Why transformToUniAndConcatenate will return a Multi?

[jiayaoO3O]

    public <O> Multi<O> transformToUniAndConcatenate(Function<? super T, Uni<? extends O>> mapper) {
        return this.transformToUni(mapper).concatenate();
    }

    public <O> Multi<O> transformToUniAndMerge(Function<? super T, Uni<? extends O>> mapper) {
        return this.transformToUni(mapper).merge();
    }

According to the tutorial :

Multi<String> concat = multi
    .onItem().transformToUniAndConcatenate(name -> invokeRemoteGreetingService(name));

If the initial items of Multi are ["a", "b"], after the above code, does it become ["Hello a", "Hello b"] ?
So where does the Uni<String> returned by this invokeRemoteGreetingService(name) method go?

[jponge]

This is (mostly) for performing asynchronous operations on items, whereas transform performs synchronous operations.

A Uni is a good abstraction for an asynchronous operation like doing a database insert or posting to a message queue.
This is why the "mapper" returns a Uni, meaning "take this item and perform some operation that will eventually provide a result".

The resulting Multi collects the result for each Uni, the only difference is the order. Concatenation preserves the original Multi order, while merging pushes the results as soon as each Uni completes.

See https://smallrye.io/smallrye-mutiny/getting-started/transforming-items-async