Run two Kotlin coroutines inside coroutine in parallel

Question

I have two suspend functions:

suspend fun sendData() : Boolean 

suspend fun awaitAcknowledge() : Boolean

and I want to wrap them in a third suspend function in which they should be executed in parallel and I want to calculate the final result by having both return values:

suspend fun sendDataAndAwaitAcknowledge() : Boolean {
    // TODO execute both in parallel and compare both results
}

However, if I write it like that,

suspend fun sendDataAndAwaitAcknowledge() : Boolean {
    val sendResult = sendData()
    val receiveAck = awaitAcknowledge()
}

the functions will be executed in a serial order, which will not work in my case.

Coming from RxJava, I would like to achieve something like the zip operator:

Single.zip(awaitAcknowledge(), sendData(), {receiveAck, sendResult -> ...})

How can I do this with Coroutines?

Alexey Soshin · Accepted Answer · 2019-08-12 09:17:40Z

65

You can use awaitAll for that purpose:

import kotlinx.coroutines.*

suspend fun sendDataAndAwaitAcknowledge() = coroutineScope {
    awaitAll(async {
        awaitAcknowledge()
    }, async {
        sendData()
    })
}

fun sendData() = true

fun awaitAcknowledge() = false

fun main() {
    runBlocking {
        println(sendDataAndAwaitAcknowledge()) // [false, true]
    }
}

answered Aug 12, 2019 at 9:17

Alexey Soshin

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1 Comment

RaJ Over a year ago

It is possible to execute all functions at the same time without a milliseconds delay?

milosmns · Accepted Answer · 2023-07-01 20:54:12Z

15

This is the general pattern:

suspend fun sendDataAndAwaitAck() = coroutineScope {
  val one = async { sendData() }
  val two = async { awaitAck() }
  println("The result is ${one.await()}, ${two.await()}")
}

The two suspending functions (sendData and awaitAck) are called from within another suspending function (sendDataAndAwaitAck). They could potentially run in parallel, depending on the environment and CPU setup. The outer suspending function will wait for the two inner functions to finish their tasks before proceeding.

Note 1: async is available only in a Coroutine Scope.

Note 2: if you don't want the whole scope to fail when one of the child coroutines fails, you should use supervisorScope instead.

edited Jul 1, 2023 at 20:54

milosmns

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answered Aug 12, 2019 at 7:30

user3089483

5 Comments

Andrei Tanana Over a year ago

you cannot just use async. You need coroutine scope for it

user3089483 Over a year ago

@AndreiTanana check the link. kotlinlang.org/docs/reference/coroutines/…

Andrei Tanana Over a year ago

this is the full code of this example github.com/kotlin/kotlinx.coroutines/blob/master/… it uses runBlocking scope

Christopher Over a year ago

Hmm, when I try to implement in this way, I get unresolved reference on the async keyword. However, I don't get an import suggestion from Android Studio.

afollestad Over a year ago

The async being used here is a static import from GlobalScope, which is not a great thing to use. Async should be launched off of a more granular coroutine scope.

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