Duolingo + KMP: A Case Study in Developer Productivity (KotlinConf 2025)

Duolingo ships weekly on iOS and Android to 40M+ Daily Active Users across 176 countries. Shipping fast is important to us and Kotlin Multiplatform is starting to play a huge role in how we accomplish this!

In this talk, we’ll:

* Discuss how we shipped Video Call, Math, Adventures and more across Android, iOS, and Web
* Propose what potential upcoming projects could benefit from KMP and the rubric we use
* Share the challenges faced and lessons learned along the way

…and more!

Co-Presented with @johnnyeet.

Video: https://kotlinconf.com/talks/812400/

1. Duolingo + KMP : A Case Study in Developer Productivity Johnny Ye & John Rodriguez @ johnnyeet @jrodbx
2. duolingo + kmp a case study in developer productivity Johnny Ye John Rodriguez
4. June 2012 (web)
5. May 2013 (Android) Nov 2012 (iOS)
6. 10 Years Later
7. Nov 2022
8. Nov 2022
9. Nov 2023
10. kmp @duolingo
11. What makes a good “module” for KMP?
12. • Stable What makes a good “module” for KMP? • No UI • Cohesive • Divergent
13. Nov 2023
14. Nov 2023 experiments-lib
15. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, )
16. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, ) rollout 0% false
17. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, ) rollout 20% true
18. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, ) "control" "experiment"
19. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, ) "control" "exp1" "exp2"
20. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, )
21. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, ) setOf("backend", "frontend") ?
22. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, )
23. data class ExperimentRecord( val eligible: Boolean, val condition: String, val treated: Boolean, val contexts: Set<String>, ) @objc public class ExperimentRecord: NSObject { @objc public let eligible: Bool @objc public let condition: String @objc public let treated: Bool @objc public let contexts: Set<String> }
24. data class TreatmentUpdate( val experimentName: String, val contexts: Set<String>, val condition: String, )
25. data class TreatmentUpdate( val experimentName: String, val contexts: Set<String>, val condition: String, ) public struct ExperimentUpdate { public let experimentName: ExperimentName public let contexts: Set<String> public let treated = true public func addingContext(_ context: String?) -> ExperimentUpdate { if let context { return addingContexts([context]) } else { return self } } public func addingContexts( _ newContexts: some Sequence<String> ) -> ExperimentUpdate {
26. class TreatmentApiClient( s: NativeNetworkStubs, engine: HttpClientEngine ) : TreatmentSender { private val client = HttpClient(engine) { install(s) } fun sendTreatment(treatment: TreatmentUpdate): ExperimentRecord { val response = client.patch( s.buildUrl("/users/${s.userId}/experiments/${treatment.name}") ) { headers { append(HttpHeaders.ContentType, "application/json") } setBody(serialize(treatment).toString()) } return Json.decodeFromString(response.bodyAsText()) }
27. class ExperimentNetworkClient: ExperimentNetworkService { let networkClient: NetworkClient func treat( userID: DUOUserID, inExperiment experimentName: ExperimentName, contexts: Set<String>, completion: @escaping (CommonResult<ExperimentRecord>) -> Void ) { let route = UpdateExperimentTreatment( contexts: Array(contexts), experimentName: experimentName, treated: true, userId: userID ) networkClient.fireRequest(route, completion: completion) } }
28. NativeFile
29. NativeFile IosNativeFile AndroidNativeFile FakeNativeFile
30. NativeFile
31. NativeFile JsonBlobFile JsonBlobTreatmentQueue TreatmentApiClient QueuedTreatmentSender ExperimentsManager ExperimentRepository
32. NativeFile JsonBlobFile kotlinx.serialization JsonBlobTreatmentQueue TreatmentApiClient QueuedTreatmentSender ktor ExperimentsManager ExperimentRepository
33. NativeFile JsonBlobFile JsonBlobTreatmentQueue TreatmentApiClient QueuedTreatmentSender ExperimentsManager ExperimentRepository NativeUserModelStubs NativeNetworkStubs kotlinx.serialization ktor
34. NativeFile JsonBlobFile JsonBlobTreatmentQueue TreatmentApiClient QueuedTreatmentSender ExperimentsManager ExperimentRepository kotlinx.serialization ktor NativeUserModelStubs NativeNetworkStubs val userId: Long, val experiments: Map<ExperimentID, ExperimentRecord> val userId: Long val baseUrl: Url val userAgent: String val jwtToken: String
36. Nov 2023 experiments-lib
37. Mar 2024 (iOS) Jun 2024 (web) Dec 2023 adventures
38. • Stable What makes a good “module” for KMP? • No UI • Cohesive • Divergent
39. rive!
