Lock-Free Consistency Control for Web 2.0 Applications

1. Lock-Free Consistency Control for Web 2.0 Applications Jiang-Ming Yang, Hai-Xun Wang, Ning Gu, Yi-Ming Liu, Chun-Song Wang, Qi-Wei Zhang 25 April 2008

2. Outline • Motivation & Challenges • Problem Setting • Our Solutions • Experiments • Conclusions

3. Outline • Motivation & Challenges • Problem Setting • Our Solutions • Experiments • Conclusions

4. Motivation • Single Site

5. Motivation • Multiple Mirror Site

6. Challenges • How to merge the conflicts in Multiple Mirror Site for Web 2.0 Application? ? ?

7. Outline • Motivation & Challenges • Problem Setting • Our Solutions • Experiments • Conclusions

8. Problem Setting • Data shared in mirrored sites can be in varied forms. Here, we assume the shared data on mirrored sites are XML documents. • Consequently, operations on the data are expressed by XML queries and updates.

9. Problem Setting <Root> <book @title="Introduction to Algorithm"> <category>CS</category> <tag>Hot</tag> </book> <book @title="Advanced Statistical Learning"> <category>UnKnow</category> </book> <book @title="Linear Algebra"> <category>Math</category> </book> </Root>

10. Problem Setting • CAUSAL RELATIONSHIPS • TRANSACTIONS • CONCURRENT OPERATIONS (Lock Free)

11. Problem Setting • CAUSAL RELATIONSHIPS U1 Change the title “Advanced Statistical Learning” to “Statistical Learning”. U2 Set the category of the “Statistical Learning” book to “Math”.

12. Problem Setting • TRANSACTIONS • Using transaction model to execute some critical operations • Should be executed serialized with others

13. Problem Setting • CONCURRENT OPERATIONS U3 Add a “Discount” tag to books in “Math” category. U4 Set the category of the “Linear Algebra” book to “CS”.

14. Outline • Motivation & Challenges • Problem Setting • Our Solutions • Causality Preservation • Transaction • Consistency Control • Experiments • Conclusions

15. Causality Preservation • Definition. (Causal Ordering Relation “→”). Given two operations Oa and Ob from local replica sites i and j respectively, we have Oa → Ob, if and only if (1) i = j, and Oa is generated before Ob is generated; (2) i ≠ j, and Oa is executed on site j before Obis generated; (3) there exists an operation Ox, such that Oa → Oxand Ox → Ob. • Definition.(Concurrent Relation “ ”). Given two operations Oa and Ob, we say Oa and Ob are concurrent or Oa Ob iff neither Oa → Ob, nor Ob → Oa.

16. Causality Preservation • Vector based Timestamp • SV = <SV1, SV2, . . . , SVn> • Both for Site & operation • Definition. (Execution Condition).Operation O (from site i) is causally ready for execution at site j (i ≠ j) if the following conditions are satisfied: (in reference [6, 14]) • SVO[i] = SVj[i] + 1 • SVO[k] ≤ SVj[k], for all 1 ≤ k ≤ N and k ≠ i.

17. Transaction • The transaction model is introduced to achieve concurrent transparency • Definition. (Serialized Transaction). Let T be a transaction, and O be an operation (O may or may not be a transaction). Transaction T is a serialized transaction only if either O is executed before T in all sites, or O is executed after T in all sites.

18. Transaction • O1 → NOOP1 → O3 and O2 → NOOP2 → O4

19. Transaction • Definition. (TOrder: total order for transactions). It’s a logical order among transactions : T1 ≺ T2 ≺ T3 ≺ … ≺ Tn.

20. Consistency Control – Basic Idea

21. Consistency Control - Operation • XPath : /root/book • XQuery & XUpdate FOR $title in /root//title $category = $book/category WHERE $title = "Advanced Statistical Learning" ..

22. Consistency Control– Storage Model

23. Consistency Control– Storage Model

24. ConsistencyControl–StorageModel

25. Consistency Control- Query Process • It is clear that there is no overlap in their timestamps, and at any time only one of them is valid. • In general, only nodes whose timestamp (tcreate, tdelete) satisfies t ∈ (tcreate, tdelete) are valid at t.

26. Consistency Control- Query Process

27. Consistency Control – Size of OHL • In our approach, we store operations in an Operation History List (OHL). • Each time a replica receives a new remote operation, it will update OHL. • When an operation is executed on all replicas, it will be removed from OHL and its related state information will be removed from the inverted list.

28. Outline • Motivation & Challenges • Problem Setting • Our Solutions • Experiments • Conclusions

29. Experiments

30. Experiments

31. Experiments

32. Outline • Motivation & Challenges • Problem Setting • Our Solutions • Experiments • Conclusions

33. Conclusions • We proposed a lock-free approach for consistency maintance in Web 2.0 environment. • We do not use the locking mechanism, so concurrent operations are executed as soon as possible upon their arrival with ensuring the convergence. • We also support the transaction semantics for critical operations without using the locking mechanism relying on the causality preservation approach. • Better load balance, high-speed access and shorter respond time

34. Thank You !