KharkivJS 2017: Коллаборативные системы и CRDT

Коллаборативные системыCollaborative systems and Conflict-Free Replicated Data Types

Максим Климишин

Head of Software Architecture at Takeoff Technologies

29 октября 2017 г.

Максим Климишин Коллаборативные системы 1 / 48

Работа

Head of Software Architecture, Takeoff Technologies, 2017CTO @ ZAKAZ.UA and CartFresh, 2012Team Lead @ oDesk (now Upwork), 2010Project Coordinator @ 42 Coffee Cups, 2009


Организация

co-organizer of PapersWeLove Kyivco-founder of KyivJSco-founder of LvivJSco-organizer of PyCon Ukraineco-organizer of Hotcodejudge at UA Web Challenge


Takeoff TechnologieseGrocery: Grocery Delivery startup with robotic warehouse


Distributed systems & Groupware systems

Онлайн игрыКалендарь, контакты, напоминалкиСофт для конференцийРедактирование онлнай несколькими людьмиГрупповые чаты


Распределенные приложения

Использование на разных устройствах в разное времяОдновременное использование на разных устройствахРабота офлайн


Data and Consistency


Serializability

serial schedule i.e. sequential with no operations overlap in time

Solution of a Problem in Concurrent Programming Control (E.W. Dijkstra,1965)

A Quorum-based Commit Protocol (Dale Skeen, 1982)


Concurrency fundamentals

divergence или расхождение: ∪ni=1o

1i ̸= ∪m

j=1o2j

causality-violations или нарушение порядка операций:o1 → o3

intention-violations или нарушение намерений: o1||o2


Что приходит в голову

LockingTransactionsTentative TransactionsSingle Active ParticipantDependency DetectionReversible Execution


Good luck with thatThe Ultimate Guide to Becoming Your Best Self


Consistency Models

Strong ConsistencyWeak ConsistencyRead Your WritesEventual ConsistencyStrong Eventual Consistency


Replication Types

PessimisticOptimistic or Eventual Consistency


Concurrency Control in Distributed Systems1989 by Ellis and Gibbs/GROVE editor

Операции должны быть коммутативными, чтобы удовлетворитьconvergence property:

op1 ⊙ op2 = op2 ⊙ op1


Кто использует OT?Wave took 2 years to write and if we rewrote it today, it

would take almost as long to write a second time

on ShareJS


Proving correctness of transformation functions incollaborative editing systems2006 Gerald Oster et al

Доказал что все алгоритмы некорректные (сконтрпримерами), упсПредложил TTF (Tombstone Transformation Functions)


Operation Transformations on Google Scholar1989-2006


OP DOH


Conflict-Free Replicated Data TypesCRDTs


Ограничения сети

ЗадержкиDrop пакетовПорядокДубликаты


Conflict-free replicated data types (CRDT)2011 Shapiro et al

коммутативность – ∀x, y : x⊙ y = y ⊙ x

ассоциативность – x⊙ (y ⊙ z) = (x⊙ y)⊙ z

идемпотентность – x⊙ x = x


Strong Eventual Consistency

C = [c1, ..., cn], ∀i, j : ci = cj ⇒ si ≡ sj


Счетчик

1 + 1


+ идемпотентность нарушена

1 + 1 ̸= 1


∪ – ништяк с сетами

1 ∪ 1 = 1


Прочие свойства в семантике множеств

(1 ∪ 2) ∪ 3 = 1 ∪ (2 ∪ 3)

1 ∪ 2 = 2 ∪ 1


CRDT

ciCRDT State(convergent)

ConflictResolutionSemantics

Applicationaccess API

Si


CRDT: GCounterGrowth-only counter: Interface

export class GCounter {constructor(counters) { this.counters = counters; }increment(id) { ... }query() { ... }merge(counter) { ... }

}


CRDT: GCounter payloadCounter State on init

let counters = {1: 02: 0...N: 0}


CRDT: GCounter increment op

export class GCounter {increment(id) {

return new GCounter(Object.assign({[id]: (this.counters[id] || 0) + 1}))}

}


CRDT: GCounter query op

export class GCounter {query() {

return Object.values(this.counters).reduce((a, b) => a + b, 0);}

}


CRDT: GCounter merge op

export class GCounter {merge(counter) {

let sites = Object.keys(counter.counters).concat(Object.keys(this.counters));

return new GCounter(sites.reduce((merged, site) => Object.assign(

merged, {[site]: Math.max(merged[site] || 0,counter.counters[site] || 0)}),

this.counters));}

}


CRDT: GCounter sum it up

export class GCounter {constructor(counters) { this.counters = counters; }increment(id) {

return new GCounter(Object.assign({[id]: (this.counters[id] || 0) + 1}))}

query() {return Object.values(this.counters)

.reduce((a, b) => a + b, 0); }merge(counter) {

let sites = Object.keys(counter.counters).concat(Object.keys(this.counters));

return new GCounter(sites.reduce((merged, site) => Object.assign(

merged, {[site]: Math.max(merged[site] || 0,counter.counters[site] || 0)}),

this.counters));}

}


CRDT: GCounter-testIncrement only counter

linenos function examples_GCounter() {linenos let log = (op, c1, c2) => console.log(linenos "[OP] " + op + ": c1=" + c1.query() + ",linenos c2=" + c2.query());linenos let counter1 = new GCounter({});linenos let counter2 = new GCounter({});linenoslinenos log("counter1", counter1, counter2);linenos counter1 = counter1.increment(0);linenos log("counter1++", counter1, counter2);linenos counter2 = counter2.merge(counter1);linenos log("counter1 U counter2", counter1, counter2);linenos }


