Implement backoff strategy in flow - kotlin

Question

I'm trying to implement a backoff strategy just using kotlin flow.
I need to fetch data from timeA to timeB
result = dataBetween(timeA - timeB)
if the result is empty then I want to increase the end time window using exponential backoff
result = dataBetween(timeA - timeB + exponentialBackOffInDays)
I was following this article which is explaining how to approach this in rxjava2.
But got stuck at a point where flow does not have takeUntil operator yet.
You can see my implementation below.
fun main() {
runBlocking {
(0..8).asFlow()
.flatMapConcat { input ->
// To simulate a data source which fetches data based on a time-window start-date to end-date
// available with in that time frame.
flow {
println("Input: $input")
if (input < 5) {
emit(emptyList<String>())
} else { // After emitting this once the flow should complete
emit(listOf("Available"))
}
}.retryWhenThrow(DummyException(), predicate = {
it.isNotEmpty()
})
}.collect {
//println(it)
}
}
}
class DummyException : Exception("Collected size is empty")
private inline fun <T> Flow<T>.retryWhenThrow(
throwable: Throwable,
crossinline predicate: suspend (T) -> Boolean
): Flow<T> {
return flow {
collect { value ->
if (!predicate(value)) {
throw throwable // informing the upstream to keep emitting since the condition is met
}
println("Value: $value")
emit(value)
}
}.catch { e ->
if (e::class != throwable::class) throw e
}
}
It's working fine except even after the flow has a successful value the flow continue to collect till 8 from the upstream flow but ideally, it should have stopped when it reaches 5 itself.
Any help on how I should approach this would be helpful.

Answer 1

Maybe this does not match your exact setup but instead of calling collect, you might as well just use first{...} or firstOrNull{...}
This will automatically stop the upstream flows after an element has been found.
For example:
flowOf(0,0,3,10)
.flatMapConcat {
println("creating list with $it elements")
flow {
val listWithElementCount = MutableList(it){ "" } // just a list of n empty strings
emit(listWithElementCount)
}
}.first { it.isNotEmpty() }
On a side note, your problem sounds like a regular suspend function would be a better fit.
Something like
suspend fun getFirstNonEmptyList(initialFrom: Long, initialTo: Long): List<Any> {
var from = initialFrom
var to = initialTo
while (coroutineContext.isActive) {
val elements = getElementsInRange(from, to) // your "dataBetween"
if (elements.isNotEmpty()) return elements
val (newFrom, newTo) = nextBackoff(from, to)
from = newFrom
to = newTo
}
throw CancellationException()
}