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Copenhagen Denmark KOTLIN NATIVE CONCURRENCY EXPLAINED KEVIN GALLIGAN @kpgalligan

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Touchlab

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[Your Org Here] reach out if interested, obv

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[email protected] @kpgalligan

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I still have a talk! and then…

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multithreaded coroutines!

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Intro

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What is Kotlin Multiplatform?

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kot·lin mul·ti·plat·form /ˌkätˈlin məltiˈplatfôrm,ˌkätˈlin məltīˈplatfôrm/ noun noun: kotlin multiplatform 1.optional, natively-integrated, open-source, code sharing platform, based on the popular, modern language kotlin. facilitates non-ui logic availability on many platforms.

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modern

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Saner Concurrency? concurrency is hard

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Kotlin Native model is controversial lots of disagreement

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JVM model is controversial many other platforms don’t let you do that

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JVM model is controversial many other platforms don’t let you do that

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JVM model is controversial many other platforms don’t let you do that

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This talk will cover •How the model works •My thoughts on concurrency and best practices •Libraries and future stuff (MT coroutines!)

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This talk will not cover •Me “selling” the model •Me complaining about the model •Debating various perspectives from the community

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Also… no “getting around” rules

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You need to understand it libraries won’t hide all details

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important to relax

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Kotlin Native Rules

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1) Mutable State = one thread thread confined

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2) Immutable state = many threads no changes mean no problems

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[email protected] @kpgalligan

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Internalize The Rules mutable = 1/immutable = many

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object BigGlobalState { var statusString = "" }

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Don’t manage concurrent state no “synchronized”, “volatile”, etc

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JVM Trusts You native does not

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Runtime Verification

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Do Things Differently not “losing” anything

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(jvm strict mode would be nice)

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You can break rules too just not a great idea

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OK. Code please?

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Worker kn concurrency queue

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Similar to ExecutorService Handler/MessageQueue/Looper on Android

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Worker job queue thread ??? execute

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Worker job queue thread ??? execute

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Worker job queue thread ??? execute

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Worker job queue thread ???

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Worker job queue thread ???

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Worker job queue thread ???

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Worker job queue thread ???

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val worker = Worker.start() worker.execute(TransferMode.SAFE, {"Hello "}) { println(it) }

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val worker = Worker.start() worker.execute(TransferMode.SAFE, {"Hello "}) { println(it) }

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val worker = Worker.start() worker.execute(TransferMode.SAFE, {"Hello "}) { println(it) }

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Hello

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val worker = Worker.start() worker.execute(TransferMode.SAFE, {"Hello "}) { println(it) }

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public fun execute( mode: TransferMode, producer: () -> T1, job: (T1) -> T2): Future { //... }

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public fun execute( mode: TransferMode, producer: () -> T1, job: (T1) -> T2): Future { //... }

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public fun execute( mode: TransferMode, producer: () -> T1, job: (T1) -> T2): Future { //... }

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val worker = Worker.start() worker.execute(TransferMode.SAFE, {"Hello "}) { println(it) }

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public fun execute( mode: TransferMode, producer: () -> T1, job: (T1) -> T2): Future { //... }

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val worker = Worker.start() worker.execute(TransferMode.SAFE, {"Hello "}) { println(it) }.result

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public fun execute( mode: TransferMode, producer: () -> T1, job: (T1) -> T2): Future { //... }

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producer: () -> T1

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{"Hello "} (Unit)->String

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data class SomeData(val s:String, val i:Int) fun printStuff2() { worker.execute(TransferMode.SAFE, {SomeData("Hello ", 2)}) { println(it) }.result }

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data class SomeData(val s:String, val i:Int) {SomeData("Hello ", 2)} (Unit)->SomeData

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data class SomeData(val s:String, val i:Int) fun printStuff2() { worker.execute(TransferMode.SAFE, {SomeData("Hello ", 2)}) { println(it) }.result }

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fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData }) { println(it) }.result }

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fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData }) { println(it) }.result }

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fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData }) { println(it) }.result } kotlin.IllegalStateException: Illegal transfer state

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fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData }) { println(it) }.result } execute wants to transfer someData between threads

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fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData }) { println(it) }.result } checks result (and children) for external refs

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fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData }) { println(it) }.result } local is still a ref till the end

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data class SomeData(val s:String, val i:Int) fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData }) { println(it) }.result } Mutable state?

