Mono

Mono is another type of Reactor can emit only one item at the most. An asynchronous task that just wants to signal completion can use a Mono. Mono mainly deals with a stream of one element, as opposed to Flux's N elements.

Both Flux and Mono make use of this semantic by coercing to the relevant type when using some operations. For example, concatenating two Monos together will produce a Flux; on the other hand, calling single() on Flux<T> will return a Mono <T>.

Both Flux and Mono are Reactive Streams (RS) publisher implementations and conform to Reactive-pull back pressure.