Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
CRDTs - The science behind Phoenix Presence
Search
Maciej Kaszubowski
May 25, 2017
Programming
2
260
CRDTs - The science behind Phoenix Presence
Maciej Kaszubowski
May 25, 2017
Tweet
Share
More Decks by Maciej Kaszubowski
See All by Maciej Kaszubowski
Error-free Elixir
mkaszubowski
0
330
Modular Design in Elixir (ElixirConf EU 2019)
mkaszubowski
2
720
The Big Ball of Nouns
mkaszubowski
0
97
Modular Design in Elixir
mkaszubowski
1
380
Our three years with Elixir
mkaszubowski
0
240
Concurrency Basics for Elixir
mkaszubowski
0
120
Distributed Elixir
mkaszubowski
0
150
Software Architecture
mkaszubowski
0
130
Let it crash - fault tolerance in Elixir/OTP
mkaszubowski
0
460
Other Decks in Programming
See All in Programming
ニーリーにおけるプロダクトエンジニア
nealle
0
780
Team topologies and the microservice architecture: a synergistic relationship
cer
PRO
0
1.2k
AI時代のソフトウェア開発を考える(2025/07版) / Agentic Software Engineering Findy 2025-07 Edition
twada
PRO
70
22k
5つのアンチパターンから学ぶLT設計
narihara
1
160
なぜ「共通化」を考え、失敗を繰り返すのか
rinchoku
1
640
猫と暮らす Google Nest Cam生活🐈 / WebRTC with Google Nest Cam
yutailang0119
0
110
チームで開発し事業を加速するための"良い"設計の考え方 @ サポーターズCoLab 2025-07-08
agatan
0
250
“いい感じ“な定量評価を求めて - Four Keysとアウトカムの間の探求 -
nealle
1
8.5k
なんとなくわかった気になるブロックテーマ入門/contents.nagoya 2025 6.28
chiilog
1
270
Select API from Kotlin Coroutine
jmatsu
1
230
Hypervel - A Coroutine Framework for Laravel Artisans
albertcht
1
110
ふつうの技術スタックでアート作品を作ってみる
akira888
0
460
Featured
See All Featured
The Cost Of JavaScript in 2023
addyosmani
51
8.5k
KATA
mclloyd
30
14k
Git: the NoSQL Database
bkeepers
PRO
430
65k
Testing 201, or: Great Expectations
jmmastey
42
7.6k
Fireside Chat
paigeccino
37
3.5k
How to Think Like a Performance Engineer
csswizardry
24
1.7k
Speed Design
sergeychernyshev
32
1k
Java REST API Framework Comparison - PWX 2021
mraible
31
8.7k
GraphQLの誤解/rethinking-graphql
sonatard
71
11k
Improving Core Web Vitals using Speculation Rules API
sergeychernyshev
18
960
How to train your dragon (web standard)
notwaldorf
94
6.1k
How to Create Impact in a Changing Tech Landscape [PerfNow 2023]
tammyeverts
53
2.9k
Transcript
The Problem
None
Server Node 1 Server Node 2
Node A Node B [User] [User] [User] [] [] [User]
User connects User disconnects
There's no global time
Node A Node B [User] [User] [User] [] [] [User]
User connects User disconnects
Node A Node B [User] [User] [User] [] [] [User]
User connects User disconnects
Node A Node B
P.track(self, "users", "U1", %{})
P.track(self, "users", "U1", %{}) P.list("users") %{"U1" %{metas: [%{phx_ref: "…"}]}}
P.track(self, "users", "U1", %{}) P.list("users") P.track(self, "users", "U2", %{}) %{"U1"
%{metas: [%{phx_ref: "…"}]}}
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: "…"}]}}
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: "…"}]}}
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: "…"}]}}
CRDTs The science behind Phoenix Presence Maciej Kaszubowski
Conflict-free Replicated Data Type
Alternatives
• Single source of truth (DB) • Consensus algorithm •
Resolving conflicts manually
Why CRDTs?
Eventually consistent Highly available Easy to use
Eventually consistent Highly available Easy to use Hard to create
:(
Features
1. Commutative 2. Associative 3. Idempotent x y = y
x (x y) z = x (y z) x x = x
Server Node 1 Server Node 2 Server Node 3
Client Client Client Client Client Client Client Client Client Server
Examples
Counters
Node A Node B +5 5 8 -2 3 +5
User connects User disconnects 0 0 5 3 8
Node A Node B +5 5 8 -2 8 +5
User connects User disconnects 0 0 5 3 13 10
G-Counter Grow-only counter
Node 2: 3 Value=8 Node 1: 5 Node 3: 1
Merge
Node 2: 3 Value=9 Node 1: 6 Node 3: 1
+1 Merge
PN-Counter Positive-Negative Counter
Node 2: P=2 N=2 Value=7 Node 1: P=5 N=2 Node
3: P=4 N=0 3 0 4 Merge
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
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
Sets
Node A Node B [User] [User] [User] [] [] [User]
User connects User disconnects
G-Set Grow-only set
Node 2: [1,2] Value=[1,2,3,4] Node 1: [1] Node 3: [3,4]
Merge
Node 2: [1,2] Value=[1,2,3,4,5] Node 1: [1,5] Node 3: [3,4]
Merge
2P-Set Two-phase set
Node 2: [1,2],[] Value=[1,2,3,4] Node 1: [1],[] Node 3: [3,4],[]
Merge
Node 2: [1,2],[] Value=[1,2,3,4] Node 1: [1],[] Node 3: [3,4],[]
Merge G-Set for adds
Node 2: [1,2],[] Value=[1,2,4] Node 1: [1],[] Node 3: [3,4],[3]
Merge G-Set for removals
Elements cannot be re-added
Removes win
Node A Node B [ ] Add Remove [1] [
] [1]
Node A Node B [ ] [1] Add Remove [1]
[1] [ ] [1] [ ] [1]
Node A Node B [ ] [1] Add Remove [1]
[1] [ ] [1] [ ] [1]
Node A Node B [ ] [1] [1] Add Remove
[1] [1] [1] [ ] [1] [1] [1] [ ] [1]
OR-Set Observed-remove set
Node A Node B [ ] Add Remove [{A,1}] [
] [{A,1}]
Node A Node B [ ] [ ] Add Remove
[{A,1}] [{A,1}] [ ] [ ] [{A,1}] [{A,1}, ]
Node A Node B [ ] [ 1 ] Add
Remove [{A,1}] [{A,1}] [ ] [ ] [{A,1}] [{A,1},{A,2}]
Node A Node B [ ] [ 1 ] Add
Remove [{A,1}] [{A,1}] [ ] [ ] [{A,1}] [{A,1},{A,2}]
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 ]
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]
Add 1000 Remove 1000 Add Remove …
ORSWOT Observed-remove set without tombstones
None
None
Use Cases
None
None
None
None
• Load balancing / routing • Mobile clients synchronisation •
Temporary data on the servers • Avoiding work duplication • Collaborative editing
None
lasp-lang.readme.io
Summary
You're (almost) always designing a distributed system
Think about failures
Choose the correct tool for the job
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
Thanks!