Spring-Flavored Kotlin Coroutines

by Roman Elizarov

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Spring-Flavored Kotlin Coroutines Roman Elizarov October 7–10, 2019 Austin Convention Center

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Speaker: Roman Elizarov Professional developer since 2000 Previously developed high-perf trading software @ Devexperts Teach concurrent & distributed programming @ St. Petersburg ITMO University Chief judge @ Northern Eurasia Contest / ICPC Now team lead in Kotlin Libraries @ JetBrains elizarov @ relizarov

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Kotlin – Programming Language for

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Kotlin – Programming Language for

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Backend evolution Starting with “good old days”

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ET 1 ET 2 ET N … DB Old-school client-server monolith Clients Executor Threads

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ET 1 ET 2 ET N … DB Incoming request Clients Executor Threads

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ET 1 ET 2 ET N … DB Blocks tread Clients Executor Threads

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ET 1 ET 2 ET N … DB Sizing threads – easy Clients N = number of DB connections Executor Threads

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Services

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ET 1 ET 2 ET N … DB Old-school client-server monolith Clients Executor Threads

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ET 1 ET 2 ET N … DB Now with Services Service Clients Executor Threads

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ET 1 ET 2 ET N … Services everywhere … Service K Service 1 Service 2 Clients Executor Threads

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Complex business logic fun placeOrder(order: Order): Response { … }

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Complex business logic fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) … }

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Complex business logic fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) val margin = if (account.isOptionsAccount) … }

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Complex business logic fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) val margin = if (account.isOptionsAccount) { marginService.loadMargin(account) } else { defaultMargin } … }

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What if a service is slow? fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) val margin = if (account.isOptionsAccount) { marginService.loadMargin(account) } else { defaultMargin } return validateOrder(order, margin) }

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Slow service @Service class MarginService { private val rest = RestTemplate() fun loadMargin(account: Account): Margin = rest.getForObject( "http://localhost:9090/margin/${account.id}", Margin::class.java)!! }

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Demo Slow service, blocking

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ET 2 ET N … Clients Blocks threads … Service K Service 1 Service 2 ET 1 Executor Threads

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ET 2 ET N … Clients Blocks threads … Service K Service 1 Service 2 ET 1 Executor Threads

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ET 2 ET N … Clients Blocks threads … Service K Service 1 Service 2 ET 1 Executor Threads

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Code that waits

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Asynchronous programming Writing code that waits

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ET 2 ET N … Clients Instead of blocking… … Service K Service 1 Service 2 ET 1 Executor Threads

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ET 2 ET N … Release the thread … Service K Service 1 Service 2 ET 1 Clients Executor Threads

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Clients ET 2 ET N … Resume operation later … Service K Service 1 Service 2 ET 1 Executor Threads

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But how? fun loadMargin(account: Account): Margin

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But how? Callbacks fun loadMargin(account: Account, callback: (Margin) -> Unit)

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But how? Callbacks Futures/Promises fun loadMargin(account: Account): CompletableFuture

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But how? Callbacks Futures/Promises/Reactive fun loadMargin(account: Account): Mono

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Reactive

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The logic fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) val margin = if (account.isOptionsAccount) { marginService.loadMargin(account) } else { defaultMargin } return validateOrder(order, margin) }

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Going reactive fun placeOrder(order: Order): Mono

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Going reactive fun placeOrder(order: Order): Mono =

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Going reactive fun placeOrder(order: Order): Mono = accountService.loadAccount(order.accountId)

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Going reactive fun placeOrder(order: Order): Mono = accountService.loadAccount(order.accountId) .flatMap { account -> … }

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Going reactive fun placeOrder(order: Order): Mono = accountService.loadAccount(order.accountId) .flatMap { account -> if (account.isOptionsAccount) { marginService.loadMargin(account) } else { Mono.just(defaultMargin) } }

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Reactive Service @Service class MarginService { private val client = WebClient.create("http://localhost:9090/") fun loadMargin(account: Account): Mono = client .get() .uri("margin/${account.id}") .retrieve() .bodyToMono(Margin::class.java) }

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Demo: Reactive Was it worth it?

