spring boot - How do I debug a Mono that never completes

Question

I have a Spring Boot application which contains a complex reactive flow (it involves MongoDB and RabbitMQ operations). Most of the time it works, but...

Some of the methods return a Mono<Void>. This is a typical pattern, in multiple layers:

fun workflowStep(things: List<Thing>): Mono<Void> =
    Flux.fromIterable(things).flatMap { thing -> doSomethingTo(thing) }.collectList().then()

Let's say doSomethingTo() returns a Mono<Void> (it writes something to the database, sends a message etc). If I just replace it with Mono.empty() then everything works as expected, but otherwise it doesn't. More specifically the Mono never completes, it runs through all processing but misses the termination signal at the end. So the things are actually written in the database, messages are actually sent, etc.

To prove that the lack of termination is the problem, here is a hack that works:

val hackedDelayedMono = Mono.empty<Void>().delayElement(Duration.ofSeconds(1))
return Mono.first(
    workflowStep(things),
    hackedDelayedMono
)

The question is, what can I do with a Mono that never completes, to figure out what's going on? There is nowhere I could put a logging statement or a brakepoint, because:

• there are no errors
• there are no signals emitted

How could I check what the Mono is waiting for to be completed?

ps. I could not reproduce this behaviour outside the application, with simple Mono workflows.

question from:https://stackoverflow.com/questions/65904851/how-do-i-debug-a-mono-that-never-completes

Answer 1

You can trace and log events in your stream by using the log() operator in your reactive stream. This is useful for gaining a better understanding about what events are occurring within your app.

Flux.fromIterable(things)
    .flatMap(thing -> doSomethingTo(thing))
    .log()
    .collectList()
    .then()

Chained inside a sequence, it peeks at every event of the Flux or Mono upstream of it (including onNext, onError, and onComplete as well as subscriptions, cancellations, and requests).

Reactor Reference Documentation - Logging a Sequence

The Reactor reference documentation also contains other helpful advice for debugging a reactive stream and can be found here: Debugging Reactor