40. Section Title
44. https://www.droidcon.com/2024/10/17/cross-platform-animations-with-rive/
45. • Immersive mini-games for language learning • Heavily interactive • Lots of Animation + Audio Adventures
46. why kmp?
47. • Android was already built • Company + Personal Interest why kmp?
48. convincing others
49. Code Reuse 🤝 Time Savings product
50. De-risked KMP by rapidly prototyping iOS
55. we made it!
56. class AdventuresStateEngine() { val sceneStateFlow: MutableStateFlow<SceneState?> = MutableStateFlow(null) fun onSceneTouchEvent(event: GridTouchEvent) fun onFrameRefresh(elapsedTime: Duration) fun onGoalButtonClicked() fun onPlayerChoice( choiceId: OptionId, updateChallengeStats: () -> Unit ) }
57. class AdventuresStateEngine() { val sceneStateFlow: MutableStateFlow<SceneState?> = MutableStateFlow(null) fun onSceneTouchEvent(event: GridTouchEvent) fun onFrameRefresh(elapsedTime: Duration) fun onGoalButtonClicked() fun onPlayerChoice( choiceId: OptionId, updateChallengeStats: () -> Unit ) }
58. walking
60. class AdventuresStateEngine() { val sceneStateFlow: MutableStateFlow<SceneState?> = MutableStateFlow(null) fun onSceneTouchEvent(event: GridTouchEvent) fun onFrameRefresh(elapsedTime: Duration) fun onGoalButtonClicked() fun onPlayerChoice( choiceId: OptionId, updateChallengeStats: () -> Unit ) }
61. class AdventuresStateEngine() { val sceneStateFlow: MutableStateFlow<SceneState?> = MutableStateFlow(null) fun onSceneTouchEvent(event: GridTouchEvent) fun onFrameRefresh(elapsedTime: Duration) fun onGoalButtonClicked() fun onPlayerChoice( choiceId: OptionId, updateChallengeStats: () -> Unit ) }
62. func onSceneGesture(_ sender: UILongPressGestureRecognizer) { guard let screenGridHelper else { return } let point = sender.location(in: sceneView.adventuresScene) let gridCoords = screenGridHelper.screenToGridF(screenCoordinates: PointF( x: Float(point.x), y: Float(point.y) ) ) let gridTouchEvent = GridTouchEvent( x: GridUnit(n: gridCoords.x), y: GridUnit(n: gridCoords.y), ) stateEngine.onSceneTouchEvent(event: gridTouchEvent) }
63. class AdventuresStateEngine() { fun onSceneTouchEvent(event: GridTouchEvent) { when (event.action) { GridTouchEvent.Action.UP -> { if (state.mode != SceneMode.ROAM) { return@updateState state } onTileSelected(eventTile) } } }
64. fun walkCharacterTo(character: InstanceId, destination: Point) { updateState { state - > val source = state.objects[character] ?: return@updateState state val bestPath = findBestPath( state, source, mapOf(destination to 0) ) setNewPath(bestPath) state } }
66. class AdventuresStateEngine() { fun onFrameRefresh(elapsedTime: Duration) { updateState { state - > handleDelayedUpdates() walk(elapsedTime) animateSpeechBubble() animateCameraUpdate(elapsedTime) animateLayoutUpdate(elapsedTime) updateNudgeIfNeeded() state.copy(currentTime = state.currentTime + elapsedTime) } } }
67. class AdventuresStateEngine() { fun walk(elapsedTime: Duration) { updateState { startingState - > val nextPathingPosition = currentPath.first() if (state.player.facing == nextPathingPosition.facing) { // walking } else { // rotating } state } } }
