Hi guys! I am trying to learn coroutines, coming from Java background and CompletableFutures: Does t...

# coroutines

[Kamila]

Hi guys! I am trying to learn coroutines, coming from Java background and CompletableFutures: Does this snippet make sense?

private suspend fun fetchExternalData(futures: List<CompletableFuture<Reply>>): List<Reply> = coroutineScope {
        futures.map {
            async {
                it.join()
            }
        }.awaitAll()
    }

I have a list of CompletableFuture, which I would run async and do action when all of them complete. Is this the right place where I call join (or get()) to get reply?
The idea is to get a bunch of request, send them in parallel (fork) and wait for all of them (join), then combine the result

[Joffrey]

You're almost there. The snippet makes sense but you shouldn't use a blocking join() in coroutines. A better option is to use the CompletableFuture.await() function provided by kotlinx-coroutines-jdk8

[Joffrey]

Actually I think you could even use asDeferred() directly instead of async { it.await() }

[ephemient]

you don't need `coroutineScope`+`async` if you use https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-jdk8/kotlinx.coroutines.future/as-deferred.html

private suspend fun fetch(futures: List<CompletableFuture<Reply>>): List<Reply> =
    futures.map { it.asDeferred() }.awaitAll()

the basic idea is sound, but asDeferred() also ties cancellation back to the original Future

[ephemient]

it's an important part of structured concurrency that Java's Futures don't really provide support for

[Joffrey]

Just to clarify, both CompletableFuture.await and asDeferred forward cancellation to the underlying future. Using join doesn't.

[ephemient]

and the easiest thing to write in Java also misses cancellation, whereas kotlinx.coroutines basically enforces structure on its own coroutines and makes it relatively easy in integrations where possible

[Joffrey]

Actually now I'm intrigued because in the docs it's mentioned that asDeferred().await() does not forward cancellation. So I guess Deferred.await() is cancellable but doesn't cancel the underlying Deferred? Weird..

[Kamila]

Thanks guys! ❤️