The document discusses Conflict-free Replicated Data Types (CRDTs) which allow for concurrent modifications to replicated data across multiple replicas without requiring synchronization or locking. It covers different types of CRDTs including operation-based (CmRDT) and state-based (CvRDT) as well as examples like last-writer-wins registers, grow-only sets, counters, and sets. Merging processes are described as taking the union or maximum of values from replicas to achieve convergence. The document suggests CRDTs can be combined and composed to build more complex data types in a modular way.

1. CRDTs
for engineers
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2. CRDT
• Conflict-Free (amongst others)
• Replicated
• Data
• Type
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3. CRDT
• Conflict-Free (amongst others)
• Replicated
• Data - obvious
• Type - obvious
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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?
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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)
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10. Types
• CmRDT message based
• CvRDT state based (v for vector)
• can be emulated
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11. CmRDT
• based on messages
• requires messages to be delivered
• needs controls of messages (complex)
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12. CvRDT
• Based on the objects state
• independent from underlaying structure
• that’s good!
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13. LWW register
VaTa VbTb
VbTb
Ta =< Tb
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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
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17. G Set
A
M1 Merge M2
AA
B B
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18. G Set
A
M1 Merge M2
AA
B B
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19. G Set
A
M1 Merge M2
AA
B B C
C
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20. G Set
A
M1 Merge M2
AA
B B C
C
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21. G Set
A
M1 Merge M2
AA
B B C
CD D
D
Union
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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
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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
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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
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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
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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
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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
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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
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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
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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
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31. now it’s like lego
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32. PN Counter
Increments Decrements
G Counter G Counter
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33. 2P Set
Additions Deletions
G Set G Set
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34. 2P Set
• Can only be added once
• Deleted for good
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35. 35Thursday, June 27, 13

36. PN Set
7 A
0 B
3 C
-2 D
PN Counter
A
C
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37. PN Set
• Partial merges can lead to negative values
• Adds might (temporarily) have no effect
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38. OR Set
M1,1 A
M1,1 A
M2,1 A
M1,2 B
M1,3 A
A
B
2P Set
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39. OR Set
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40. OR Set
M1,1 A M1,1 A A
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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
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43. OR Set
M1,1 A M1,1 A A
M1,2 B BM2,1 B
M1,2 B
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44. OR Set
M1,1 A M1,1 A A
M1,2 B BM2,1 B
M1,2 B
M1,3 B
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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
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46. OR Set
• Elements have a unique ID
• delete
• Elements might ‘reappear’ after a merge
• lots of trash
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47. Questions?
• Code?
• C-x C-+
• https://github.com/Licenser/ecrdt
• more links there!
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