1. CRDTs
for engineers
1Thursday, June 27, 13

2. CRDT
• Conflict-Free (amongst others)
• Replicated
• Data
• Type
2Thursday, June 27, 13

3. CRDT
• Conflict-Free (amongst others)
• Replicated
• Data - obvious
• Type - obvious
3Thursday, June 27, 13

4. Replicated
• Build to be used in distributed systems
• multiple replicas can be changed
concurrently
• do not require locking
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5. Conflict-Free
• the big one!
• replica merges with predictable behavior!
• predictable =:= desired?
5Thursday, June 27, 13

6. big words
• Associative
• Commutative
• Idempotent
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7. Associative
• operations can be applied in any order
• x ∧ (y ∧ z) = (x ∧ y) ∧ z
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8. Commutative
• The values of operations can be swapped.
• x ∧ y = y ∧ x
• together
• we don’t need to care about order
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9. Idempotent
• not impotent!
• applying twice does not change a thing.
• f(f(x)) = f(x)
9Thursday, June 27, 13

10. Types
• CmRDT message based
• CvRDT state based (v for vector)
• can be emulated
10Thursday, June 27, 13

11. CmRDT
• based on messages
• requires messages to be delivered
• needs controls of messages (complex)
11Thursday, June 27, 13

12. CvRDT
• Based on the objects state
• independent from underlaying structure
• that’s good!
12Thursday, June 27, 13

13. LWW register
VaTa VbTb
VbTb
Ta =< Tb
13Thursday, June 27, 13

14. G Set
M1 Merge M2
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15. G Set
A
M1 Merge M2
AA
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16. G Set
A
M1 Merge M2
AA
16Thursday, June 27, 13

17. G Set
A
M1 Merge M2
AA
B B
17Thursday, June 27, 13

18. G Set
A
M1 Merge M2
AA
B B
18Thursday, June 27, 13

19. G Set
A
M1 Merge M2
AA
B B C
C
19Thursday, June 27, 13

20. G Set
A
M1 Merge M2
AA
B B C
C
20Thursday, June 27, 13

21. G Set
A
M1 Merge M2
AA
B B C
CD D
D
Union
21Thursday, June 27, 13

22. G Counter
Master 1
Master 2
Master 3
0 0 0
0 0 0
0 0 0
M1 M2 Merge
0
0
0
M3
0 0 0 0
= Max
= Max
= Max
SumSumSumSum
22Thursday, June 27, 13

23. G Counter
Master 1
Master 2
Master 3
1 1 1
0 0 0
0 0 0
M1 M2 Merge
1
0
0
M3
1 1 1 1
= Max
= Max
= Max
SumSumSumSum
23Thursday, June 27, 13

24. G Counter
Master 1
Master 2
Master 3
1 1 1
4 4 4
0 0 0
M1 M2 Merge
1
4
0
M3
5 5 5 5
= Max
= Max
= Max
SumSumSumSum
24Thursday, June 27, 13

25. G Counter
Master 1
Master 2
Master 3
1 1 1
4 4 4
0 0 0
M1 M2 Merge
1
4
0
M3
5 5 5 5
= Max
= Max
= Max
SumSumSumSum
25Thursday, June 27, 13

26. G Counter
Master 1
Master 2
Master 3
1 1 1
4 4 4
5 0 5
M1 M2 Merge
1
4
5
M3
10 5 10 10
= Max
= Max
= Max
SumSumSumSum
26Thursday, June 27, 13

27. G Counter
Master 1
Master 2
Master 3
1 1 1
4 4 4
5 0 5
M1 M2 Merge
1
4
5
M3
10 5 10 10
= Max
= Max
= Max
SumSumSumSum
27Thursday, June 27, 13

28. G Counter
Master 1
Master 2
Master 3
1 1 1
4 4 4
5 0 5
M1 M2 Merge
1
4
5
M3
10 5 10 10
= Max
= Max
= Max
SumSumSumSum
28Thursday, June 27, 13

29. G Counter
Master 1
Master 2
Master 3
1 1 1
4 4 4
5 7 7
M1 M2 Merge
1
4
7
M3
10 12 12 12
= Max
= Max
= Max
SumSumSumSum
29Thursday, June 27, 13

30. G Counter
Master 1
Master 2
Master 3
1 1 1
4 4 4
5 7 7
M1 M2 Merge
1
4
7
M3
10 12 12 12
= Max
= Max
= Max
SumSumSumSum
30Thursday, June 27, 13

31. now it’s like lego
31Thursday, June 27, 13

32. PN Counter
Increments Decrements
G Counter G Counter
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33. 2P Set
Additions Deletions
G Set G Set
33Thursday, June 27, 13

34. 2P Set
• Can only be added once
• Deleted for good
34Thursday, June 27, 13

35. 35Thursday, June 27, 13

36. PN Set
7 A
0 B
3 C
-2 D
PN Counter
A
C
36Thursday, June 27, 13

37. PN Set
• Partial merges can lead to negative values
• Adds might (temporarily) have no effect
37Thursday, June 27, 13

38. OR Set
M1,1 A
M1,1 A
M2,1 A
M1,2 B
M1,3 A
A
B
2P Set
38Thursday, June 27, 13

39. OR Set
39Thursday, June 27, 13

40. OR Set
M1,1 A M1,1 A A
40Thursday, June 27, 13

41. OR Set
M1,1 A M1,1 A A
M1,2 B B
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42. OR Set
M1,1 A M1,1 A A
M1,2 B BM2,1 B
42Thursday, June 27, 13

43. OR Set
M1,1 A M1,1 A A
M1,2 B BM2,1 B
M1,2 B
43Thursday, June 27, 13

44. OR Set
M1,1 A M1,1 A A
M1,2 B BM2,1 B
M1,2 B
M1,3 B
44Thursday, June 27, 13

45. OR Set
M1,1 A M1,1 A A
M1,2 B M2,1 B
M1,2 B
M1,3 B
M2,1 B
M1,3 B
M2,1 B
M1,3 B
45Thursday, June 27, 13

46. OR Set
• Elements have a unique ID
• delete
• Elements might ‘reappear’ after a merge
• lots of trash
46Thursday, June 27, 13

47. Questions?
• Code?
• C-x C-+
• https://github.com/Licenser/ecrdt
• more links there!
47Thursday, June 27, 13