Kotlin Coroutines in Practice @ KotlinConf 2018

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elizarov@ Roman Elizarov Kotlin Coroutines in Practice @relizarov

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Coroutines recap

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Coroutines are like light-weight threads

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suspend fun main() { val jobs = List(100_000) { GlobalScope.launch { delay(5000) print(".") } } jobs.forEach { it.join() } } Coroutines are like light-weight threads

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suspend fun main() { val jobs = List(100_000) { GlobalScope.launch { delay(5000) print(".") } } jobs.forEach { it.join() } } Coroutines are like light-weight threads

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suspend fun main() { val jobs = List(100_000) { GlobalScope.launch { delay(5000) print(".") } } jobs.forEach { it.join() } } Coroutines are like light-weight threads

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Quantity

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Quantity → Quality

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A practical challenge suspend fun downloadContent(location: Location): Content

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fun processReferences(refs: List) References

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References fun processReferences(refs: List) { for (ref in refs) { … } }

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References Locations resolve fun processReferences(refs: List) { for (ref in refs) { val location = ref.resolveLocation() … } }

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References Locations Contents resolve download fun processReferences(refs: List) { for (ref in refs) { val location = ref.resolveLocation() val content = downloadContent(location) processContent(ref, content) } }

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References Locations Contents resolve download suspend fun processReferences(refs: List) { for (ref in refs) { val location = ref.resolveLocation() val content = downloadContent(location) processContent(ref, content) } }

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References Locations Contents resolve download fun processReferences(refs: List) { for (ref in refs) { val location = ref.resolveLocation() GlobalScope.launch { val content = downloadContent(location) processContent(ref, content) } } } Parallel

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fun processReferences(refs: List) { for (ref in refs) { val location = ref.resolveLocation() GlobalScope.launch { val content = downloadContent(location) processContent(ref, content) } } } Coroutines are cheap! What could go wrong?

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ref1 location1 Launch download ref2 location2 Launch download ref3 Crash! Crash! Leak! fun processReferences(refs: List) { for (ref in refs) { val location = ref.resolveLocation() GlobalScope.launch { val content = downloadContent(location) processContent(ref, content) } } }

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Structured concurrency

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fun processReferences(refs: List)

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suspend fun processReferences(refs: List)

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suspend fun processReferences(refs: List) = coroutineScope { … }

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suspend fun processReferences(refs: List) = coroutineScope { for (ref in refs) { val location = ref.resolveLocation() GlobalScope.launch { val content = downloadContent(location) processContent(ref, content) } } }

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suspend fun processReferences(refs: List) = coroutineScope { for (ref in refs) { val location = ref.resolveLocation() launch { val content = downloadContent(location) processContent(ref, content) } } }

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Crash? suspend fun processReferences(refs: List) = coroutineScope { for (ref in refs) { val location = ref.resolveLocation() launch { val content = downloadContent(location) processContent(ref, content) } } }

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Crash? cancels Waits for completion suspend fun processReferences(refs: List) = coroutineScope { for (ref in refs) { val location = ref.resolveLocation() launch { val content = downloadContent(location) processContent(ref, content) } } }

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The state

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References Contents Download process

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Reference 1 Content Reference 2 Location Download process State

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class Downloader { }

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class Downloader { private val requested = mutableSetOf() }

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class Downloader { private val requested = mutableSetOf() fun downloadReference(ref: Reference) { val location = ref.resolveLocation() … } }

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class Downloader { private val requested = mutableSetOf() fun downloadReference(ref: Reference) { val location = ref.resolveLocation() if (requested.add(location)) { // schedule download } } }

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class Downloader { private val requested = mutableSetOf() fun downloadReference(ref: Reference) { val location = ref.resolveLocation() if (requested.add(location)) { // schedule download } // ... wait for result ... processContent(ref, content) } }

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Shared Mutable State Share by Communicating

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Synchronization Primitives Communication Primitives

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classes coroutines

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launch { val requested = mutableSetOf() … } Does not share mutable state

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launch { val requested = mutableSetOf() for (ref in references) { val location = ref.resolveLocation() if (requested.add(location)) { // schedule download } // ... wait for result ... processContent(ref, content) } }

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Channel References Downloader

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fun CoroutineScope.downloader( references: ReceiveChannel, ) = launch { … }

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fun CoroutineScope.downloader( references: ReceiveChannel, ) = launch { … }

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fun CoroutineScope.downloader( references: ReceiveChannel, ) = launch { … }

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fun CoroutineScope.downloader( references: ReceiveChannel, ) = launch { … } Convention

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fun CoroutineScope.downloader( references: ReceiveChannel, ) = launch { val requested = mutableSetOf() for (ref in references) { val location = ref.resolveLocation() if (requested.add(location)) { // schedule download } // ... wait for result ... processContent(ref, content) } }

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fun CoroutineScope.downloader( references: ReceiveChannel, ) = launch { val requested = mutableSetOf() for (ref in references) { val location = ref.resolveLocation() if (requested.add(location)) { launch { … } } // ... wait for result ... processContent(ref, content) } } Coroutines are cheap! What could go wrong?

