CRDTs - The science behind Phoenix Presence

Transcript

1. The Problem
3. Server Node 1 Server Node 2
4. Node A Node B [User] [User] [User] [] [] [User] User connects User disconnects
5. There's no global time
6. Node A Node B [User] [User] [User] [] [] [User] User connects User disconnects
7. Node A Node B [User] [User] [User] [] [] [User] User connects User disconnects
8. Node A Node B
9. P.track(self, "users", "U1", %{})
10. P.track(self, "users", "U1", %{}) P.list("users") %{"U1"  %{metas: [%{phx_ref: "…"}]}}
11. P.track(self, "users", "U1", %{}) P.list("users") P.track(self, "users", "U2", %{}) %{"U1"  %{metas: [%{phx_ref: "…"}]}}
12. P.track(self, "users", "U1", %{}) P.list("users") P.track(self, "users", "U2", %{}) P.list("users") %{"U1"  %{metas: [%{phx_ref: "…"}]}, "U2"  %{metas: [%{phx_ref: "…"}]}} %{"U1"  %{metas: [%{phx_ref: "…"}]}}
13. P.track(self, "users", "U1", %{}) P.list("users") P.track(self, "users", "U2", %{}) P.list("users") %{"U1"  %{metas: [%{phx_ref: "…"}]}, "U2"  %{metas: [%{phx_ref: "…"}]}} P.track(self, "users", "U1", %{}) P.untrack(self, "users", "U1") %{"U1"  %{metas: [%{phx_ref: "…"}]}}
14. P.track(self, "users", "U1", %{}) P.list("users") P.track(self, "users", "U2", %{}) P.list("users") %{"U1"  %{metas: [%{phx_ref: "…"}]}, "U2"  %{metas: [%{phx_ref: "…"}]}} P.track(self, "users", "U1", %{}) P.untrack(self, "users", "U1") P.list("users") %{"U1"  %{metas: [%{phx_ref: "…"}]}, "U2"  %{metas: [%{phx_ref: "…"}]}} P.list("users") %{"U1"  %{metas: [%{phx_ref: "…"}]}, "U2"  %{metas: [%{phx_ref: "…"}]}} %{"U1"  %{metas: [%{phx_ref: "…"}]}}
15. CRDTs The science behind Phoenix Presence Maciej Kaszubowski
16. Conflict-free Replicated Data Type
17. Alternatives
18. • Single source of truth (DB) • Consensus algorithm • Resolving conflicts manually
19. Why CRDTs?
20. Eventually consistent Highly available Easy to use
21. Eventually consistent Highly available Easy to use Hard to create :(
22. Features
23. 1. Commutative 2. Associative 3. Idempotent x y = y x (x y) z = x (y z) x x = x
24. Server Node 1 Server Node 2 Server Node 3
25. Client Client Client Client Client Client Client Client Client Server
26. Examples
27. Counters
28. Node A Node B +5 5 8 -2 3 +5 User connects User disconnects 0 0 5 3 8
29. Node A Node B +5 5 8 -2 8 +5 User connects User disconnects 0 0 5 3 13 10
30. G-Counter Grow-only counter
31. Node 2: 3 Value=8 Node 1: 5 Node 3: 1 Merge
32. Node 2: 3 Value=9 Node 1: 6 Node 3: 1 +1 Merge
33. PN-Counter Positive-Negative Counter
34. Node 2: P=2 N=2 Value=7 Node 1: P=5 N=2 Node 3: P=4 N=0 3 0 4 Merge
35. Node 2: P=2 N=2 Value=6 Node 1: P=5 N=3 Node 3: P=4 N=0 2 0 4 +1 Merge
36. Node 2: P=2 N=2 Value=8 Node 1: P=5 N=3 Node 3: P=6 N=0 2 0 6 +2 +1 Merge
37. Sets
38. Node A Node B [User] [User] [User] [] [] [User] User connects User disconnects
39. G-Set Grow-only set
40. Node 2: [1,2] Value=[1,2,3,4] Node 1: [1] Node 3: [3,4] Merge
41. Node 2: [1,2] Value=[1,2,3,4,5] Node 1: [1,5] Node 3: [3,4] Merge
42. 2P-Set Two-phase set
43. Node 2: [1,2],[] Value=[1,2,3,4] Node 1: [1],[] Node 3: [3,4],[] Merge
44. Node 2: [1,2],[] Value=[1,2,3,4] Node 1: [1],[] Node 3: [3,4],[] Merge G-Set for adds
45. Node 2: [1,2],[] Value=[1,2,4] Node 1: [1],[] Node 3: [3,4],[3] Merge G-Set for removals
46. Elements cannot be re-added
47. Removes win
48. Node A Node B [ ] Add Remove [1] [ ] [1]
49. Node A Node B [ ] [1] Add Remove [1] [1] [ ] [1] [ ] [1]
50. Node A Node B [ ] [1] Add Remove [1] [1] [ ] [1] [ ] [1]
51. Node A Node B [ ] [1] [1] Add Remove [1] [1] [1] [ ] [1] [1] [1] [ ] [1]
52. OR-Set Observed-remove set
53. Node A Node B [ ] Add Remove [{A,1}] [ ] [{A,1}]
54. Node A Node B [ ] [ ] Add Remove [{A,1}] [{A,1}] [ ] [ ] [{A,1}] [{A,1}, ]
55. Node A Node B [ ] [ 1 ] Add Remove [{A,1}] [{A,1}] [ ] [ ] [{A,1}] [{A,1},{A,2}]
56. Node A Node B [ ] [ 1 ] Add Remove [{A,1}] [{A,1}] [ ] [ ] [{A,1}] [{A,1},{A,2}]
57. Node A Node B [ ] [ 1 ] Add Remove [{A,1}] [{A,1}] [ ] [ ] [{A,1}] [{A,1},{A,2}] [ 1 ] [{A,1},{A,2}] [{A,1},{A,2}] [ 1 ]
58. Node A Node B [ ] [ 1 ] Add Remove [{A,1}] [{A,1}] [ ] [ ] [{A,1}] [{A,1},{A,2}] [ 1 ] [{A,1},{A,2}] [{A,1},{A,2}] [ 1 ] [A] [A]
59. Add 1000 Remove 1000 Add Remove …
60. ORSWOT Observed-remove set without tombstones
63. Use Cases
68. • Load balancing / routing • Mobile clients synchronisation • Temporary data on the servers • Avoiding work duplication • Collaborative editing
70. lasp-lang.readme.io
71. Summary
72. You're (almost) always designing a distributed system
73. Think about failures
74. Choose the correct tool for the job
75. References • https://medium.com/@istanbul_techie/a-look-at-conflict-free-replicated-data- types-crdt-221a5f629e7e • http://basho.com/posts/technical/distributed-data-types-riak-2-0/ • http://highscalability.com/blog/2014/10/13/how-league-of-legends-scaled- chat-to-70-million-players-it-t.html • https://hal.inria.fr/inria-00609399v1/document • https://developers.soundcloud.com/blog/roshi-a-crdt-system-for- timestamped-events
76. Thanks!