RxJava Flowable.Interval backpressure when flatmap with single

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()