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More Transaction Management

More Transaction Management. Concurrency control and recovery Resolving deadlocks Logical logging. Source: slides by Hector Garcia-Molina. Concurrency control & recovery. …. …. Example: T j T i w j (A) r i (A) Commit T i Abort T j. …. …. ….

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More Transaction Management

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  1. More Transaction Management Concurrency control and recovery Resolving deadlocks Logical logging Source: slides by Hector Garcia-Molina

  2. Concurrency control & recovery … … Example: Tj Ti wj(A) ri(A) Commit Ti Abort Tj … … … …  Cascading rollback (Bad!)

  3. Schedule is conflict serializable: Tj Ti • But not recoverable

  4. Need to make “final" decision for each transaction: • commit decision - system guarantees transaction will or has completed, no matter what • abort decision - system guarantees transaction will or has been rolled back (has no effect)

  5. To model this, two new actions: • Ci - transaction Ti commits • Ai - transaction Ti aborts • Each transaction Ti has exactly one, either Ci or Ai • Every read and write of Ti precedes the Ci (or Ai)

  6. Back to example: Tj Ti Wj(A) ri(A) Cican wecommit here? ... ... ... ...

  7. Definition Ti reads from Tj in S (denoted Tj STi) if • there is a write wj(A) in S that is followed by a readri(A) in S • there is no abort of Tj in S before the ri(A) in S • if there is a write to A in S between wj(A) and ri(A) by another transaction Tk, then Tk aborts before the ri(A)

  8. Definition Schedule S is recoverable if whenever Ti reads from another transaction Tj, Ti does not commit until after Tj commits.

  9. How to achieve recoverable schedules?

  10.  With 2PL, hold write locks until transaction commits (strict 2PL) Tj Ti Wj(A) Cj uj(A) ri(A) ... ... ... ... ... ... ...

  11. S is recoverable if each transaction commits only after all transactions from which it read have committed. • S avoids cascading rollback if each transaction may read only those values written by committed transactions.

  12. S is strict if each transaction may read and write only items previously written by committed transactions. RC Avoids cascading rollback ST SERIAL ACR

  13. RC Avoids cascading rollback ST SERIAL ACR Where are serializable schedules? SERIALIZABLE

  14. Examples • Recoverable: • w1(A) w1(B)w2(A) r2(B)c1c2 • Avoids Cascading Rollback: • w1(A) w1(B)w2(A)c1r2(B)c2 • Strict: • w1(A) w1(B) c1w2(A) r2(B) c2

  15. RC Avoids cascading rollback ST SERIAL ACR More Examples w2(A) w1(B) w1(A) r2(B) c1 c2 SERIALIZABLE w1(A) w1(B)w2(A) r2(B) c2c1

  16. Deadlocks • Detection • Wait-for graph • Prevention • Resource ordering • Timeout • Wait-die • Wound-wait

  17. Deadlock Detection • Build Wait-For graph • Use lock table structures • Build incrementally or periodically • When cycle found, rollback victim T5 T2 T1 T7 T4 T6 T3

  18. Resource Ordering • Order all elements A1, A2, …, An • Each transaction must acquire locks on elements in this order (i.e., T cannot lock Aj before Ai if j > i) Problem : Ordered lock requests not realistic in most cases

  19. Timeout • If transaction waits more than L sec., roll it back! • Simple scheme • Hard to select L

  20. Wait-die • Each transaction Ti is given a timestamp when it starts, denoted ts(Ti) • Suppose Ti requests a lock currently held by Tj • if ts(Ti)< ts(Tj) then Ti waits for Tj(older waits for younger) else Ti dies (aborts) (younger dies) • If Ti dies then it later restarts with the same timestamp

  21. Example: T1 (ts =10) T2 (ts =20) T3 (ts =25) wait wait? wait No.

  22. Wound-wait • Each transaction Ti is given a timestamp when it starts, denoted ts(Ti) • Suppose Ti requests a lock currently held by Tj • If ts(Ti)< ts(Tj) then Ti wounds Tj (younger yields lock to older) else Ti waits (younger waits for older) “Wound”: Tj rolls back and gives lock to Ti • If Tj dies then later it restarts with the same timestamp

  23. Example: T1 (ts =25) T2 (ts =20) T3 (ts =10) wait wait? No. wait (Note that timestamps are different than in previous example)

  24. Comparing Deadlock Management Schemes • Wait-die and Wound-wait ensure no starvation (unlike the others) • Wait-die tends to roll back more transactions than Wound-wait but they tend to have done less work • Wait-die and Wound-wait are easier to implement than waits-for graph • Waits-for graph technique only aborts transactions if there really is a deadlock (unlike the others)

  25. Managing Rollbacks Using Locking What are we really locking?

  26. Example: ... Ti Read record r1 Read record r1 do record locking Modify record r3 ... ... ...

  27. If we lock all data involved in read of R1, we may prevent an update to R2 (which may require reorganization within block) But underneath: record id R1 R2 R3 Disk pages

  28. Solution: view DB at two levels Top level: record actions record locks undo/redo actions — logical e.g., Insert record(X,Y,Z) Redo: insert(X,Y,Z) Undo: delete Low level: deal with physical details

  29. Note: undo does not return physical DB to original state; only same logical state e.g., Insert R3 Undo (delete R3) R1 R1 R1 R2 R2 R2 R3

  30. Logging Logical Actions • Logical action typically span one block • Undo/redo log entry specifies undo/redo logical action • Challenge: making actions idempotent • Example (bad): redo insert key inserted multiple times!

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