[[coroutines]]
= Coroutines

Kotlin {kotlin-docs}/coroutines-overview.html[Coroutines] are Kotlin
lightweight threads allowing to write non-blocking code in an imperative way. On language side,
suspending functions provides an abstraction for asynchronous operations while on library side
{kotlin-github-org}/kotlinx.coroutines[kotlinx.coroutines] provides functions like
{kotlin-coroutines-api}/kotlinx-coroutines-core/kotlinx.coroutines/async.html[`async { }`]
and types like {kotlin-coroutines-api}/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/index.html[`Flow`].

Spring Framework provides support for Coroutines on the following scope:

* {kotlin-coroutines-api}/kotlinx-coroutines-core/kotlinx.coroutines/-deferred/index.html[Deferred] and {kotlin-coroutines-api}/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/index.html[Flow] return values support in Spring MVC and WebFlux annotated `@Controller`
* Suspending function support in Spring MVC and WebFlux annotated `@Controller`
* Extensions for WebFlux {spring-framework-api-kdoc}/spring-webflux/org.springframework.web.reactive.function.client/index.html[client] and {spring-framework-api-kdoc}/spring-webflux/org.springframework.web.reactive.function.server/index.html[server] functional API.
* WebFlux.fn {spring-framework-api-kdoc}/spring-webflux/org.springframework.web.reactive.function.server/co-router.html[coRouter { }] DSL
* WebFlux {spring-framework-api-kdoc}/spring-web/org.springframework.web.server/-co-web-filter/index.html[`CoWebFilter`]
* Suspending function and `Flow` support in RSocket `@MessageMapping` annotated methods
* Extensions for {spring-framework-api-kdoc}/spring-messaging/org.springframework.messaging.rsocket/index.html[`RSocketRequester`]
* Spring AOP


[[dependencies]]
== Dependencies

Coroutines support is enabled when `kotlinx-coroutines-core` and `kotlinx-coroutines-reactor`
dependencies are in the classpath:

`build.gradle.kts`
[source,kotlin,indent=0]
----
dependencies {

	implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:${coroutinesVersion}")
	implementation("org.jetbrains.kotlinx:kotlinx-coroutines-reactor:${coroutinesVersion}")
}
----

Version `1.4.0` and above are supported.


[[how-reactive-translates-to-coroutines]]
== How Reactive translates to Coroutines?

For return values, the translation from Reactive to Coroutines APIs is the following:

* `fun handler(): Mono<Void>` becomes `suspend fun handler()`
* `fun handler(): Mono<T>` becomes `suspend fun handler(): T` or `suspend fun handler(): T?` depending on if the `Mono` can be empty or not (with the advantage of being more statically typed)
* `fun handler(): Flux<T>` becomes `fun handler(): Flow<T>`

For input parameters:

* If laziness is not needed, `fun handler(mono: Mono<T>)` becomes `fun handler(value: T)` since a suspending functions can be invoked to get the value parameter.
* If laziness is needed, `fun handler(mono: Mono<T>)` becomes `fun handler(supplier: suspend () -> T)` or `fun handler(supplier: suspend () -> T?)`

{kotlin-coroutines-api}/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/index.html[`Flow`] is `Flux` equivalent in Coroutines world, suitable for hot or cold stream, finite or infinite streams, with the following main differences:

* `Flow` is push-based while `Flux` is push-pull hybrid
* Backpressure is implemented via suspending functions
* `Flow` has only a {kotlin-coroutines-api}/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/collect.html[single suspending `collect` method] and operators are implemented as {kotlin-docs}/extensions.html[extensions]
* {kotlin-github-org}/kotlinx.coroutines/tree/master/kotlinx-coroutines-core/common/src/flow/operators[Operators are easy to implement] thanks to Coroutines
* Extensions allow to add custom operators to `Flow`
* Collect operations are suspending functions
* {kotlin-coroutines-api}/kotlinx-coroutines-core/kotlinx.coroutines.flow/map.html[`map` operator] supports asynchronous operation (no need for `flatMap`) since it takes a suspending function parameter

Read this blog post about {spring-site-blog}/2019/04/12/going-reactive-with-spring-coroutines-and-kotlin-flow[Going Reactive with Spring, Coroutines and Kotlin Flow]
for more details, including how to run code concurrently with Coroutines.


[[controllers]]
== Controllers

Here is an example of a Coroutines `@RestController`.

