Kotlin Coroutines in Practice Roman Eliza zarov !"#$%&( - - PowerPoint PPT Presentation

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Kotlin Coroutines in Practice Roman Eliza zarov !"#$%&( - - PowerPoint PPT Presentation

Jun 25, 2023 154 likes •1.45k views

Kotlin Coroutines in Practice Roman Eliza zarov !"#$%&( ) )&!"#$%&( Coroutines recap Coroutines are like light-weight threads suspend fun main() { val jobs = List (100_000) { GlobalScope. launch { delay (5000)

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!"#$%&’( ) Roman Eliza zarov

Kotlin Coroutines in Practice

)&!"#$%&’(

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Coroutines recap

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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 f un main() { val jobs = List(100_000) { GlobalScope.launch { delay(5000) print(". ") } } jobs.forEach { i t .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<Reference>)

References

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References

f un processReferences(refs: List<Reference>) { f or (ref i n refs) { … } }

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References Locations

resolve

fun processReferences(refs: List<Reference>) { for (ref in refs) { val location = ref.resolveLocation() … } }

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References Locations Contents

resolve download

fun processReferences(refs: List<Reference>) { 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<Reference>) { 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<Reference>) { for (ref in refs) { val location = ref.resolveLocation() val content = downloadContent(location) processContent(ref, content) } }

Sequential

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References Locations Contents

resolve download

fun processReferences(refs: List<Reference>) { for (ref in refs) { val location = ref.resolveLocation() val content = downloadContent(location) processContent(ref, content) } }

Parallel

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References Locations Contents

resolve download

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

ref1 location1 Launch download ref2 location2 Launch download ref3 Crash! Crash!

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ref1 location1 Launch download ref2 location2 Launch download ref3 Crash! Crash! Leak!

fun processReferences(refs: List<Reference>) { 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<Reference>)

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suspend fun processReferences(refs: List<Reference>)

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

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

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

Child

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Crash?

suspend fun pr ocessRef er ences( r ef s: Li st <Ref er ence>) = cor out i neScope { for ( r ef in r ef s) { val l ocat i on = r ef . r esol veLocat i on( ) l aunch { val cont ent = downl oadCont ent ( l ocat i on) pr ocessCont ent ( r ef , cont ent ) } } }

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

Crash?

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

Crash? cancels

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Crash? cancels Waits for completion

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

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

Never leaks jobs

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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<Location>() }

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

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

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

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

Concurrent

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

Concurrent

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

Shared mutable state

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

Shared mutable state Needs Syn ynchroniza zation

Shared + Mutable =

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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 r equest ed = mutableSetOf<Locat i on>( ) … }

Does not share mutable state

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

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launch { val requested = mutableSetOf<Location>() 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<Reference>, ) = launch { … }

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

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

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

Convention

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fun Cor out i neScope. downl oader ( r ef er ences: Recei veChannel <Ref er ence>, ) = launch { val r equest ed = mutableSetOf<Locat i on>( ) for ( r ef in r ef er ences) { val l ocat i on = r ef . r esol veLocat i on( ) if ( r equest ed. add( l ocat i on) ) { // schedule download } // ... wait for result ... processContent( r ef , cont ent ) } }

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

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f un Cor out i neScope. downl oader ( r ef er ences: Recei veChannel <Ref er ence>, ) = launch { val r equest ed = mutableSetOf<Locat i on>( ) f or ( r ef i n r ef er ences) { val l ocat i on = r ef . r esol veLocat i on( ) i f ( r equest ed. add( l ocat i on) ) { // schedule download } // ... wait for result ... processContent( r ef , cont ent ) } }

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fun CoroutineScope.downloader( references: ReceiveChannel<Reference>, ) = launch { val requested = mutableSetOf<Location>() 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<Reference>, ) = launch { val requested = mutableSetOf<Location>() 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<Reference>, ) = l aunch { val requested = m ut abl eSet O f <Location>() for (ref in references) { val location = ref.resolveLocation() if (requested.add(location)) { l aunch { … } } / / . . . wai t f or r esul t . . . pr ocessCont ent (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 Cor out i neScope. downl oader ( r ef er ences: Recei veChannel <Ref er ence>, l ocat i ons: SendChannel <Locat i on> )

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

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fun CoroutineScope.worker( locations: ReceiveChannel<Location> )

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fun CoroutineScope.worker( locations: ReceiveChannel<Location> )

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

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f un CoroutineScope.worker( locations: ReceiveChannel<Location> ) = launch { f or (loc i n locations) { val content = downloadContent(loc) processContent(ref, content) } }

Fan-out

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

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

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fun Cor out i neScope. wor ker ( l ocat i ons: Recei veChannel <Locat i on> ) = launch { for ( l oc in l ocat i ons) { val cont ent = downloadContent( l oc) processContent( r ef , cont ent ) } }

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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

…

Ref er ences Downl oader Ref s ↔ Locs l ocat i on & cont ent

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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<Location>, contents: SendChannel<LocContent> )

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

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fun Cor out i neScope. downl oader ( r ef er ences: Recei veChannel <Ref er ence>, l ocat i ons: SendChannel <Locat i on>, cont ent s: Recei veChannel <LocCont ent > ) = launch { val r equest ed = mutableSetOf<Locat i on>( ) for ( r ef in r ef er ences) { val l ocat i on = r ef . r esol veLocat i on( ) if ( r equest ed. add( l ocat i on) ) { l ocat i ons. send( l ocat i on) } } }

Hmm….

