A A+100;Write(A) l1(B); u1(A) l2(A);Read(A) A Ax2;Write(A);l2(B) Read(B);B B+100 Write(B); u1(B) request delayed Notes 02

  13. Schedule G T1 T2 l1(A);Read(A) A A+100;Write(A) l1(B); u1(A) l2(A);Read(A) A Ax2;Write(A);l2(B) Read(B);B B+100 Write(B); u1(B) l2(B); u2(A);Read(B) B Bx2;Write(B);u2(B); request delayed Notes 02

  14. Schedule H (T2 reversed) T1 T2 l1(A); Read(A) l2(B);Read(B) A A+100;Write(A) B Bx2;Write(B) l1(B) l2(A) Deadlock! delayed delayed Notes 02

  15. Solution • Assume deadlocked transactions are rolled back • They have no effect • They do not appear in schedule Notes 02

  16. Next step: Show that rules #1,2,3  conflict- serializable schedules Notes 02

  17. New Conflict rules for li(A), ui(A): • Check the semantics of the operations, e.g., • li(A), uj(A) conflict • li(A), ri(A) conflict; li(A), wi(A) conflict • ri(A), ui(A) conflict; wi(A), ui(A) conflict • Some actions can never become adjacent after legal swaps • <ui(A), uj(A)>, <li(A), rj(A)>,... Example: l1(A)r1(A)l1(B)u1(A)l2(A)w2(A)w1(B)u2(A)u1(B) Notes 02

  18. Theorem Rules #1,2,3  conflict (2PL) serializable schedule To help in proof: Definition Shrink(Ti) = SH(Ti) = first unlock action of Ti Notes 02

  19. By rule 3: SH(Ti) SH(Tj) So, SH(Ti) <S SH(Tj) Proof of lemma: Ti  Tj means that S = … pi(A) … qj(A) …; p,q conflict By rules 1,2: ui(A) <S lj(A) S = … pi(A) … ui(A) … lj(A) ... qj(A) … Lemma Ti  Tj in S  SH(Ti) <S SH(Tj) Notes 02

  20. Theorem Rules #1,2,3  conflict (2PL) serializable schedule Proof: (1) Assume P(S) has cycle T1 T2 …. Tn T1 (2) By lemma: SH(T1) <SH(T2) < ... <SH(T1) (3) Impossible, so P(S) acyclic (4)  S is conflict serializable Notes 02

  21. 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 concurrency control mechanisms Notes 02

  22. Shared locks S = ...l1(A) r1(A) u1(A) … l2(A) r2(A) u2(A) … read only, noconflict Use “shared” lock: S=... ls1(A) r1(A) ls2(A) r2(A) …. us1(A) us2(A) Notes 02

  23. Lock actions l-ti(A): lock A in t mode (t is S or X) u-ti(A): unlock t mode (t is S or X) “S” = “Shared”, “X” = “Exclusive” Shorthand: ui(A): unlock whatever modes Ti has locked A Notes 02

  24. Rule #1 Well formed transactions Ti =... l-S1(A) … r1(A) …u1 (A) … Ti =... l-X1(A) … w1(A) …u1 (A) … Lock an element before accessing it Release the lock afterwards Notes 02

  25. What about transactions that read and write same object? Option 1: Request exclusive lock Ti = ...l-X1(A) … r1(A) ... w1(A) ... u(A) … Notes 02

  26. Option 2: Upgrade (E.g., need to read, but don’t know if will write…) Ti=... l-S1(A) … r1(A) ...l-X1(A) …w1(A) ...u(A)… Think of - Get 2nd lock on A, or - Drop S, get X lock Notes 02

  27. Rule #2 Legal scheduler S = ....l-Si(A) … … ui(A) … l-Sj(A) OK, but no l-Xj(A) S = ... l-Xi(A) … … ui(A) … no l-Xj(A) no l-Sj(A) Notes 02

  28. A way to summarize Rule #2 Compatibility matrix: “comp” S X S true false X false false Notes 02

  29. Rule # 3 2PL transactions No change: • Locking must precede unlocking, i.e., • No action sli(X) or xli(X) can be preceded by ui(X) Notes 02

  30. Theorem Rules 1,2,3  Conf.serializable for S/X locks schedules Notes 02