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freeze() runtime immutability designation

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public fun T.freeze(): T {...}

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Freeze • Recursively freezes everything • One-way (no “unfreeze”) • Data may be shared between threads Int String Float Float String Int MoreData val strData var width SomeData val moreData var count

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data class SomeData(val s:String, val i:Int) fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData }) { println(it) }.result }

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data class SomeData(val s:String, val i:Int) fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData.freeze() }) { println(it) }.result }

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data class SomeData(val s:String, val i:Int) fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData.freeze() }) { println(it) }.result }

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data class SomeData(val s:String, val i:Int) fun printStuff2() { val someData = SomeData("Hello ", 2) worker.execute(TransferMode.SAFE, { someData.freeze() }) { println(it) }.result }

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data class SomeData(val s:String, val i:Int) fun printStuff2() { val someData = SomeData("Hello ", 2).freeze() worker.execute(TransferMode.SAFE, { }) { println(someData) }.result } Worker.execute must take an unbound, non-capturing function or lambda

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fun background(block:(Unit)->Unit){ worker.execute(TransferMode.SAFE, {}, block) }

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fun background(block:(Unit)->Unit){ worker.execute(TransferMode.SAFE, {}, block) } Worker.execute must take an unbound, non-capturing function or lambda

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fun background(block:(Unit)->Unit){ worker.execute(TransferMode.SAFE, {}, block.freeze()) } Worker.execute must take an unbound, non-capturing function or lambda

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fun background(block:()->Unit){ worker.execute(TransferMode.SAFE, {block}) { it() } } kotlin.IllegalStateException: Illegal transfer state

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fun background(block:()->Unit){ worker.execute(TransferMode.SAFE, {block.freeze()}) { it() } }

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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class CounterModel { var count = 0 fun countClicked() { count++ } }

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class CounterModel { var count = 0 fun countClicked() { background { count++ } } }

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class CounterModel { var count = 0 fun countClicked() { background { count++ } } }

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class CounterModel { var count = 0 fun countClicked() { background { count++ } } }

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class CounterModel { var count = 0 fun countClicked() { background { count++ } } }

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class CounterModel { var count = 0 fun countClicked() { background { count++ } } } InvalidMutabilityException: mutation attempt of frozen sample.CounterModel

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Functions Have State careful what you capture

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Global State some special rules

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val mainOnly = SomeData("a", 1)

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val mainOnly = SomeData("a", 1) object GlobalObject { val data = SomeData("b", 2) }

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@ThreadLocal val mainOnly = SomeData("a", 1) @ThreadLocal object GlobalObject { val data = SomeData("b", 2) }

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@SharedImmutable val mainOnly = SomeData("a", 1) @ThreadLocal object GlobalObject { val data = SomeData("b", 2) }

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Why Special Rules? you can access it from anywhere

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Tips and New Stuff

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Debugging Issues living with the new model

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InvalidMutabilityException changing frozen state

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class CounterModel { init { ensureNeverFrozen() } var count = 0 fun countClicked() { background { count++ } } }

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Rethinking architecture intentional mutability

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Main Thread Other Thread (maybe?) Data Data Data Content Model iOS View Model

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Atomics mutating frozen state

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class CounterModel { val count = AtomicInt(0) fun countClicked() { background { count.value++ } } }

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class CounterModel { val count = AtomicInt(0) fun countClicked() { background { count.value++ } } }

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class CounterModel { val count = AtomicInt(0) fun countClicked() { background { count.value++ } } }

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val atom = AtomicReference(SomeData("", 0))

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class SomeModel { val atom = atomic(SomeData("abc", 123)) fun update(sd:SomeData){ atom.value = sd } init { freeze() } }

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class SomeModel { val atom = atomic(SomeData("abc", 123)) fun update(sd:SomeData){ atom.value = sd } init { freeze() } }

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class SomeModel { val atom = atomic(SomeData("abc", 123)) fun update(sd:SomeData){ atom.value = sd } init { freeze() } }

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class SomeModel { val atom = atomic(SomeData("abc", 123)) fun update(sd:SomeData){ atom.value = sd } init { freeze() } }

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Atomic-fu/Stately

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Don’t use too many atomics AtomicRef can leak memory

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DetachedObjectGraph transfer state

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DetachedObjectGraph transfer state

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Worker dispatched state mutable state thread local

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State Thread State Worker ??? State Container Data

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State Thread State Worker ??? State Container ??? Data

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Multiplatform State freeze in common

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freeze() is native only

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expect fun T.freeze(): T expect fun T.isFrozen(): Boolean expect fun T.isNativeFrozen(): Boolean expect fun Any.ensureNeverFrozen()

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expect fun T.freeze(): T expect fun T.isFrozen(): Boolean expect fun T.isNativeFrozen(): Boolean expect fun Any.ensureNeverFrozen()

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Multithreaded Coroutines

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General Thoughts what does this mean?

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State Model Clarity strict mode is sticking around

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Library Development was a blocker

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KMP is more “real”

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Disclaimers just fyi

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Overview coroutine always bound to a single thread create with newSingleThreadContext on native that uses Worker Default has single background thread Main defined for Apple targets Windows/Linux ¯\_(ツ)_/¯

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Switching Threads use withContext (or equivalents) data passed in (or captured) is frozen data returned is frozen

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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suspend fun printStuffBg() { val someData = SomeData("Hello ", 2) withContext(workerDispatcher) { println(someData) } }

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suspend fun printStuffBg() { val someData = SomeData("Hello ", 2) withContext(workerDispatcher) { println(someData) } }

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suspend fun printStuffForeground() { val someData = SomeData("Hello ", 2) val someMoreData = withContext(workerDispatcher) { SomeData("${someData.s}, Dogs!", someData.i + 1) } println(someMoreData) println("I am frozen? ${someMoreData.isFrozen()}") }