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fun CoroutineScope.downloader( references: ReceiveChannel, ) = launch { val requested = mutableSetOf() for (ref in references) { val location = ref.resolveLocation() if (requested.add(location)) { launch { … } } // ... wait for result ... processContent(ref, content) } } Child

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fun CoroutineScope.downloader( references: ReceiveChannel, ) = launch { val requested = mutableSetOf() for (ref in references) { val location = ref.resolveLocation() if (requested.add(location)) { launch { … } } // ... wait for result ... processContent(ref, content) } } Coroutines are cheap! But the work they do…

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Limiting concurrency

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Worker 1 Worker 2 Worker 3 Worker N Worker pool … References Downloader references locations

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fun CoroutineScope.downloader( references: ReceiveChannel, locations: SendChannel )

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fun CoroutineScope.downloader( references: ReceiveChannel, locations: SendChannel ) = launch { val requested = mutableSetOf() for (ref in references) { val location = ref.resolveLocation() if (requested.add(location)) { locations.send(location) } } }

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fun CoroutineScope.worker( locations: ReceiveChannel )

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fun CoroutineScope.worker( locations: ReceiveChannel )

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fun CoroutineScope.worker( locations: ReceiveChannel ) = launch { for (loc in locations) { val content = downloadContent(loc) processContent(ref, content) } }

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fun CoroutineScope.worker( locations: ReceiveChannel ) = launch { for (loc in locations) { val content = downloadContent(loc) processContent(ref, content) } } Fan-out

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fun CoroutineScope.worker( locations: ReceiveChannel ) = launch { for (loc in locations) { val content = downloadContent(location) processContent(ref, content) } }

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fun CoroutineScope.worker( locations: ReceiveChannel ) = launch { for (loc in locations) { val content = downloadContent(loc) processContent(ref, content) } }

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fun CoroutineScope.worker( locations: ReceiveChannel ) = launch { for (loc in locations) { val content = downloadContent(loc) processContent(ref, content) } }

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Worker 1 Worker 2 Worker 3 Worker N … References Downloader

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Worker 1 Worker 2 Worker 3 Worker N … References Downloader Refs ↔ Locs location & content

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data class LocContent(val loc: Location, val content: Content)

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data class LocContent(val loc: Location, val content: Content) fun CoroutineScope.worker( locations: ReceiveChannel, contents: SendChannel )

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data class LocContent(val loc: Location, val content: Content) fun CoroutineScope.worker( locations: ReceiveChannel, contents: SendChannel ) = launch { for (loc in locations) { val content = downloadContent(loc) contents.send(LocContent(loc, content)) } }

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Worker 1 Worker 2 Worker 3 Worker N … References Downloader locations contents

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fun CoroutineScope.downloader( references: ReceiveChannel, locations: SendChannel, contents: ReceiveChannel )

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fun CoroutineScope.downloader( references: ReceiveChannel, locations: SendChannel, contents: ReceiveChannel ) = launch { val requested = mutableSetOf() for (ref in references) { val location = ref.resolveLocation() if (requested.add(location)) { locations.send(location) } } } Hmm….

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Select

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select { references.onReceive { ref -> … } contents.onReceive { (loc, content) -> … } }

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select { references.onReceive { ref -> … } contents.onReceive { (loc, content) -> … } }

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select { references.onReceive { ref -> … } contents.onReceive { (loc, content) -> … } }

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launch { val requested = mutableMapOf>() … }

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launch { val requested = mutableMapOf>() while (true) { select { references.onReceive { ref -> … } contents.onReceive { (loc, content) -> … } } } }

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launch { val requested = mutableMapOf>() while (true) { select { references.onReceive { ref -> … } contents.onReceive { (loc, content) -> … } } } }

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launch { val requested = mutableMapOf>() while (true) { select { references.onReceive { ref -> val loc = ref.resolveLocation() … } contents.onReceive { (loc, content) -> … } } } }

