Coroutines

Kotlin Coroutines are instances of suspendable computations allowing to write non-blocking code imperatively. On language side, suspend functions provides an abstraction for asynchronous operations while on library side kotlinx.coroutines provides functions like async { } and types like Flow.

Spring Data modules provide support for Coroutines on the following scope:

  • Deferred and Flow return values support in Kotlin extensions

Dependencies

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

Dependencies to add in Maven pom.xml
<dependency>
	<groupId>org.jetbrains.kotlinx</groupId>
	<artifactId>kotlinx-coroutines-core</artifactId>
</dependency>

<dependency>
	<groupId>org.jetbrains.kotlinx</groupId>
	<artifactId>kotlinx-coroutines-reactive</artifactId>
</dependency>

<dependency>
	<groupId>org.jetbrains.kotlinx</groupId>
	<artifactId>kotlinx-coroutines-reactor</artifactId>
</dependency>
Supported versions 1.3.0 and above.

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>

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 single suspending collect method and operators are implemented as extensions

  • Operators are easy to implement thanks to Coroutines

  • Extensions allow adding custom operators to Flow

  • Collect operations are suspending functions

  • map operator supports asynchronous operation (no need for flatMap) since it takes a suspending function parameter

Read this blog post about Going Reactive with Spring, Coroutines and Kotlin Flow for more details, including how to run code concurrently with Coroutines.

Repositories

Here is an example of a Coroutines repository:

interface CoroutineRepository : CoroutineCrudRepository<User, String> {

    suspend fun findOne(id: String): User

    fun findByFirstname(firstname: String): Flow<User>

    suspend fun findAllByFirstname(id: String): List<User>
}

Coroutines repositories are built on reactive repositories to expose the non-blocking nature of data access through Kotlin’s Coroutines. Methods on a Coroutines repository can be backed either by a query method or a custom implementation. Invoking a custom implementation method propagates the Coroutines invocation to the actual implementation method if the custom method is suspend-able without requiring the implementation method to return a reactive type such as Mono or Flux.

Note that depending on the method declaration the coroutine context may or may not be available. To retain access to the context, either declare your method using suspend or return a type that enables context propagation such as Flow.

  • suspend fun findOne(id: String): User: Retrieve the data once and synchronously by suspending.

  • fun findByFirstname(firstname: String): Flow<User>: Retrieve a stream of data. The Flow is created eagerly while data is fetched upon Flow interaction (Flow.collect(…)).

  • fun getUser(): User: Retrieve data once blocking the thread and without context propagation. This should be avoided.

Coroutines repositories are only discovered when the repository extends the CoroutineCrudRepository interface.