Papers we Love PDX

Detection of Mutual Inconsistency in Distributed Systems

Transcript

1. Detection of Mutual Inconsistency in Distributed Systems Papers We Love PDX - June 2016
2. Caitie McCaffrey @caitie Distributed Systems Engineer caitiem.com
8. Replication for Availability
9. “In some environments it is desirable or necessary to permit users to continue modifying resources such as files when the network is partitioned”
10. “Files are not stored redundantly but are kept on removable storage media which can be carried around during prolonged partitions. Thus availability & consistency are simultaneously achieved” Disk Toting
11. The Partitioning Problem
12. The Partitioning Problem
13. The Partitioning Problem
14. The Partitioning Problem
15. How Do We Detect Inconsistencies? The Partitioning Problem
16. Paper Goals Do Not Restrict Availability of Systems during Partition Effectively Detect Mutual Inconsistencies
17. Origin Points Version Vectors &
18. Origin Points • System wide unique identifier generated when the file is created • Immutable attribute of the file • Suggested: Creation Time & Creation Site Pair • Detect Name Conflicts
19. Name Conflicts B A
20. Name Conflicts B A
21. Name Conflicts B A Conflict!
22. Version Conflicts C C
23. Version Conflicts C C
24. Version Conflicts C C
25. Version Conflicts C C Origin Points are Not Enough
26. What about TimeStamps?
27. Version Conflicts T0 T0
28. Version Conflicts T0 T0
29. Version Conflicts T0 T0 No Conflict!
30. Version Conflicts T0 T0
31. Version Conflicts T0 T1
32. Version Conflicts T0 T1
33. Version Conflicts T1 T1 No Conflict!
34. “Each partition can break into sub partitions and/or merge with other partitions many times before the entire network finally becomes connected” The Partition Problem
35. ABCD Partition Graph T0
36. ABCD AB CD Partition Graph T0 T0 T0
37. ABCD AB CD Partition Graph T0 T0 T1 T2
38. ABCD AB CD D BC A Partition Graph T0 T0 T0 T1 T1
39. ABCD AB CD D BC A Partition Graph T0 T0 T0 T1 T1 No Conflict! T1
40. ABCD AB CD D BC A Partition Graph T0 T0 T0 T1 T1 T2
41. ABCD AB CD D BC A BCD Partition Graph T0 T0 T0 T1 T1 T2
42. ABCD AB CD D BC A BCD Partition Graph T0 T0 T0 T1 T1 T2 Conflict!
43. ABCD AB CD D BC A BCD Partition Graph T0 T0 T0 T2 T1 T3 T2
44. ABCD AB CD D BC A BCD ABCD Partition Graph T0 T0 T0 T2 T1 T3 T2 T2
45. ABCD AB CD D BC A BCD ABCD Partition Graph T0 T0 T0 T2 T1 T3 T2 T2 Conflict!
46. “What is more difficult is to find a mechanism which detects version conflicts only when they are real”
47. Version Vectors “A Version Vector for a file f is a sequence of n pairs, where n is the number of sites at which f is stored … the ith vector entry counts the number Vi of updates to f made at site Si” <A:9, B:7, C:22, D:3>
48. Compatible Version Vectors <A:1, B:2, C:4, D:3> <A:0, B:2, C:2, D:3>
49. Incompatible Version Vectors <A:1, B:2, C:4, D:3> <A:1, B:2, C:3, D:4>
50. Version Vectors • For each update of f on site Si we increment the Sith component of the version vector • When conflicts are reconciled the Max Sith entry is set in the version vector • Version vectors encode the partial order defined by the partition graph
51. ABCD Partition Graph <A:0, B:0, C:0, D:0>
52. ABCD AB CD Partition Graph <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0>
53. ABCD AB CD Partition Graph <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0>
54. ABCD AB CD D BC A Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:1, B:0, C:0, D:0>
55. ABCD AB CD D BC A Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> No Conflict!
56. ABCD AB CD D BC A Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:1, B:0, C:0, D:0>
57. ABCD AB CD D BC A Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:2, B:0, C:0, D:0> <A:1, B:0, C:1, D:0>
58. ABCD AB CD D BC A BCD Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:2, B:0, C:0, D:0> <A:1, B:0, C:1, D:0>
59. ABCD AB CD D BC A BCD Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:2, B:0, C:0, D:0> <A:1, B:0, C:1, D:0> No Conflict!
60. ABCD AB CD D BC A BCD Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:2, B:0, C:0, D:0> <A:1, B:0, C:1, D:0> <A:1, B:0, C:1, D:0>
61. ABCD AB CD D BC A BCD Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:2, B:0, C:0, D:0> <A:1, B:0, C:1, D:0> <A:1, B:0, C:1, D:0> ABCD
62. ABCD AB CD D BC A BCD ABCD Partition Graph T2 <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:0, B:0, C:0, D:0> <A:1, B:0, C:0, D:0> <A:2, B:0, C:0, D:0> <A:1, B:0, C:1, D:0> <A:1, B:0, C:1, D:0> Conflict!
63. Conflict Resolution “A conflict detection mechanism, while valuable, has increased effect if there is also a method for reconciling conflicts automatically”
64. Conflict Resolution “Clearly, conflict reconciliation must take into account the semantics of the operations which were done to the data object in conflict”
67. Thank You! @caitie
68. Image Credits • https://thenounproject.com/search/?q=laptop +user&i=512528 • https://thenounproject.com/term/user/512525/ • https://thenounproject.com/search/? q=database&i=9658