Tuesday, March 23, 2021

Coroutine based Spring boot webflux application

I have worked with Spring Framework for ages and it still manages to surprise me with how cutting edge it continues to be but at the same time enabling a developer to put together a fairly sane app.

The most recent surprise was with how it enables programming a web application with Kotlin coroutines. Coroutines is a fairly complicated concept to get my head around, but it is starting to click now and while trying out some samples I thought it may be a good idea to put an end to end web application in place. 

Thanks to the excellent Spring Boot starters it was not difficult at all. Along the way I also decided to experiment with r2dbc which is another involved technology to interact with a database using reactive streams. Combining reactive streams for interacting with the database but using coroutines in the rest of the layers was not difficult at all. In this post I will not be covering the nuances of what I had to do to get the sample to work, but will cover one thin slice of what it looks like. The sample is here in my github repo and should be fairly self explanatory. 

I have to acknowledge that Nicolas Frankel's blog post provided me a lot of pointers in getting the working code just right

A Slice of functionality

The slice of functionality that I will consider in this post is to return a list of entities and an entity from an embedded database that I have used for the application. 

Let's start from bottom up. So at the lowest level I have to query the database and return a list of entities, this is made dirt simple using the Spring Data based repositories. This is the entirety of the repository code that returns coroutine types.

import org.springframework.data.repository.kotlin.CoroutineCrudRepository
import samples.geo.domain.City

interface CityRepo : CoroutineCrudRepository<City, Long>
Just by doing this the CRUD operations now become suspendable functions. So to return a list of an entity or a specific entity, the signature looks something like this:

fun getCities(): Flow<City> {
    return cityRepo.findAll()

suspend fun getCity(id: Long): City? {
    return cityRepo.findById(id)
Any list operations now return the Coroutine Flow type and getting an entity is a suspendable function.

Moving to the web layer(I have a service layer, but it is just a passthrough to the repo in this instance), I like to have an handler for handling the Webflux ServerRequest and ServerResponse types the following way:
suspend fun getCities(request: ServerRequest): ServerResponse {
    val cities = cityService.getCities()
    return ServerResponse.ok().bodyValueAndAwait(cities)

suspend fun getCity(request: ServerRequest): ServerResponse {
    val id = request.pathVariable("id").toLong()
    val city = cityService.getCity(id)

    return city
        ?.let { ServerResponse.ok().bodyValueAndAwait(it) }
        ?: ServerResponse.notFound().buildAndAwait()
which is then composed at the web layer the following way:
object AppRoutes {
    fun routes(cityHandler: CityHandler): RouterFunction<*> = coRouter {
        accept(MediaType.APPLICATION_JSON).nest {
            GET("/cities", cityHandler::getCities)
            GET("/cities/{id}", cityHandler::getCity)
The "coRouter" dsl above provides the functionality to convert the Kotlin coroutine types to the Spring WebFlux RouterFunction type This is essentially it. The code and tests for all this fairly sophisticated set of technology(r2dbc, coroutines, webflux, reactive streams etc) that this encompasses is fairly small as can be seen from the github repository


Getting a working end to end web application with Coroutines and Spring Webflux is just a "Spring" board to further exploration of Coroutines for me and I hope to gain deeper insights into this fascinating technology over time. Having been involved in the Spring community for so long, it is fascinating to note that it continues to remain one of the best frameworks to develop applications in, mainly because of the constant innovation and its focus on developer happiness.

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