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Select

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sel ect { r ef er ences. onRecei ve { r ef - > … } cont ent s. onRecei ve { ( l oc, cont ent ) - > … } }

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

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

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launch { val requested = mutableMapOf<Location, MutableList<Reference>>() … }

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launch { val r equest ed = mutableMapOf<Locat i on, M ut abl eLi st <Ref er ence>>( ) while ( true) { sel ect <Uni t > { r ef er ences. onReceive { r ef -> … } cont ent s. onReceive { ( l oc, cont ent ) -> … } } } }

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launch { val r equest ed = mutableMapOf<Locat i on, M ut abl eLi st <Ref er ence>>( ) while ( true) { sel ect <Uni t > { r ef er ences. onReceive { r ef -> … } cont ent s. onReceive { ( l oc, cont ent ) -> … } } } }

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

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

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launch { val requested = mutableMapOf<Location, MutableList<Reference>>() while (true) { select<Unit> { 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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l aunch { val requested = m ut abl eM apO f <Location, MutableList<Reference>>() while (true) { select<Unit> { references.onReceive { ref -> val loc = ref.resolveLocation() val refs = requested[loc] if (refs != null) { requested[loc] = m ut abl eLi st O f (ref) locations.send(loc) } else { refs.add(ref) } } contents.onReceive { (loc, content) -> … } } } }

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launch { val requested = mutableMapOf<Location, MutableList<Reference>>() while (true) { select<Unit> { 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 r equest ed = mutableMapOf<Locat i on, M ut abl eLi st <Ref er ence>>( ) while ( true) { sel ect <Uni t > { r ef er ences. onReceive { r ef -> … } cont ent s. onReceive { ( l oc, cont ent ) -> val r ef s = r equest ed. r em

  • ve( l oc) ! !

for ( r ef in r ef s) { processContent( r ef , cont ent ) } } } } }

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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<Reference> )

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fun Cor out i neScope. pr ocessRef er ences( r ef er ences: Recei veChannel <Ref er ence> )

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

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fun CoroutineScope.processReferences( references: ReceiveChannel<Reference> ) { val locations = Channel<Location>() val contents = Channel<LocContent>() 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

…

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fun CoroutineScope.processReferences(…)

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Root CoroutineScope

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class Som et hi ngW i t hLi f ecycl e { }

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class SomethingWithLifecycle : CoroutineScope { }

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class SomethingWithLifecycle : CoroutineScope {

  • verride val coroutineContext: CoroutineContext

get() = … }

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class SomethingWithLifecycle : CoroutineScope { private val job = Job()

  • verride val coroutineContext: CoroutineContext

get() = … }

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

  • verride val coroutineContext: CoroutineContext

get() = … }

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

  • verride val coroutineContext: CoroutineContext

get() = … }

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

  • verride val coroutineContext: CoroutineContext

get() = … }

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

  • verride val coroutineContext: CoroutineContext

get() = job }

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class Som et hi ngW i t hLi f ecycl e : Cor out i neScope { private val job = Job( ) fun cl ose( ) {

  • job. cancel ( )

}

  • verride val coroutineContext: Cor out i neCont ext

get( ) = job + Di spat cher s. Main }

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class SomethingWithLifecycle : CoroutineScope { …

  • verride val coroutineContext: CoroutineContext

get() = job + Dispatchers.Main fun doSomething() { } }

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cl ass SomethingWithLifecycle : CoroutineScope { …

  • ver r i de val

cor out i neCont ext : CoroutineContext get () = j ob + Dispatchers.M ai n f un doSomething() { l aunch { … } } }

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class SomethingWithLifecycle : CoroutineScope { …

  • verride 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 downl oadCont ent ( l ocat i on: Locat i on) : Cont ent

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suspend fun downloadContent(location: Location): Content

Does something long & waits for it to complete without

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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

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Takeaway

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Coroutines are like light-weight threads

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Coroutines are NO

NOT like

threads

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Coroutines are NO

NOT like

threads

Rethink the way you structure your code

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Thank k yo you

Any y quest stions? s?

elizarov@ Roman Eliza zarov @relizarov