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launch { val requested = mutableMapOf>() while (true) { select { references.onReceive { ref -> val loc = ref.resolveLocation() val refs = requested[loc] … } contents.onReceive { (loc, content) -> … } } } }

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launch { val requested = mutableMapOf>() while (true) { select { references.onReceive { ref -> val loc = ref.resolveLocation() val refs = requested[loc] if (refs == null) { requested[loc] = mutableListOf(ref) locations.send(loc) } } contents.onReceive { (loc, content) -> … } } } }

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launch { val requested = mutableMapOf>() while (true) { select { references.onReceive { ref -> val loc = ref.resolveLocation() val refs = requested[loc] if (refs == null) { requested[loc] = mutableListOf(ref) locations.send(loc) } else { refs.add(ref) } } contents.onReceive { (loc, content) -> … } } } }

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launch { val requested = mutableMapOf>() while (true) { select { references.onReceive { ref -> val loc = ref.resolveLocation() val refs = requested[loc] if (refs == null) { requested[loc] = mutableListOf(ref) locations.send(loc) } else { refs.add(ref) } } contents.onReceive { (loc, content) -> … } } } } No concurrency No synchronization

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launch { val requested = mutableMapOf>() while (true) { select { references.onReceive { ref -> … } contents.onReceive { (loc, content) -> val refs = requested.remove(loc)!! for (ref in refs) { processContent(ref, content) } } } } }

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Putting it all together

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Worker 1 Worker 2 Worker 3 Worker N … References Downloader locations contents references

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Worker 1 Worker 2 Worker 3 Worker N … References Downloader locations contents references

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fun CoroutineScope.processReferences( references: ReceiveChannel )

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fun CoroutineScope.processReferences( references: ReceiveChannel )

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fun CoroutineScope.processReferences( references: ReceiveChannel ) { val locations = Channel() val contents = Channel() repeat(N_WORKERS) { worker(locations, contents) } downloader(references, locations, contents) }

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Worker 1 Worker 2 Worker 3 Worker N … References Downloader locations contents references

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Worker 1 Worker 2 Worker 3 Worker N … References Downloader locations contents references processReferences

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Worker 1 Worker 2 Worker 3 Worker N … References Downloader locations contents references processReferences Patterns everywhere Worker pool Actor

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fun CoroutineScope.processReferences(…)

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Root CoroutineScope

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class SomethingWithLifecycle { }

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class SomethingWithLifecycle : CoroutineScope { }

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class SomethingWithLifecycle : CoroutineScope { override val coroutineContext: CoroutineContext get() = … }

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class SomethingWithLifecycle : CoroutineScope { private val job = Job() override val coroutineContext: CoroutineContext get() = … }

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class SomethingWithLifecycle : CoroutineScope { private val job = Job() fun dispose() { … } override val coroutineContext: CoroutineContext get() = … }

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class SomethingWithLifecycle : CoroutineScope { private val job = Job() fun close() { … } override val coroutineContext: CoroutineContext get() = … }

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class SomethingWithLifecycle : CoroutineScope { private val job = Job() fun close() { job.cancel() } override val coroutineContext: CoroutineContext get() = … }

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class SomethingWithLifecycle : CoroutineScope { private val job = Job() fun close() { job.cancel() } override val coroutineContext: CoroutineContext get() = job }

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class SomethingWithLifecycle : CoroutineScope { private val job = Job() fun close() { job.cancel() } override val coroutineContext: CoroutineContext get() = job + Dispatchers.Main }

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class SomethingWithLifecycle : CoroutineScope { … override val coroutineContext: CoroutineContext get() = job + Dispatchers.Main fun doSomething() { } }

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class SomethingWithLifecycle : CoroutineScope { … override val coroutineContext: CoroutineContext get() = job + Dispatchers.Main fun doSomething() { launch { … } } }

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class SomethingWithLifecycle : CoroutineScope { … override val coroutineContext: CoroutineContext get() = job + Dispatchers.Main fun doSomething() { processReferences(references) } } Never leak any coroutines

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suspend vs scope

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suspend fun downloadContent(location: Location): Content

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suspend fun downloadContent(location: Location): Content Does something long & waits for it to complete without blocking

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suspend fun downloadContent(location: Location): Content fun CoroutineScope.processReferences(…)

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suspend fun downloadContent(location: Location): Content fun CoroutineScope.processReferences(…) Launches new coroutines & quickly returns, does not wait for them