How to execute concurrent void method calls

Question

I'm working with a database and trying to figure out how I can concurrently execute writes which have a void return type. I've already wrapped the blocking calls as per the docs.

I want to:

1. Get the item from the database which returns a Mono<ObjectToReturn>
2. Concurrently call 2 methods which return Mono<Void> which I need to complete before I return
3. Return the response from get() to the subscriber

Functionally I have this working with:

    return repository
            .get(param)
            .flatMap(response -> Mono.zip(
                    method1(response),
                    method2(response),
                    (x, y) -> response
            ))

However:

• I feel like I'm abusing Mono.zip to subscribe to the response since I'm ignoring x and y

• I've had to change the method1/2 interfaces to arbitrarily return a value after the write is successful

What's the right way of doing this?

Answer 1

The problem with your code is that if any mono is empty, the zip operation won't emit any value, resulting in no output.

I think it is better to use .thenReturn() fallback:

 return repository.get(param)
                  .flatMap(response -> Mono.zip(method1(response), method2(response))
                        .thenReturn(response)
                  )

The response will be returned after Mono.zip is completed