Debugging "NoSuchElementException: Source was empty" errors with project reactor - reactive-programming

Question

I am looking for tips on how to pinpoint such errors as:
NoSuchElementException: Source was empty
whilst using project reactor. This indicates that the Mono/Flux did not emit any result but how I am supposed to find out the reason for that?
I currently use Hooks.onOperatorDebug(); which is very helpful, but I am looking for other ways of spotting the cause of such errors.
Any advice or recommendation on best practices welcome.
P.S. I have opened another question here: Issue with use of project reactor's flatMap and switchIfEmpty operators related to this one.

Answer 1

Just using log() can go a long way here.
Take something like the following:
Flux.range(0, 5)
.log("Initial")
.filter(x -> x%2==0)
.log("Even only")
.filter(x -> x<3)
.log("Less than 3 only")
.subscribe(System.out::println);
Which, taking the relevant bit out the log, will show:
21:08:28.809 [main] INFO Initial - | onNext(0)
21:08:28.809 [main] INFO Even only - | onNext(0)
21:08:28.809 [main] INFO Less than 3 only - | onNext(0)
0
21:08:28.809 [main] INFO Initial - | onNext(1)
21:08:28.809 [main] INFO Initial - | onNext(2)
21:08:28.810 [main] INFO Even only - | onNext(2)
21:08:28.810 [main] INFO Less than 3 only - | onNext(2)
2
21:08:28.810 [main] INFO Initial - | onNext(3)
21:08:28.810 [main] INFO Initial - | onNext(4)
21:08:28.810 [main] INFO Even only - | onNext(4)
This allows us to deduce where each element is being filtered, or if it was even emitted in the first place. From the above we can deduce:
0 was emitted, made it past the "Even only" filter, then past the "Less than 3 only" filter;
1 was emitted, but didn't get past the first filter
2 and 3 have the same patterns as 0 and 1 respectively
4 made it past the "Even only" filter, but went no further (so failed at the second filter)
5 and above, or any other elements, weren't ever emitted from the initial Flux.range() call.
With something like the above approach on your previous question, you may have noticed that userMono was never emitting anything the second time it was called, which may have helped narrow down the problem.