69. conversations
70. fun animateSpeechBubble() { updateState { state - > val prevSpan = ttsSpans.lastOrNull { it.startTime.seconds <= elapsedTime } val currentRange = prevSpan.startIndex until prevSpan.endIndex } state.copy( speechBubbles = mapOf( speaker to bubble.copy( text = bubbleText.copy(audioSpan = currentRange))) ) } }
71. animations
72. data class ScreenGridHelper( val tileHeight: Float, val tileWidth: Float, val gridOrigin: PointF, val environmentBounds: Rect, ) { // Maps a position on the grid to screen coordinates fun gridToScreen(gridCoordinates: Point): Point // Maps a tile on the grid to its bounding rectangle on the screen fun getTileScreenBounds(gridCoordinates: Point): Rect }
73. extension AdventuresSceneView { /// Sets the given RiveView's position, hide/unhide, z position, etc. /// according to given layout. func setObjectViewLayout( object: AdventuresObjectModel, gridHelper: ScreenGridHelper ) { // Calculate view rect let p1 = gridHelper.gridToScreenF(gridCoordinates: corner) let p2 = gridHelper.gridToScreenF(gridCoordinates: otherCorner) let rect = CGRect( origin: CGPoint(x: CGFloat(p1.x), y: CGFloat(p2.y)), size: CGSize( width: CGFloat(p2.x - p1.x), height: CGFloat(p1.y - p2.y) ) riveView.frame = rect } )
74. challenges
75. fun interface Closeable { fun close() } class CFlow<T>(private val origin: Flow<T>) : Flow<T> by origin { fun watch(block: (T) -> Unit): Closeable { val job = Job() onEach { block(it) } .launchIn(CoroutineScope(Dispatchers.Unconfined + job)) return Closeable { job.cancel() } } } fun <T : Any> Flow<T>.wrap(): CFlow<T> = CFlow(this)
76. class PriorityQueue<T>(val comparator: Comparator<T>) { fun add(element: T) { ... } fun remove(): T { ... } }
77. good results!
78. • Performant • Faster iteration good results!
79. web!
81. @JsExport class AdventuresEngineHelper(private val engine: AdventuresStateEngine) { var state: SceneState? = null val scope = CoroutineScope(Dispatchers.Unconfined) fun observeEngine() { scope.launch { engine.observeState().collect { value - > state = value } } } }
82. 9 months Android Time = time per engineer × # of engineers
83. 9 months iOS + KMP 5 months Time = time per engineer × # of engineers
84. 9 months Web 5 months 1.5 months Time = time per engineer × # of engineers
85. Mar 2024 (iOS) Jun 2024 (web) Dec 2023 adventures
86. Q3 2024 (iOS) Q4 2024 (Android) Jul 2024 video-call
88. • AI-speaking feature • Build on iOS first • Wanted to communicate with backend via WebSockets video call
89. class VideoCallSession { val webSocketSessionManager = WebSocketSessionManager<UnifiedRequestMessage, UnifiedResponseMessage> fun receiveMessage(message: UnifiedResponseMessage) { ... } fun create(createSessionRequest: CreateSessionRequest) { ... } fun interrupt(interruptEvent: InterruptEvent) { ... } fun tts(ttsEvent: TtsEvent) { ... } fun end(endSessionRequest: EndSessionRequest) { ... } }
90. class VideoCallSession { val webSocketSessionManager = WebSocketSessionManager<UnifiedRequest, UnifiedResponse> fun interrupt(interruptEvent: InterruptEvent) { webSocketSessionManager.sendMessage( InterruptEventMessage(interruptEvent) ) } }
