Using Mono and Optional together

Question

One of the things I'm trying to wrap my head around with reactive programs is dealing with no results appropriately.

Let's say I had an API that optionally retrieved the latest order for a user. I want to leave the behavior of if there is no user found up to the caller. In plain old java, I would write something like:

Optional<Order> getLatestOrder(int userId) { ... }

In webflux, I'm trying to wrap my head around if an equivalent function should be returning Mono<Optional<Order>> or Mono<Order>.

If I add getLatestOrder() to a reactive flow, it might stop if I emit an empty mono. It feels weird also to do something like:

getLatestOrder().defaultIfEmpty(new Order());

since that now feels like just a sentinel value that's representing that it doesn't exist.

Is there guidance on a pattern with using Mono and Optional? Is there a proper way to ditch Optional completely when you really want to express something is not present?

Answer 1

I started using webflux for a small project and we faced the same question. We decided to return Mono<Order> Most of the time we then use switchIfEmpty(Mono<? extends T> alternate) to throw an error or recover with an alternative

In our usecases switchIfEmpty() and defaultIfEmpty() give us enough control to handle empty results.

Example from the reference guide: https://projectreactor.io/docs/core/release/reference/index.html#_checking_the_execution_path_with_code_publisherprobe_code