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suspend fun printStuffForeground() { val someData = SomeData("Hello ", 2) val someMoreData = withContext(workerDispatcher) { SomeData("${someData.s}, Dogs!", someData.i + 1) } println(someMoreData) println("I am frozen? ${someMoreData.isFrozen()}") }

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suspend fun printStuffForeground() { val someData = SomeData("Hello ", 2) val someMoreData = withContext(workerDispatcher) { SomeData("${someData.s}, Dogs!", someData.i + 1) } println(someMoreData) println("I am frozen? ${someMoreData.isFrozen()}") }

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SomeData(s=Hello , Dogs!, i=3) I am frozen? true

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class DbModel(private val dbId:Int){ fun showDbStuff() = mainScope.launch { val sd = withContext(workerDispatcher){ DB.find(dbId) } println(sd) } init { ensureNeverFrozen() } }

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class DbModel(private val dbId:Int){ fun showDbStuff() = mainScope.launch { val capturedId = dbId val sd = withContext(workerDispatcher){ DB.find(capturedId) } println(sd) } init { ensureNeverFrozen() } }

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class DbModel(private val dbId: Int) { fun showDbStuff() = mainScope.launch { val sd = loadDbInfo(dbId) println(sd) } private suspend fun loadDbInfo(id: Int) = withContext(workerDispatcher) { DB.find(id) } init { ensureNeverFrozen() } }

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class DbModel(private val dbId: Int) { fun showDbStuff() = mainScope.launch { val sd = loadDbInfo(dbId) println(sd) } private suspend fun loadDbInfo(id: Int) = withContext(workerDispatcher) { DB.find(id) } init { ensureNeverFrozen() } }

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class DbModel(private val dbId: Int) { fun showDbStuff() = mainScope.launch { val sd = loadDbInfo(dbId) println(sd) } private suspend fun loadDbInfo(id: Int) = withContext(workerDispatcher) { DB.find(id) } init { ensureNeverFrozen() } }

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Flow, Channel, etc all freezable

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Main Thread Other Thread (maybe?) Data Data Data Content Model iOS View Model

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init { ensureNeverFrozen() mainScope.launch { query.asFlow() .map { assertNotMainThread() extractData(it) } .flowOn(ServiceRegistry.backgroundDispatcher) .collect { vt -> view?.let { it.update(vt) } } } }

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init { ensureNeverFrozen() mainScope.launch { query.asFlow() .map { assertNotMainThread() extractData(it) } .flowOn(ServiceRegistry.backgroundDispatcher) .collect { vt -> view?.let { it.update(vt) } } } }

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init { ensureNeverFrozen() mainScope.launch { query.asFlow() .map { assertNotMainThread() extractData(it) } .flowOn(ServiceRegistry.backgroundDispatcher) .collect { vt -> view?.let { it.update(vt) } } } }

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init { ensureNeverFrozen() mainScope.launch { query.asFlow() .map { assertNotMainThread() extractData(it) } .flowOn(ServiceRegistry.backgroundDispatcher) .collect { vt -> view?.let { it.update(vt) } } } }

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init { ensureNeverFrozen() mainScope.launch { query.asFlow() .map { assertNotMainThread() extractData(it) } .flowOn(ServiceRegistry.backgroundDispatcher) .collect { vt -> view?.let { it.update(vt) } } } }

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init { ensureNeverFrozen() mainScope.launch { query.asFlow() .map { assertNotMainThread() extractData(it) } .flowOn(ServiceRegistry.backgroundDispatcher) .collect { vt -> view?.let { it.update(vt) } } } }

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Community Libraries

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Stately?

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Stately v1 (Probably) deprecated a lot of it, anyway

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@Deprecated Stately Collections

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@Discouraged Stately Collections

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January that whole “holidays” thing

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github.com/Autodesk/coroutineworker

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github.com/Badoo/reaktive

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Next Steps

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go.touchlab.co/knthreads

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JetBrains Links • https://github.com/JetBrains/kotlin-native/blob/ master/IMMUTABILITY.md • https://github.com/JetBrains/kotlin-native/blob/ master/CONCURRENCY.md • https://www.youtube.com/watch?v=nw6YTfEyfO0

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go.touchlab.co/mtco

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Kotlin Koans for native and state

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January or whenever MT coroutines are stable(ish)

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Try out MT Coroutines? some assembly required

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Build and deploy local assuming no updates from kotlinx.coroutines

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> git clone -b native-mt \ https://github.com/Kotlin/kotlinx.coroutines.git > cd kotlinx.coroutines/ > ./gradlew build publishToMavenLocal

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Samples from slides https://github.com/kpgalligan/MTCoroutines Droidcon app https://github.com/touchlab/DroidconKotlin/ (look for the kpg/mt_coroutines branch)

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KMP evaluation kit

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KaMP-Kit go.touchlab.co/KaMP-Kit

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Stickers!

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#KotlinConf THANK YOU AND REMEMBER TO VOTE Kevin Galligan @kpgalligan

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Thanks! @kpgalligan touchlab.co

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Thanks! @kpgalligan touchlab.co Join the team !