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Choice 1: Direct code, blocking fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) val margin = if (account.isOptionsAccount) { marginService.loadMargin(account) } else { defaultMargin } return validateOrder(order, margin) }

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Choice 2: Complex code, non-blocking fun placeOrder(order: Order): Mono = accountService.loadAccount(order.accountId) .flatMap { account -> if (account.isOptionsAccount) { marginService.loadMargin(account) } else { Mono.just(defaultMargin) } } .map { margin -> validateOrder(order, margin) }

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Why not both?

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But how? Callbacks Futures/Promises/Reactive fun loadMargin(account: Account): Mono

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But how? Callbacks Futures/Promises/Reactive Kotlin Coroutines suspend fun loadMargin(account: Account): Margin

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Direct code: Blocking fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) val margin = if (account.isOptionsAccount) { marginService.loadMargin(account) } else { defaultMargin } return validateOrder(order, margin) }

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Direct code: Coroutines suspend fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) val margin = if (account.isOptionsAccount) { marginService.loadMargin(account) } else { defaultMargin } return validateOrder(order, margin) } Write regular code! Call suspending funs

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Coroutines Service @Service class MarginService { private val client = WebClient.create("http://localhost:9090/") suspend fun loadMargin(account: Account): Margin = client .get() .uri("margin/${account.id}") .retrieve() .awaitBody() }

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Demo: Coroutines

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No content

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Suspend behind the scenes suspend fun loadMargin(account: Account): Margin

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Suspend behind the scenes suspend fun loadMargin(account: Account): Margin fun loadMargin(account: Account, cont: Continuation) But why callback and not future/mono?

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Performance! Callback is a simple, low-level primitive Integration with async IO libraries is easy

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Service integration suspend fun loadMargin(account: Account): Margin

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Service integration suspend fun loadMargin(account: Account): Margin = suspendCoroutine { cont -> // install callback & use cont to resume }

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Server integrated with coroutines @PostMapping("/order") suspend fun placeOrder(order: Order): Response { … }

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Server not integrated with coroutines @PostMapping("/order") fun placeOrder(order: Order): Mono = mono { … } Coroutine builder

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Blocking integration @Component class Initializer { @EventListener(ApplicationStartedEvent::class) fun initData() { … } }

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Blocking integration @Component class Initializer { @EventListener(ApplicationStartedEvent::class) fun initData() = runBlocking { … } } Coroutine builder

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Suspend is efficient

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Suspend is efficient suspend fun placeOrder(order: Order): Response { val account = accountService.loadAccout(order.accountId) val margin = if (account.isOptionsAccount) { marginService.loadMargin(account) } else { defaultMargin } return validateOrder(order, margin) } One object allocated

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Reactive fun placeOrder(order: Order): Mono = accountService.loadAccount(order.accountId) .flatMap { account -> if (account.isOptionsAccount) { marginService.loadMargin(account) } else { Mono.just(defaultMargin) } } .map { margin -> validateOrder(order, margin) } Lambda allocated* Mono allocated Lambda allocated Mono allocated

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Let’s go deeper fun placeOrder(params: Params): Mono { // check pre-conditions return actuallyPlaceOrder(order) } fun actuallyPlaceOrder(order: Order): Mono

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Let’s go deeper (with coroutines) suspend fun placeOrder(params: Params): Response { // check pre-conditions return actuallyPlaceOrder(order) } suspend fun actuallyPlaceOrder(params: Params): Response Tail call optimization Tail call

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Call stack with coroutines Coroutine Builder placeOrder actuallyPlaceOrder moreLogic marginService.loadMargin suspendCoroutine

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Call stack with coroutines Coroutine Builder placeOrder actuallyPlaceOrder moreLogic marginService.loadMargin suspendCoroutine unwind Continuation in heap

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Learn more KotlinConf San Francisco 2017 GOTO Copenhagen 2018

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Scaling with coroutines With thread pools

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ET 2 ET N … Clients Thread pools ET 1 Executor Threads

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ET 2 ET N … Clients Thread pools ET 1 ST 2 ST M1 … S1 1 N = number of CPU cores M1 = depends Service 1 Threads Executor Threads

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IO-bound (blocking) fun loadAccount(order: Order): Account { // some blocking code here.... }

