How to call long running blocking void returning method with Mutiny reactive programming?

I have a chain of Async and Sync method invocation on Mutiny’s Uni, some methods are a long-running process with return type void.

What is the proper way of invoking/calling them without blocking downstream?

Below is the simple analogy code.

class Root {

    public static void main(String[] args) {

        final Response response = getResponsePayload(); // Gets the the Payload from upstream service
        Uni.createFrom().item(response)
            .onItem().invoke(() -> System.out.println("Process Started"))
            .onItem().call(res -> {
            longRunningMethodAsync(res);    // long running blocking method, I want to run on a worker thread
            return Uni.createFrom().voidItem(); // This line I created, because of the ppipeline will be broken if the Uni is not returned from here
        })
            .onItem().transform(item -> item.hello + " mutiny")
            .onItem().transform(String::toUpperCase)
            .subscribe().with(
            item -> System.out.println(">> " + item));  // This is printed to the console
    }



    // Boilerplate method - I created to invoke/call the actual method actual method - `longRunningMethod`, this method basically an adapter
    // This is the best apprach I could come up, but I'm looking for better thatn this as I'm not conviced I'm doing it right
    private static UniSubscribe<Void> longRunningMethodAsync(final Response response) {

        
        return Uni.createFrom().voidItem().invoke(() -> longRunningMethod(response))
            .runSubscriptionOn(Infrastructure.getDefaultExecutor()).subscribe();
    }


    // Important - this is the method I want to run asynchronously independently of main *event-loop* thread.
    private static void longRunningMethod(final Response response) {
  
        System.out.println("Long running process started"); // Doesn't get printed, which means this is never called at all, not even in the blocked manner by the main even-loop thread
    }




   // Not as importatnt, I provded this in case if you like to run on your local box
    private static Response getResponsePayload() {
        return new Response();
    }

    private static class Response {
        public final String hello = "hello";
    }
}

Answer

In general, use runSubscriptionOn and pass a specific executor:

longRunningMethodAsync
   .runSubscriptionOn(executor);

Note that it will constraint the concurrency to the number of threads available in the executor.

Reference:

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Answer