CRDT: GCounter example output

linenos node_modules/.bin/babel-node --presets es2016,es2015 main.jslinenos [OP] counter1: c1=0, c2=0linenos [OP] counter1++: c1=1, c2=0linenos [OP] counter1 U counter2: c1=1, c2=1


CRDT: PNCounterPositive-Negative Counter

class PNCounter {constructor(p, n) { this.n = n; this.p = p}increment() { return new PNCounter(this.p.increment(), this.n); }decrement() { return new PNCounter(this.p, this.n.increment()); }query() { return this.p.query() - this.n.query(); }merge(pncounter) {

return new PNCounter(this.p.merge(pncounter.p),this.n.merge(pncounter.n)); }

}


CRDT: MVRegisterMulti-Value Register

export class MVRegister {constructor(id, register) { this.id = id; this.register = register; }set(value) {

return new MVRegister(this.id,Object.assign(this.register, {[this.id]: value}))}

query() {return Object.values(this.register); }merge(register) {

let state = this.register[this.id] === undefined ?{} : {[this.id]: this.register[this.id]};

return new MVRegister(this.id,Object.assign(register.register, state))

}}


CRDT: MVRegister Testconst repr = (arr) => arr.length === 0 ? '[]' :

'["' + arr.join('", "') + '"]';

function example_mvregister() {let log = (op, r1, r2) => console.log("[OP] " + op +

": r1=" + repr(r1.query()) +", r2=" + repr(r2.query()));

let r1 = new MVRegister(1, {});let r2 = new MVRegister(2, {});

log("register1", r1, r2);r1 = r1.set("key1")log("r1.set(key1)", r1, r2);r2 = r2.merge(r1);log("r1 U r2", r1, r2);r1.set("v1");log("r1.set(v1)", r1, r2)r2.set("v2")log("[CONCURRENT] r2.set(v2)", r1, r2)r2 = r2.merge(r1);log("[MERGED]", r1, r2);

}Максим Климишин Коллаборативные системы 38 / 48

CRDT: MVRegister Output

[OP] register1: r1=[], r2=[][OP] r1.set(key1): r1=["key1"], r2=[][OP] r1 U r2: r1=["key1"], r2=["key1"][OP] r1.set(v1): r1=["v1"], r2=["key1"][OP] [CONCURRENT] r2.set(v2): r1=["v1"], r2=["key1", "v2"][OP] [MERGED]: r1=["v1", "v2"], r2=["v1", "v2"]


JSON CRDTA Conflict-Free Replicated JSON Datatype (August 15, 2017)


JSON CRDTConcurrent mutation on same key with different types


CRDT Shopping Cart2017: redesign of Shopping Cart API


CRDTs: CvRDT, CmRDTstate-based and op-based replication

Рис.: State-basedConvergent Replicated Data

Type or CvRDT

Рис.: Op-based CommutativeReplicated Data Type or

CmRDT


CRDT: TypesRegisters, Counters, Sets

Register: LWW or Multi-Value (Dynamo or Couchdb-like)Counter (growth-only) and Counter w/decrementingG-Set – growth-only set2P-Set – remove only once set (G-Set + Tombstones set)LWW-Element-Set – vector clocksOR-Set – unique-tagged elements and list of tags withinTombstones setWOOT, LOGOOT, Treedoc, RGA, LSEQ for ordered lists


Инструменты

Y-js – framework for offline-first p2p shared editing on structureddataRiak 2.0: Counters, Flags, Sets, Registers, MapsRoshi by SoundcloudShareDB (and fall of Google Wave)Swarm (and forever-in-pre-alpha tool)OT.js (Operational Transformation for JS)CRDT – github.com/dominictarr/crdtreplikativ.io – p2p distributed system frameworkGUN framework: p2p distributed framework


Cвой CRDT

+ Мало кода+ Относительно легко дополнять+ Предсказуемо работает+ Надежно при любом качестве связи+ Работа оффлайн- Сложная ментальная модель- Семантическое разрешение конфликтов- Сложно оптимально подобрать параметры GC- Реально сложно работать с большим количеством данных(δ-mutation)


У кого в проде?

FacebookTomTomLeague of LegendsSoundCloudBet265RIAK Distributed Database


https://dzone.com/articles/facebook-announces-apollo-qcon

https://speakerdeck.com/ajantis/practical-demystification-of-crdts

http://highscalability.com/blog/2014/10/13/how-league-of-legends-scaled-chat-to-70-million-players-it-t.html

https://developers.soundcloud.com/blog/roshi-a-crdt-system-for-timestamped-events

http://www.erlang-factory.com/static/upload/media/1434558446558020erlanguserconference2015bet365michaelowen.pdf

http://docs.basho.com/riak/kv/2.2.3/developing/data-types/

Thanks

@maxmaxmaxmax


KharkivJS 2017: Коллаборативные системы и CRDT

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