Folks, I am writing my first coroutine. In the follow piece of code, I am fetching a bunch of objec...

# coroutines

[Mani]

Folks, I am writing my first coroutine. In the follow piece of code, I am fetching a bunch of objects and then doing a IO call with other service. Need some feedback whether my understanding of coroutine is correct.

val allWarehouses: List<Warehouse> = getWarehouses()

 GlobalScope.launch(<http://Dispatchers.IO|Dispatchers.IO>) {
        allWarehouses.forEach { warehouse ->
        	val notes = getNotes()
			// I want to run this loop in parallel
			notes.forEach {
			   serviceClient.markDone(note)  // this is an IO call 
			}
        }
}

I want to run the forEach operation in parallel. Is this the right way of doing?

[Allan Wang]

Everything is within a single launch, so markDone will not run in parallel. This job however will be sent to an io thread relative to the contents outside of the launch. If you want to run all markDone in parallel, launch a separate task for each one.

[Mani]

separate task as in, put this another launch {}?

[Allan Wang]

Yes. You should essentially have your launch inside the for loop

[gildor]

Wrap every operation to async { } using map:
notes.map { async { markDone() } }.awaitAll()

[gildor]

But you should understand that markDone must be non-blocking, so it's not how parallel stream works, which created to parallel blocking operation using forkJoin

[Mani]

thank you both 🙂 So, is the outer launch(<http://Dispatchers.IO|Dispatchers.IO>) in the right place?

[Mani]

Also, if one of the markDone request fails, what will happen to others?

[gildor]

It will throw exception when it fail and cancel other requests, but you can handle it, by replacing awaitAll() with map ++ await() and try/catch