Mutiny: How to cancel a step in a chained pipeline?

[flowt-au]

Hi.
I have a question about cancelling in a Uni pipeline. I have a sequence where I do something before the main thing, do the main thing, do something after the main thing. The "before" step needs to be cancellable so my business logic can say "No, you cant do the main thing for some reason".

If I have a Uni pipeline like this pseudocode:

this.beforeSomething(arg1)
    .onCancellation(handleBeforeCancellation())
    .chain(resultFromBefore -> doSomething(resultFromBefore))
    .onCancellation(handleCancellation())
    .chain(resultFromSomething -> afterSomething(resultFromSomething))
    .onCancellation(handleAfterCancellation())
    .subscribe().with(
      res -> System.out.println("Test flow success=" + res),
      failure-> System.out.println("Test flow failure=" + failure)
    );

Each of my methods returns a Uni<String>

how do I emit a "cancel" from my methods so the onCancellation will fire? Something like:

Uni<String> beforeSomething (String data) {
  if (allOk) {
    return Uni.createFrom().item("DataForSomething");
  } else {
    // A method like this fake cancelItem()...
    return Uni.createFrom().cancelItem("DataFromCancellation");
  }
}

I couldn't find anything in the docs.

Thanks,
Murray

[quarkus-bot[bot]]

/cc @cescoffier, @jponge

[jponge]

Subscriptions are forwarded by operators from the source to the downstream subscriber. If you have an operator in the middle that triggers a subscription.cancel(), then a cancellation signal is sent up to the source publisher so it eventually stops emitting events.

You might want to look at onSubscription() to catch the subscription somewhere in your pipeline, and pass it somehow to your code so it can perform a cancellation request.

[flowt-au]

Thanks for your reply @jponge
I realised I was misunderstanding the role that a cancellation plays.
I found a way to do this with just item and failure which I will post once I am happy it is a solution.
Cheers,
Murray

[flowt-au]

@jponge As mentioned, I have tested my approach and documented it here for comment and feedback.
#28621
Thanks again, Julien.