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Carnegie Mellon Univ. Dept. of Computer Science 15-415 - Database Applications

Carnegie Mellon Univ. Dept. of Computer Science 15-415 - Database Applications. C. Faloutsos Concurrency control - deadlocks. General Overview. Relational model - SQL Functional Dependencies & Normalization Physical Design &Indexing Query optimization Transaction processing

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Carnegie Mellon Univ. Dept. of Computer Science 15-415 - Database Applications

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  1. Carnegie Mellon Univ.Dept. of Computer Science15-415 - Database Applications C. Faloutsos Concurrency control - deadlocks

  2. General Overview • Relational model - SQL • Functional Dependencies & Normalization • Physical Design &Indexing • Query optimization • Transaction processing • concurrency control • recovery 15-415 - C. Faloutsos

  3. Transactions - dfn = unit of work, eg. move $10 from savings to checking Atomicity (all or none) Consistency Isolation (as if alone) Durability recovery concurrency control 15-415 - C. Faloutsos

  4. Isolation - concurrency control • serializability <-> correctness • precedence graph • automatically correct interleavings: • locks + protocol (2PL, 2PLC) • but: deadlocks! 15-415 - C. Faloutsos

  5. Deadlocks • detection • handling • (prevention) 15-415 - C. Faloutsos

  6. Deadlock detection time T1 T2 lock manager L(A) Yes L(B) Yes ... L(B) No <waits> L(A) No DEADLOCK 15-415 - C. Faloutsos

  7. Algo for deadlock detection? 15-415 - C. Faloutsos

  8. Algo for deadlock detection? • wait-for graph: • nodes -> transactions • arcs -> Tsource waits for Tsink • if cycle, then deadlock! 15-415 - C. Faloutsos

  9. Eg: time T1 T2 lock manager L(A) Yes L(B) Yes ... L(B) No <waits> L(A) No for ‘B’ T1 T2 ‘A’ ‘B’ for ‘A’ 15-415 - C. Faloutsos

  10. T1 T2 T4 T3 Another example • is there a deadlock? • if yes, which xacts are involved? 15-415 - C. Faloutsos

  11. T1 T2 T4 T3 Another example • now, is there a deadlock? • if yes, which xacts are involved? 15-415 - C. Faloutsos

  12. Deadlock detection • how often should we run the algo? • how many transactions are typically involved? 15-415 - C. Faloutsos

  13. T1 T2 T4 T3 Deadlock handling • Q: what to do? 15-415 - C. Faloutsos

  14. T1 T2 T4 T3 Deadlock handling • Q0: what to do? • A: select a ‘victim’ & ‘rollback’ • Q1: which/how to choose? 15-415 - C. Faloutsos

  15. T1 T2 T4 T3 Deadlock handling • Q1: which/how to choose? • A1.1: by age • A1.2: by progress • A1.3: by # items locked already... • A1.4: by # xacts to rollback • Q2: How far to rollback? 15-415 - C. Faloutsos

  16. T1 T2 T4 T3 Deadlock handling • Q2: How far to rollback? • A2.1: completely • A2.2: minimally • Q3: Starvation?? 15-415 - C. Faloutsos

  17. T1 T2 T4 T3 Deadlock handling • Q3: Starvation?? • A3.1: include #rollbacks in victim selection criterion. 15-415 - C. Faloutsos

  18. SQL statement • usually, conc. control is transparent to the user, but • LOCK <table-name> [EXCLUSIVE|SHARED] 15-415 - C. Faloutsos

  19. Concurrency control - conclusions • serializability <-> correctness • automatically correct interleavings: • locks + protocol (2PL, 2PLC, ...) • deadlock detection + handling 15-415 - C. Faloutsos

  20. Quiz: • is there a serial schedule (= interleaving) that is not serializable? • is there a serializable schedule that is not serial? • can 2PL produce a non-serializable schedule? (assume no deadlocks) 15-415 - C. Faloutsos

  21. Quiz - cont’d • is there a serializable schedule that can not be produced by 2PL? • a xact obeys 2PL - can it be involved in a non-serializable schedule? • all xacts obey 2PL - can they end up in a deadlock? 15-415 - C. Faloutsos

  22. Quiz - hints: Q: 2PLC?? serializable schedules serializable schedules 2PL schedules serial sch’s 15-415 - C. Faloutsos

  23. Quiz - hints: 2PL schedules serializable schedules serializable schedules 2PLC serial sch’s 15-415 - C. Faloutsos

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