Java Project Reactor subscribeOn Mono 链的行为

Question

I am learning about project reactor and get confused on some questions regarding subscribeOn()

My blocking code looks like (say getA(), getB(), getC(A, B), getD(A, B), getE(C, D) are both time complex functions)

public E someMethod() {
  A a = getA();
  B b = getB();
  C c = getC(a, b);
  D d = getD(a, b);
  return getE(c, d);
}

Now I want to change it into a unblocking implementation, I wrote

public E someMethodUnblocking() {
  Mono<A> a = Mono.fromCallable(getA).subscribeOn(Schedulers.boundedElastic());
  Mono<B> b = Mono.fromCallable(getB).subscribeOn(Schedulers.boundedElastic());
  Mono<C> c = Mono.zip(a, b, (aa, bb) -> getC(aa,bb)).subscribeOn(Schedulers.boundedElastic());
  Mono<D> d = Mono.zip(a, b, (aa, bb) -> getD(aa,bb)).subscribeOn(Schedulers.boundedElastic());
  return Mono.zip(c, d, (cc, dd) -> getE(cc, dd)).block()
}

Does this look like the correct implementation? will there be a difference if I remove the subscribeOn() when generating Mono a and b?

Answer 1

Your implementation subscribes twice to the (cold) monos a and b - and will run your costly getA() and getB() methods two times.

To avoid that, you should cache() the first two monos.