rxjs back pressure https://github.com/ReactiveX/rxjs/issues/71#issuecomment-228824763

backpressure.js

// this behavior subject is basically your "give me the next batch" mechanism.
// in this example, we're going to make 5 async requests back to back before requesting more.
const BATCH_SIZE = 5;
const requests = new BehaviorSubject(BATCH_SIZE); // start by requesting five items

// for every request, pump out a stream of events that represent how many you have left to fulfill
requests.flatMap((count) => Observable.range(0, count).map(n => count - n - 1))
  // then concat map that into an observable of what you want to control with backpressure
  // you might have some parameterization here you need to handle, this example is simplified
  // handle side effects with a `do` block
  .concatMap(() => getSomeObservableOfDataHere().do(stuffWithIt), (remaining) => remaining)
  // narrow it down to when there are no more left to request,
  // and pump another batch request into the BehaviorSubject
  .filter(remaining => remaining === 0)
  .mapTo(BATCH_SIZE)
  .subscribe(requests);