91. func sendInterruptionEvent(_ interruptionEvent: VideoCallInterruptEvent) throws { let handle: VideoCallSession = try self.getRunningHandle() handle.interrupt(interruptEvent: interruptionEvent) }
92. process improvements
93. • Cleaner code • Better build process process improvements
94. developer experience
95. • Mostly maintained by Android devs • Easier to debug for Backend devs developer experience
96. 9 months iOS Time = time per engineer × # of engineers
97. 9 months KMP / WebSockets 1 month Time = time per engineer × # of engineers
98. 9 months Android 1 month 6 months Time = time per engineer × # of engineers
99. Q3 2024 (iOS) Q4 2024 (Android) Jul 2024 video-call
100. math-grading Dec 2024
101. math-grading Dec 2024
102. data class GradingResult( ... )
103. data class GradingResult( val isCorrect: Boolean, val feedbackMessage: String?, )
104. class MathGrader { }
105. class MathGrader { fun gradePlacePoint( userGuess: MathEntity.Point, gradingSpecification: GradingSpecification, ): GradingResult = PlacePointChallengeGrader.grade(userGuess, gradingSpecification) }
106. class MathGrader { fun gradePlacePoint( userGuess: MathEntity.Point, gradingSpecification: GradingSpecification, ): GradingResult = PlacePointChallengeGrader.grade(userGuess, gradingSpecification) } fun gradedAsEqual(other: MathEntity): Boolean { return this == other }
107. class MathGrader { fun gradePlacePoint( userGuess: MathEntity.Point, gradingSpecification: GradingSpecification, ): GradingResult = PlacePointChallengeGrader.grade(userGuess, gradingSpecification) }
108. class MathGrader { fun gradePolygon( userGuess: MathEntity.Polygon, gradingSpecification: GradingSpecification, ): PolygonGradingResult = PolygonChallengeGrader.grade(userGuess, gradingSpecification) }
109. class MathGrader { fun gradeFactorTree( userGuess: RiveHandle, gradingSpecification: GradingSpecification, ): GradingResult = FactorTreeChallengeGrader.grade(userGuess, gradingSpecification) }
110. class MathGrader { }
111. object FactorTreeChallengeGrader { fun grade(userGuess, gradingSpec): GradingResult { val fields = gradingSpec.answerFormat.type.indexFields val treeNodeForTile = fields.map { userGuess.getNumber(property = it) } val isCorrect = isCorrectByGradingSpec( listOf(treeNodeForTile.map { Integer(it) }), gradingSpec, ) if (isCorrect) { return GradingResult(isCorrect, feedbackMessage = null) } val correctNode = closestCorrectAnswer(treeNodeForTile, gradingSpec.rules) val first = firstIncorrectBlankIndex(treeNodeForTile, correctNode) val feedbackMessage = (gradingSpec.gradingFeedback as FactorTree) .factorTreeFeedback .feedbackOptions .getOrNull(first) return GradingResult(isCorrect = false, feedbackMessage = feedbackMessage) } } 0 1 2 3 4 0 1 2 3 4
112. 1.5 months KMP Time = time per engineer × # of engineers
113. 1.5 months iOS Time = time per engineer × # of engineers 1 week
114. 1.5 months Android Time = time per engineer × # of engineers 1 week 1 week
115. math-grading Jan 2025 (iOS, Android)
