(reposting this question since i did it late at ni...

# coroutines

[Brian Dilley]

(reposting this question since i did it late at night and it didn’t get any responses):

I’ve built a sharding library and i’m trying to add coroutine functionality to it. In the following snippet i’m hoping that it’s doing what I’m thinking it does (returns the first true result that it finds):

override fun emailExists(email: String): Boolean {
        val results = shards.asyncAll { userDao.emailExists(email) }
        return runBlocking {
            results.map { it.await() }
                .firstOrNull { it }
        } ?: false
    }

the shards.asyncAll method is:

fun <T> async(
        shardId: Long,
        context: CoroutineContext = EmptyCoroutineContext,
        start: CoroutineStart = CoroutineStart.DEFAULT,
        block: suspend CoroutineScope.() -> T): Deferred<T> {
        return scope.async(context, start) {
            selectShard(shardId)
            block()
        }
    }

    fun <T> asyncAll(
        shardIds: Collection<Long> = this.shardIds,
        context: CoroutineContext = EmptyCoroutineContext,
        start: CoroutineStart = CoroutineStart.DEFAULT,
        block: suspend CoroutineScope.() -> T): List<Deferred<T>> {
        return shardIds.map { async(it, context, start, block) }
    }

So first question is if that code above (emailExists) will work, and the second question is: how can i change it so that the first Deferred<Boolean> result that returns that is true returns from the method (rather than it testing them in order as they come back - I’d like it so that the very first one that comes back with a result of true triggers a return)

[marstran]

You should probably use a flow, which is lazy (cold). Then you will only trigger the calls you need.

shards.asFlow()
    .map { userDao.emailExists(it) }
    .firstOrNull { it }

With just a list, you will always trigger all of the emailExists calls.

[Brian Dilley]

I need to trigger all of them

[Brian Dilley]

I just d on my need to wait or check on the others once the first true is returned.

[marstran]

One problem here is that returning early will leave the other coroutines running in the background. You lose control over them by returning early . The emailExists function should take a CoroutineScope to control that.

[marstran]

By the way, where is the scope variable in async coming from?

[Brian Dilley]

from the object that this method is on