Kotlin/Native Concurrency for Mobile Multiplatform

If you're going to build shared Kotlin code with Kotlin/Native, you'll need to understand the concurrency and state model. In this talk, I give you the basics (and a bit more), plus next steps.

Transcript

1. Kotlin Native Concurrency Kevin Galligan
4. Copenhagen Denmark KOTLIN NATIVE CONCURRENCY EXPLAINED KEVIN GALLIGAN @kpgalligan
6. Intro
7. What is Kotlin Multiplatform?
8. 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.
9. modern
10. Saner Concurrency? concurrency is hard
12. Kotlin Native model is controversial lots of disagreement
13. JVM model is controversial many other platforms don’t let you do that
14. JVM model is controversial many other platforms don’t let you do that
15. JVM model is controversial many other platforms don’t let you do that
16. You need to understand it libraries won’t hide all details
17. important to relax
18. Kotlin Native Rules
19. 1) Mutable State = one thread thread confined
20. 2) Immutable state = many threads no changes mean no problems
21. kevin@touchlab.co @kpgalligan
22. Internalize The Rules mutable = 1/immutable = many
23. object BigGlobalState { var statusString = "" }
24. Don’t manage concurrent state no “synchronized”, “volatile”, etc
25. JVM Trusts You native does not
26. Runtime Verification
27. Do Things Differently not “losing” anything
28. (jvm strict mode would be nice)
29. You can break rules too just not a great idea
30. Stages of grief skip ahead to acceptance
31. Core Concepts
32. Worker kn concurrency queue
33. Similar to ExecutorService Handler/MessageQueue/Looper on Android
34. Worker kn concurrency queue
35. Mutability & State
36. 1) Mutable State = one thread simple state is simple
37. class SimpleState { private var i = 0 val currVal: Int get() = i fun doStuff(arg: Int) { i += arg } } fun main(){ val s = SimpleState() s.doStuff(22) s.doStuff(33) println("My val is ${s.currVal}") }
38. 1) Mutable State = one thread you can transfer
39. 1) Mutable State = one thread you can transfer
40. 2) Immutable state = many threads not source immutable
41. data class SomeData(val s:String, val i:Int) Mutable state?
42. freeze() runtime immutability designation
43. public fun <T> T.freeze(): T {...}
44. Freeze • Recursively freezes everything • One-way (no “unfreeze”) • Data may be shared between threads Int String Float MoreData val strData var width SomeData val moreData var count
45. Int String Float Float String Int MoreData val strData var width SomeData val moreData var count
46. Crossing Threads
47. fun background(block:()->Unit)
48. fun background(block:()->Unit) block.freeze()
49. fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }
50. fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }
51. fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }
52. fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }
53. class CounterModel { var count = 0 fun countClicked() { count++ } }
54. class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }
55. class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }
56. class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }
57. class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }
58. class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } } InvalidMutabilityException: mutation attempt of frozen sample.CounterModel
59. Functions Have State careful what you capture
60. class CounterModel { var count = 0 fun countClicked() { count++ save(count) } private fun save(count:Int) = background { saveToDb(count) } }
61. Global State some special rules
62. var someData = SomeData("a", 1)
63. var someData = SomeData("a", 1) object GlobalObject { var data = SomeData("b", 2) }
64. @ThreadLocal var someData = SomeData("a", 1) @ThreadLocal object GlobalObject { var data = SomeData("b", 2) }
65. @SharedImmutable var someData = SomeData("a", 1) @ThreadLocal object GlobalObject { var data = SomeData("b", 2) }
66. var someData:SomeData set(v:SomeData)… get():SomeData = …
67. Why Special Rules? you can access it from anywhere
68. Debugging Issues
69. InvalidMutabilityException changing frozen state
70. Why is this frozen() common question
71. class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }
72. ensureNeverFrozen()
73. class CounterModel { init { ensureNeverFrozen() } var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }
74. Int String Float Int MoreData val strData var width SomeData val moreData var count
75. Rethinking architecture intentional mutability
76. Main Thread Other Thread (maybe?) Data Data Data Content Model iOS View Model
77. This can get weird subtle things
78. class BreedModel( private val viewUpdate: (ItemDataSummary) -> Unit, private val errorUpdate: (String) -> Unit ) : BaseModel() { private val dbHelper: DatabaseHelper by inject() private val settings: Settings by inject() init { ensureNeverFrozen() scope.launch { dbHelper.selectAllItems().asFlow() .map { q -> val itemList = q.executeAsList() ItemDataSummary(/* etc */) } .flowOn(Dispatchers.Default) .collect { summary -> viewUpdate(summary) } } }