[source,kotlin,indent=0]
----
@RestController
class CoroutinesRestController(client: WebClient, banner: Banner) {

	@GetMapping("/suspend")
	suspend fun suspendingEndpoint(): Banner {
		delay(10)
		return banner
	}

	@GetMapping("/flow")
	fun flowEndpoint() = flow {
		delay(10)
		emit(banner)
		delay(10)
		emit(banner)
	}

	@GetMapping("/deferred")
	fun deferredEndpoint() = GlobalScope.async {
		delay(10)
		banner
	}

	@GetMapping("/sequential")
	suspend fun sequential(): List<Banner> {
		val banner1 = client
				.get()
				.uri("/suspend")
				.accept(MediaType.APPLICATION_JSON)
				.awaitExchange()
				.awaitBody<Banner>()
		val banner2 = client
				.get()
				.uri("/suspend")
				.accept(MediaType.APPLICATION_JSON)
				.awaitExchange()
				.awaitBody<Banner>()
		return listOf(banner1, banner2)
	}

	@GetMapping("/parallel")
	suspend fun parallel(): List<Banner> = coroutineScope {
		val deferredBanner1: Deferred<Banner> = async {
			client
					.get()
					.uri("/suspend")
					.accept(MediaType.APPLICATION_JSON)
					.awaitExchange()
					.awaitBody<Banner>()
		}
		val deferredBanner2: Deferred<Banner> = async {
			client
					.get()
					.uri("/suspend")
					.accept(MediaType.APPLICATION_JSON)
					.awaitExchange()
					.awaitBody<Banner>()
		}
		listOf(deferredBanner1.await(), deferredBanner2.await())
	}

	@GetMapping("/error")
	suspend fun error() {
		throw IllegalStateException()
	}

	@GetMapping("/cancel")
	suspend fun cancel() {
		throw CancellationException()
	}

}
----

View rendering with a `@Controller` is also supported.

[source,kotlin,indent=0]
----
@Controller
class CoroutinesViewController(banner: Banner) {

	@GetMapping("/")
	suspend fun render(model: Model): String {
		delay(10)
		model["banner"] = banner
		return "index"
	}
}
----


[[webflux-fn]]
== WebFlux.fn

Here is an example of Coroutines router defined via the
{spring-framework-api-kdoc}/spring-webflux/org.springframework.web.reactive.function.server/co-router.html[coRouter { }]
DSL and related handlers.

[source,kotlin,indent=0]
----
@Configuration
class RouterConfiguration {

	@Bean
	fun mainRouter(userHandler: UserHandler) = coRouter {
		GET("/", userHandler::listView)
		GET("/api/user", userHandler::listApi)
	}
}
----

[source,kotlin,indent=0]
----
class UserHandler(builder: WebClient.Builder) {

	private val client = builder.baseUrl("...").build()

	suspend fun listView(request: ServerRequest): ServerResponse =
			ServerResponse.ok().renderAndAwait("users", mapOf("users" to
			client.get().uri("...").awaitExchange().awaitBody<User>()))

	suspend fun listApi(request: ServerRequest): ServerResponse =
				ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).bodyAndAwait(
				client.get().uri("...").awaitExchange().awaitBody<User>())
}
----


[[transactions]]
== Transactions

Transactions on Coroutines are supported via the programmatic variant of the Reactive
transaction management.

For suspending functions, a `TransactionalOperator.executeAndAwait` extension is provided.

[source,kotlin,indent=0]
----
	import org.springframework.transaction.reactive.executeAndAwait

	class PersonRepository(private val operator: TransactionalOperator) {

		suspend fun initDatabase() = operator.executeAndAwait {
			insertPerson1()
			insertPerson2()
		}

		private suspend fun insertPerson1() {
			// INSERT SQL statement
		}

		private suspend fun insertPerson2() {
			// INSERT SQL statement
		}
	}
----

For Kotlin `Flow`, a  `Flow<T>.transactional` extension is provided.

[source,kotlin,indent=0]
----
	import org.springframework.transaction.reactive.transactional

	class PersonRepository(private val operator: TransactionalOperator) {

		fun updatePeople() = findPeople().map(::updatePerson).transactional(operator)

		private fun findPeople(): Flow<Person> {
			// SELECT SQL statement
		}

		private suspend fun updatePerson(person: Person): Person {
			// UPDATE SQL statement
		}
	}
----

[[coroutines.propagation]]
== Context Propagation

Spring applications are xref:integration/observability.adoc[instrumented with Micrometer for Observability support].
For tracing support, the current observation is propagated through a `ThreadLocal` for blocking code,
or the Reactor `Context` for reactive pipelines. But the current observation also needs to be made available
in the execution context of a suspended function. Without that, the current "traceId" will not be automatically
prepended to logged statements from coroutines.

The {spring-framework-api-kdoc}/spring-core/org.springframework.core/-propagation-context-element/index.html[`PropagationContextElement`] operator generally ensures that the
{micrometer-context-propagation-docs}/[Micrometer Context Propagation library] works with Kotlin Coroutines.

It requires the `io.micrometer:context-propagation` dependency and optionally the
`org.jetbrains.kotlinx:kotlinx-coroutines-reactor` one.

Applications can then use the `PropagationContextElement` to augment the `CoroutineContext`
with the context propagation mechanism:

include-code::./ContextPropagationSample[tag=context,indent=0]

Here, assuming that Micrometer Tracing is configured, the resulting logging statement will show the current "traceId"
and unlock better observability for your application.

