As we already know, the Reactive Streams specification defines only four interfaces: Publisher<T>, Subscriber<T>, Subscription, and Processor<T, R>. More or less, we are going to follow this list and look at the interface implementations provided by the library.

First of all, Project Reactor provides two implementations of the Publisher<T> interface: Flux<T> and Mono<T>. Such an approach adds additional contextual meaning to reactive types.  Here, to investigate the behavior of reactive types (Flux and Mono), we are going to use some reactive operators without a detailed explanation of how these operators work. Operators are covered later in this chapter.