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IO-bound suspend fun loadAccount(order: Order): Account { // some blocking code here.... }

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IO-bound withContext suspend fun loadAccount(order: Order): Account = withContext(dispatcher) { // some blocking code here.... }

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IO-bound withContext suspend fun loadAccount(order: Order): Account = withContext(dispatcher) { // some blocking code here.... } val dispatcher = Executors.newFixedThreadPool(M2).asCoroutineDispatcher()

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CPU-bound code fun validateOrder(order: Order, margin: Margin): Response { // perform CPU-consuming computation }

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CPU-bound code suspend fun validateOrder(order: Order, margin: Margin): Response = withContext(compute) { // perform CPU-consuming computation } val compute = Executors.newFixedThreadPool(M3).asCoroutineDispatcher()

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ET 2 ET N … Clients Fine-grained control and encapsulation ET 1 S1 1 ST M1 Service 2 Threads S1 1 ST M2 Service 3 Threads S1 1 ST M3 Async IO-bound CPU- bound Never blocked Service 1 Threads Executor Threads

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But there’s more!

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Cancellation

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withTimeout suspend fun placeOrder(order: Order): Response = withTimeout(1000) { // code before loadMargin(account) // code after }

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withTimeout propagation suspend fun placeOrder(order: Order): Response = withTimeout(1000) { // code before loadMargin(account) // code after } suspend fun loadMargin(account: Account): Margin = suspendCoroutine { cont -> // install callback & use cont to resume }

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withTimeout propagation suspend fun placeOrder(order: Order): Response = withTimeout(1000) { // code before loadMargin(account) // code after } suspend fun loadMargin(account: Account): Margin = suspendCancellableCoroutine { cont -> // install callback & use cont to resume }

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withTimeout propagation suspend fun placeOrder(order: Order): Response = withTimeout(1000) { // code before loadMargin(account) // code after } suspend fun loadMargin(account: Account): Margin = suspendCancellableCoroutine { cont -> // install callback & use cont to resume cont.invokeOnCancellation { … } }

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Concurrency Multiple things at the same time

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Example fun placeOrder(order: Order): Response { val account = accountService.loadAccount(order) val margin = marginService.loadMargin(order) return validateOrder(order, account, margin) }

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Example fun placeOrder(order: Order): Response { val account = accountService.loadAccount(order) val margin = marginService.loadMargin(order) return validateOrder(order, account, margin) } No data dependencies

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Concurrency with async (futures) fun placeOrder(order: Order): Response { val account = accountService.loadAccountAsync(order) val margin = marginService.loadMarginAsync(order) return validateOrder(order, account.await(), margin.await()) }

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Structured concurrency

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Concurrency with coroutines suspend fun placeOrder(order: Order): Response = coroutineScope { val account = async { accountService.loadAccount(order) } val margin = async { marginService.loadMargin(order) } validateOrder(order, account.await(), margin.await()) }

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Enforcing structure

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Without coroutine scope? suspend fun placeOrder(order: Order): Response { val account = async { accountService.loadAccount(order) } val margin = async { marginService.loadMargin(order) } return validateOrder(order, account.await(), margin.await()) }

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Extensions of CoroutineScope fun CoroutineScope.async( context: CoroutineContext = EmptyCoroutineContext, start: CoroutineStart = CoroutineStart.DEFAULT, block: suspend CoroutineScope.() -> T ): Deferred

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Convention fun CoroutineScope.bg(params: Params) = launch { // … } Launches new coroutine

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Types as documentation fun foo(params: Params): Response suspend fun foo(params: Params): Response fun CoroutineScope.foo(params: Params): Response Fast, local Remote, or slow Side effect - bg

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Types are enforced fun foo(params: Params): Response suspend fun foo(params: Params): Response fun CoroutineScope.foo(params: Params): Response Not allowed But must provide scope explicitly Using coroutineScope { … } Fast, local Remote, or slow Side effect - bg

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Green threads / fibers Alternative way to async

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Green threads / Fibers ET 2 ET N … ET 1 F 2 F M … Fibers F 1 ~ Coroutines Hidden from developer Executor Threads

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But marking with suspend pays off at scale Fibers – everything suspending

