RxJava Flowable.Interval backpressure when flatmap with single - kotlin

Question

I'm having a scenario where I need to periodically call an API to check for a result. I'm using Flowable.interval to create an interval function which calls the API.
However, I'm having trouble with backpressure. In my example below, a new single is created on each tick in the interval. The desired effect is to only call the API if a call is not already in progress
Flowable.interval(1, 1, TimeUnit.SECONDS).flatMap {
System.out.println("Delay $it")
//simulates API call
Single.just(1L).doAfterSuccess {
System.out.println("NEW SINGLE!!!")
}.delay(4, TimeUnit.SECONDS).doAfterSuccess {
System.out.println("SINGLE SUCCESS!!!")
}.toFlowable()
}.subscribeOn(Schedulers.io()).observeOn(Schedulers.computation()).blockingFirst()
I can solve this using a filter variable like so:
var filter = true
Flowable.interval(1, 1, TimeUnit.SECONDS).filter {
filter
}.flatMap {
System.out.println("Delay $it")
Single.just(1L).doOnSubscribe {
filter = true
}.doAfterSuccess {
System.out.println("NEW SINGLE!!!")
}.delay(4, TimeUnit.SECONDS).doAfterSuccess {
System.out.println("SINGLE!!!")
filter = true
}.toFlowable()
}.subscribeOn(Schedulers.io()).observeOn(Schedulers.computation()).blockingFirst()
But it seems like a hacky solution. I've tired applying onBackPressureDrop after the interval function, but it has no effect.
Any suggestions?

Answer 1

You have to constrain flatMap as well:
Flowable.interval(1, 1, TimeUnit.SECONDS)
.onBackpressureDrop()
.flatMapSingle({
System.out.println("Delay $it")
//simulates API call
Single.just(1L).doAfterSuccess {
System.out.println("NEW SINGLE!!!")
}.delay(4, TimeUnit.SECONDS).doAfterSuccess {
System.out.println("SINGLE SUCCESS!!!")
}
}, false, 1) // <----------------------------------------------------------
.subscribeOn(Schedulers.io())
.observeOn(Schedulers.computation())
.subscribe()