Kotlin coroutines within a Spring REST endpoint

Question

Given a REST endpoint and two asynchronous coroutines each returning an integer, I want this endpoint to return their sum. The two functions (funA and funB) should run in parallel, in such a way that the whole computation should take ~3secs. I'm using SpringBoot 2.6.3 and Kotlin Coroutines 1.6.0. Here is my attempt:

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RestController

@RestController
class Controllers {

    @GetMapping(
        value = ["/summing"]
    )
    fun summing(): String {

        val finalResult = runBlocking {
            val result = async { sum() }
            println("Your result: ${result.await()}")
            return@runBlocking result
        }
        println("Final Result: $finalResult")
        return "$finalResult"
    }

    suspend fun funA(): Int {
        delay(3000)
        return 10
    }

    suspend fun funB(): Int {
        delay(2000)
        return 90
    }

    fun sum() = runBlocking {
        val resultSum = async { funA().await() + funB().await() }
        return@runBlocking resultSum
    }
}

The problem is that this code does not compile as await() is not recognised as a valid method. If I remove await() the two functions are executed in series (total time ~5 secs) and instead of the expected result (100), I get:

Your result: DeferredCoroutine{Completed}@1c1fe804
Final Result: DeferredCoroutine{Completed}@48a622de

and so the endpoint returns "DeferredCoroutine{Completed}@48a622de".

I want the endpoint to return "100" instead and within ~3secs. How can I achieve this?

user_448875 · Answer

You really messed this up ;-) There are several problems with your code:

Use runBlocking() only to use another runBlocking() inside it.
Use async() and immediately call await() on it (in summing()) - it does nothing.
funA().await() and funB().await() don't make any sense really. These functions return integers, you can't await() on already acquired integers.
Generally, using much more code than needed.

The solution is pretty simple: use runBlocking() once to jump into coroutine world and then use async() to start both functions concurrently to each other:

runBlocking {
    val a = async { funA() }
    val b = async { funB() }
    a.await() + b.await()
}

Or alternatively:

runBlocking {
    listOf(
        async { funA() },
        async { funB() },
    ).awaitAll().sum()
}

Or (a little shorter, but I consider this less readable):

runBlocking { 
    val a = async { funA() }
    funB() + a.await()
}

Also,runBlocking() is not ideal. I believe Spring has support for coroutines, so it would be better to make summing() function suspend and use coroutineScope() instead of runBlocking() - this way the code won't block any threads.

Kotlin coroutines within a Spring REST endpoint

1 Answers