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Thread switching And how to avoid it

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ET 2 ET N … Clients Threads ET 1 S1 1 ST M1 S1 1 ST M2 S1 1 ST M3 Service 2 Threads Service 3 Threads Service 1 Threads Executor Threads

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ET 2 ET N … Clients Solution – shared thread pool ET 1 Executor Threads

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ET 2 ET N … Clients Solution – shared thread pool ET 1 Executor Threads

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ET 2 ET N … Clients Solution – shared thread pool ET 1 ET N+1 Executor Threads

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ET 2 ET N … Clients Solution – shared thread pool ET 1 ET N+1 ET N+2 Executor Threads

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ET 2 ET N … Clients Solution – shared thread pool ET 1 ET N+1 … ET M ET N+2 ET N+M Executor Threads

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withContext for IO suspend fun loadAccount(order: Order): Account = withContext(dispatcher) { // some blocking code here.... } val dispatcher = Executors.newFixedThreadPool(M2).asCoroutineDispatcher()

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withContext for Dispatсhers.IO suspend fun loadAccount(order: Order): Account = withContext(Dispatchers.IO) { // some blocking code here.... } No thread switch from Dispatchers.Default pool

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ET 2 ET N … Clients Solution – shared thread pool ET 1 Dispatchers.Default Executor Threads

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ET 2 ET N … Clients Solution – shared thread pool ET 1 ET N+1 … ET M ET N+2 ET N+M Dispatchers.Default Dispatchers.IO Executor Threads

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Coroutines and data streams

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Returning many responses suspend fun foo(params: Params): Response One response suspend fun foo(params: Params): List Many responses

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Returning many responses suspend fun foo(params: Params): Response One response suspend fun foo(params: Params): List Many responses fun foo(params: Params): Flow Many responses async

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Kotlin Flows Reactive streams for coroutines

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fun foo(): Flow ~ Reactive Publisher / Flux

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fun foo(): Flow = flow { … }

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fun foo(): Flow = flow { for (i in 1..10) { emit(i) delay(100) } }

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fun foo(): Flow = flow { for (i in 1..10) { emit(i) delay(100) } } suspend fun main() { foo().collect { x -> println(x) } }

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fun foo(): Flow = flow { for (i in 1..10) { emit(i) delay(100) } } suspend fun main() { foo() } Flow is cold: describes the data, does not run it until collected

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Flow operators fun foo(): Flow = flow { for (i in 1..10) { emit(i) delay(100) } } suspend fun main() { foo() .map { it * it } .toList() }

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Why flow?

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A A’ mapper fun map(mapper: (T) -> R): Flux fun flatMap(mapper: (T) -> Publisher): Flux Synchronous Asynchronous Flux

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A A’ mapper fun map(mapper: (T) -> R): Flux fun flatMap(mapper: (T) -> Publisher): Flux fun filter(predicate: (T) -> Boolean): Flux A A predicate Synchronous Asynchronous Synchronous Asynchronous Flux fun filterWhen(predicate: (T) -> Publisher): Flux

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A A’ Flow fun map(transform: suspend (T) -> R): Flow transform

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A A’ transform Flow fun map(transform: suspend (T) -> R): Flow

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A A’ transform fun map(transform: suspend (T) -> R): Flow Flow fun filter(predicate: suspend (T) -> Boolean): Flow A predicate A

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Operator avoidance startWith(value) onStart { emit(value) } delaySubscription(time) onStart { delay(time) } startWith(flow) onStart { emitAll(flow) } delayElements(time) onEach { delay(time) } onErrorReturn(value) catch { emit(value) } onErrorResume(flow) catch { emitAll(flow) } generate(…) flow { … } Composable

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Reactive + = ❤

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Spring ❤ Flow @GetMapping("/sse/{n}", produces = [MediaType.TEXT_EVENT_STREAM_VALUE]) fun greetings(@PathVariable n: String): Flow

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Spring ❤ Flow @GetMapping("/sse/{n}", produces = [MediaType.TEXT_EVENT_STREAM_VALUE]) fun greetings(@PathVariable n: String): Flow = flow { while(true) { emit(Greeting("Hello from coroutines $n @ ${Instant.now()}")) delay(1000) } }