79. class BreedModel( private val viewUpdate: (ItemDataSummary) -> Unit, private val errorUpdate: (String) -> Unit ) : BaseModel() { private val dbHelper: DatabaseHelper by inject() private val settings: Settings by inject() init { ensureNeverFrozen() scope.launch { dbHelper.selectAllItems().asFlow() .map { q -> val itemList = q.executeAsList() settings.set("Hey", "Bad idea") ItemDataSummary(/* etc */) } .flowOn(Dispatchers.Default) .collect { summary -> viewUpdate(summary) } } }
80. class BreedModel( private val viewUpdate: (ItemDataSummary) -> Unit, private val errorUpdate: (String) -> Unit ) : BaseModel() { private val dbHelper: DatabaseHelper by inject() private val settings: Settings by inject() init { ensureNeverFrozen() scope.launch { dbHelper.selectAllItems().asFlow() .map { q -> val itemList = q.executeAsList() settings.set("Hey", "Bad idea") ItemDataSummary(/* etc */) } .flowOn(Dispatchers.Default) .collect { summary -> viewUpdate(summary) } } }
82. This may be frustrating but should make you think
83. class BreedModel( private val viewUpdate: (ItemDataSummary) -> Unit, private val errorUpdate: (String) -> Unit ) : BaseModel() { private val dbHelper: DatabaseHelper by inject() private val settings: Settings by inject() init { ensureNeverFrozen() scope.launch { dbHelper.selectAllItems().asFlow() .map { q -> val itemList = q.executeAsList() ItemDataSummary(/* etc */) } .flowOn(Dispatchers.Default) .collect { summary -> viewUpdate(summary) } } }
84. Concurrent Mutability
85. Atomics mutating frozen state
86. class CounterModel { val count = AtomicInt(0) fun countClicked() { count.value++ background { saveToDb(count.value) } } }
87. class CounterModel { val count = AtomicInt(0) fun countClicked() { count.value++ background { saveToDb(count.value) } } }
88. val atom = AtomicReference(SomeData(“abc", 123))
89. class SomeModel { val atom = AtomicReference(SomeData(“abc", 123)) fun update(sd:SomeData){ atom.value = sd } init { freeze() } }
90. Don’t use too many atomics AtomicRef can leak memory
91. Worker dispatched state mutable state thread local
92. State Thread IsolateState ??? State Container Data
93. State Thread IsolateState ??? State Container ??? Data
94. open class IsolateState<T : Any>
95. open class IsolateState<T : Any> constructor(producer: () -> T)
96. open class IsolateState<T : Any> constructor(producer: () -> T) fun <R> access(block: (T) -> R): R
97. IsoArrayDeque IsoMutableCollection IsoMutableIterator IsoMutableList IsoMutableMap IsoMutableSet
98. fun <K, V> IsoMutableMap<K, V>.checkFor2(first: K, second: K, ins: V)= access { map -> if (map.containsKey(first)) map.put(first, ins) else map.put(second, ins) }
99. Objective-C/C/C++ just do what you want!
100. Multiplatform & State
101. Multiplatform State
102. freeze() is native only
103. expect fun <T> T.freeze(): T expect val <T> T.isFrozen: Boolean expect fun Any.ensureNeverFrozen() expect annotation class Throws
104. freeze() on iOS threading rules apply
105. Swift/Objc can’t be frozen be aware of concurrency issues
106. Concurrency Libraries
107. Official Native kotlinx.coroutines single-threaded
108. ktor on native called on main thread
109. multithreaded coroutines!
110. multithreaded coroutines?
111. Switching Threads use withContext (or equivalents) data passed in (or captured) is frozen data returned is frozen
112. fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }
113. suspend fun printStuffBg() { val someData = SomeData("Hello ", 2) withContext(workerDispatcher) { println(someData) } }
114. suspend fun printStuffBg() { val someData = SomeData("Hello ", 2) withContext(workerDispatcher) { println(someData) } }
115. 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? ${someMoreData.isFrozen()}") }
116. suspend fun printStuffForeground() { val someData = SomeData("Hello ", 2) val someMoreData = withContext(workerDispatcher) { SomeData("${someData.s}, Dogs!", someData.i + 1) } println(someMoreData) println("I am frozen? ${someMoreData.isFrozen()}") }
117. suspend fun printStuffForeground() { val someData = SomeData("Hello ", 2) val someMoreData = withContext(workerDispatcher) { SomeData("${someData.s}, Dogs!", someData.i + 1) } println(someMoreData) println("I am frozen? ${someMoreData.isFrozen()}") }
118. SomeData(s=Hello , Dogs!, i=3) I am frozen? true
119. Flow, Channel, etc all freezable
120. Main Thread Other Thread (maybe?) Data Data Data Content Model iOS View Model
121. iOS Suspend Functions new for 1.4!
122. Stately
123. Stately (Probably) deprecated
124. Stately v1! (ish)
125. Modules • stately-common • stately-collections • stately-concurrency
126. Modules • stately-isolate • stately-iso-collections • alpha. Feedback please!
127. github.com/Autodesk/coroutineworker
128. github.com/Badoo/reaktive
129. Next Steps
130. go.touchlab.co/kotlinlang
131. go.touchlab.co/kndevto
132. go.touchlab.co/knthreads
133. 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
134. github.com/touchlab/KaMPKit
135. Thanks! @kpgalligan touchlab.co