Reactor Core Features

The Reactor project main artifact is reactor-core, a reactive library that focuses on the Reactive Streams specification and targets Java 8.

Reactor introduces composable reactive types that implement Publisher but also provide a rich vocabulary of operators: Flux and Mono. A Flux object represents a reactive sequence of 0..N items, while a Mono object represents a single-value-or-empty (0..1) result.

This distinction carries a bit of semantic information into the type, indicating the rough cardinality of the asynchronous processing. For instance, an HTTP request produces only one response, so there is not much sense in doing a count operation. Expressing the result of such an HTTP call as a Mono<HttpResponse> thus makes more sense than expressing it as a Flux<HttpResponse>, as it offers only operators that are relevant to a context of zero items or one item.

Operators that change the maximum cardinality of the processing also switch to the relevant type. For instance, the count operator exists in Flux, but it returns a Mono<Long>.