The Reactor project is divided into different modules. The reactor-core module is the central library, aimed at providing implementations for Reactive Streams. The library provides Flux and Mono, which are two different implementations of the Publisher interface. The two publishers are different in terms of the number of events that they can emit. Flux can emit infinite sequences of elements, but the Mono API makes it possible to emit a maximum of one element. Let's cover these APIs in detail, in the following sections.