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Consistency Conditions for STM

Consistency Conditions for STM. Sandeep Hans. Agenda. Database Consistency Conditions STM Consistency Conditions A different perspective Consistency with other STM properties. Conclusion. Database Consistency Conditions. Recoverability Avoiding Cascading Aborts Strictness Rigorousness.

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Consistency Conditions for STM

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  1. Consistency Conditions for STM Sandeep Hans

  2. Agenda • Database Consistency Conditions • STM Consistency Conditions • A different perspective • Consistency with other STM properties. • Conclusion

  3. Database Consistency Conditions • Recoverability • Avoiding Cascading Aborts • Strictness • Rigorousness

  4. Dirty Reads • T1 writes x. • T2 reads x written by T1 . • T2 commits, T1 aborts. w1(x) a T1 r2(x) c2 T2

  5. Recoverability • T1 writes x. • T2 reads x written by T1 . • T1 should commit beforeT2 commits. w1(x) c1 T1 r2(x) c2 T2

  6. Problem with Recoverability • Cascading aborts w1(x) c1 a1 T1 r2(x) c2 a2 T2

  7. Avoiding Cascading Aborts • T1 should commit beforeT2 commits reads x. w1(x) c1 T1 r2(x) c2 T2

  8. Undo Problem • Initially, x=1. • T1 writes x=2. • T2 writes x=3 and commits. • T1 aborts. w1(x=2) a1 T1 w2(x=3) c2 T2

  9. Strictness • No data item is read or overwritten unless the transaction that wrote it has ended. w1(x) c1/a1 T1 r2(x) T2 w3(x) T3

  10. Rigorousness • No data item is read or overwritten unless the transaction that read/wrote it has ended. w1(x) c1/a1 T1 r2(x) c2/a2 T2 w3(x) T3

  11. Landscape RC ACA ST RG [Transactional Information Systems. Gerhard Weikum, Gottfried Vossen.]

  12. STM Conditions • Opacity • Guerraoui and Kapalka[PPoPP’08] • Sequential specification of shared objects. • Virtual World Consistency • Imbs and Raynal [SIROCCO’09] • Causal past of a transaction. • Weakest Reasonable Condition • Doherty , Groves, Luchangco, Moir[REFINE’09]

  13. Database vs. STM • Rigorousness ⊆ Opacity ⊂ Strictness • Rigorousness ⊆ VWC ⊂ Strictness ⊆ ⊆ Are these inclusions strict?

  14. Another Perspective • How do we view the system? • Operation level. • Database consistency conditions. • Recoverability, ACA, Strictness, Rigorousness • Transaction level. • STM consistency conditions. • Opacity, VWC • Is there another level?

  15. Snapshot Isolation • Everyone will have a snapshot of the whole system. • A snapshot must be consistent at every point of time. • Different snapshots need not be consistent with each other. • Updating of snapshots is allowed.

  16. How consistency conditions affect STM properties. • DAP + Invisible Reads + Wait-free + Serializability = Impossible. 1 • We have seen the proof on Wednesday. • How do Opacity/VWC affect such results. • DAP + Wait-free + VWC = possible? • DAP + Wait-free + Opacity = impossible? • 1. Hagit Attiya, Eshcar Hillel, AlessiaMilani. Inherent Limitations on Disjoint-Access Parallel Transactional Memory. SPAA 2009.

  17. Conclusion • Motivation for database and STM consistency conditions is same yet perspectives differ. • STM consistency conditions affect other properties.

  18. Thank You

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