How to get the first response of calling 5 requests with Http call? The use case is sending 5 http r...

# coroutines

[Hank]

How to get the first response of calling 5 requests with Http call? The use case is sending 5 http requests then get the first response and cancel others. Any suggestions?

[Jan Skrasek]

Use select function. Register all deferred in it and after first receive cancel others.

[Hank]

That's really helpful. Thank you so much.

[Richard Gomez]

@Jan Skrasek: Would you mind sharing a snippet of what that would look like? (For learning purposes.)

I'm assuming you're referring to "Selecting deferred values", but am not familiar with the select { } function. Perhaps something like:

// Ugly code
val requests = List<Deferred<String>>(5) {
    delay(Random.nextInt(0, 1000).toLong())
    async { "Response #$it" }
}
val response = select<String> {
    requests.withIndex().forEach { (index, deferred) ->
        deferred.onAwait { answer ->
            requests.forEach { it.cancel() }
            "Deferred $index produced answer '$answer'"
        }
    }
}

[Nick Allen]

Alternative with Flow

withTimeout(limit) {
 merge(::func1.asFlow(),::func2.asFlow(),... ). first()

}
(formatting not working on phone)

[Jan Skrasek]

@Richard Gomez your's example seems reasonable, what's not working?
You may take a look at some official samples:
https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-core/jvm/test/guide/example-select-04.kt

[Richard Gomez]

your's example seems reasonable, what's not working?

Nothing in particular, just confirming that I have the right idea. 🙂

Is there a more idiomatic way to cancel a list of jobs than requests.forEach(Job::cancel) ?
I suppose you could run them in an ad-hoc context and cancel that once the first is completed.