Change dynamically the concurrency factor in mergeMap operator - rxjs

Question

I have an Observable, mySourceObs, and the following code
concurrencyFactor = 10;
mySourceObs.pipe(
mergeMap(data => generateAnotherObservable(data), concurrencyFactor)
).subscribe();
Is there a way to change dynamically the concurrencyFactor once the chain of Observables has been subscribed?

Answer 1

You can do it like this, but inner observable will restart when concurrencyFactor changes.
concurrencyFactor = new BehaviorSubject(10);
combineLatest(mySourceObs,concurrent).pipe(
switchMap(([data,concurrent]) => generateAnotherObservable(data), concurrent)
).subscribe();
concurrencyFactor.next(5)

Answer 2

I have figured out a way of controlling the concurrency of mergeMap with a combination of an external Subject, switchMap and defer.
This is the sample code
// when this Observable emits, the concurrency is recalculated
const mySwitch = new Subject<number>();
let myConcurrency = 1;
// this block emits five times, every time setting a new value for the concurrency
// and the making mySwitch emit
merge(interval(1000).pipe(map(i => i + 2)), of(1))
.pipe(
delay(0),
take(5),
tap(i => {
myConcurrency = i * 2;
console.log("switch emitted", i);
mySwitch.next(i);
})
)
.subscribe();
// source is an hot Observable which emits an incresing number every 100 ms
const source = new Subject<number>();
interval(100)
.pipe(take(100))
.subscribe(i => source.next(i));
// this is the core of the logic
// every time mySwitch emits the old subscription is cancelled and a new one is created
mySwitch
.pipe(
switchMap(() =>
// defer is critical here since we want a new Observable to be generated at subscription time
// so that the new concurrency value is actually used
defer(() =>
source.pipe(
mergeMap(
i =>
// this is just a simulation of a an inner Observable emitting something
interval(100).pipe(
delay(100),
map(j => `Observable of ${i} and ${j}`),
take(10)
),
myConcurrency
)
)
)
)
)
.subscribe({
next: data => console.log(data)
});