116. Q1 2025 chess!
117. • Our most recent course
118. • Our most recent course • iOS first, KMP for Android port
119. • Our most recent course • iOS first, KMP for Android port • KMP + Rive to Drive UI
120. data class ChessGameState( val chessBoardState: ChessBoardState, val setupModel: ChessGameSetupModel, val userColor: ChessPlayerColor = ChessPlayerColor.WHITE, val currentPlayer: ChessPlayerColor = ChessPlayerColor.WHITE, val moveHistory: List<ChessMove> = emptyList(), val selectedPosition: ChessCoordinate? = null, val validMovesForSelectedPosition: List<ChessMove> = emptyList(), ... )
121. data class ChessGameState( val chessBoardState: ChessBoardState, val setupModel: ChessGameSetupModel, val userColor: ChessPlayerColor = ChessPlayerColor.WHITE, val currentPlayer: ChessPlayerColor = ChessPlayerColor.WHITE, val moveHistory: List<ChessMove> = emptyList(), val selectedPosition: ChessCoordinate? = null, val validMovesForSelectedPosition: List<ChessMove> = emptyList(), ... ) { fun getValidMoveFromString(moveString: String): ChessMove? { .. . } fun getEligibleMovesForPosition( position: ChessCoordinate, color: ChessPlayerColor? = null, ): List<ChessMove> { ... }
122. public class ChessRiveUseCase(dispatcher: CoroutineDispatcher) { private val coroutineScope = CoroutineScope(SupervisorJob() + dispatcher) private lateinit var gameUseCase: ChessGameUseCase public fun initializeGame(setupModel: ChessGameSetupModel) { gameUseCase = ChessGameUseCase(setupModel) chessGameState = gameUseCase.gameState.value } private var chessGameState: ChessGameState? = null set(value) { value ?. let { state - > field = value updateCurrentPlayerColor(state.currentPlayer) setKingState() updateRiveAssetDataForGameState(state) } }
123. public class ChessRiveUseCase(dispatcher: CoroutineDispatcher) { private val coroutineScope = CoroutineScope(SupervisorJob() + dispatcher) private lateinit var gameUseCase: ChessGameUseCase public fun initializeGame(setupModel: ChessGameSetupModel) { gameUseCase = ChessGameUseCase(setupModel) chessGameState = gameUseCase.gameState.value } private var chessGameState: ChessGameState? = null set(value) { value ?. let { state - > field = value updateCurrentPlayerColor(state.currentPlayer) setKingState() updateRiveAssetDataForGameState(state) } }
125. 20 months iOS Time = time per engineer × # of engineers
126. 20 months KMP Library 1 month Time = time per engineer × # of engineers
127. 20 months Android 1 month 12 months* *estimate Time = time per engineer × # of engineers
130. Q1 2025 chess!
131. maker Q2 2025
132. going kmp-first
135. • Mp4 needs to be compressed • Switch from MP4 -> Rive
136. class ModularRiveEngine { private fun handleEventIfNeeded() { private fun handleSubtitleEvent(event: SubtitleEvent) { updateState { state - > state.copy(currentSubtitle = event.text) } } }
137. func updateSubtitle(with newState: ModularRiveState) { let oldSubtitle = state.currentSubtitle let newSubtitle = newState.currentSubtitle guard let newSubtitle, let subtitleRiveVM = riveMapping[ModularRiveSubtitleVM.subtitleId], oldSubtitle != newSubtitle else { // Subtitle did not change return } subtitleRiveVM.triggerInput( "enter_trig", path: nextSubtitleArtboard.rawValue ) }
140. 1 month KMP Time = time per engineer × # of engineers
141. 1 month iOS Time = time per engineer × # of engineers 2 weeks
142. 1 month Android Time = time per engineer × # of engineers 2 weeks 1 week
144. recap
145. Started iOS Android experiments-lib Nov 2023 - - adventures Dec 2023 Mar 2024 n/a video-call Jul 2024 Q3 2024 Q4 2024 math-grading Dec 2024 Jan 2025 (partial) Jan 2025 (partial) chess Q1 2025 Q2 2025 soon! maker Q2 2025 Q2 2025 Q2 2025
146. KMP Familiarity KMP Usefulness
147. what’s next?
148. "KMP first" strategy
150. ai + kmp
153. thank you! @jrodbx @johnnyeet
154. Thank You, and Don ’ t Forget to Vote @twitter