Make iterations of a loop sequentially in Mutiny

user_17047177 0
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I am new in the reactive programming world. I am currently working in a Java reactive application using the Mutiny library.

I need to develop a loop that waits for the previous iteration to finish in order to start the next one. For instance:

List<Uni<T>> uniList = new ArrayList<>();
for (T item : items) { //items is an already fulfilled collection
    uniList.add(this.doSomethingAndReturnInUni(item));
}


return Uni.combine().all().unis(uniList).combinedWith(unisToCombine -> {
    List<T> list = new ArrayList<>();
    unisToCombine.forEach(x ->list.add(x));
    return list;
  });

The for loop in the example, generates a thread per iteration. I am wondering how to order the i-th call to the method doSomethingAndReturnInUni() waits for the (i-1) call to trigger the event, that is, make the for loop sequentially. It is possible to suscribe those events in such a way?

vert.x mutiny quarkus reactive-programming
3 Answers

Could you try something like this?

Builder<Item> items = Uni.join().builder();
for (Item item : items) {
    builder.add(this.doSomethingAndReturnInUni(item));
}
return builder.joinAll().andCollectFailures()
    .flatMap(itemList -> do whatever you need ...)  //itemList type is List<Item>
user_18427310

I don't know why you are using uni, as this should just handle one operation, for loops you should use multi, where you can handle the back pressure, and only get the next event, when one event is finished. Multi can be run sequentially and in parallel.

see https://quarkus.io/blog/mutiny-back-pressure/

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