[[webflux-reactive-libraries]]
= Reactive Libraries

`spring-webflux` depends on `reactor-core` and uses it internally to compose asynchronous
logic and to provide Reactive Streams support. Generally, WebFlux APIs return `Flux` or
`Mono` (since those are used internally) and leniently accept any Reactive Streams
`Publisher` implementation as input. The use of `Flux` versus `Mono` is important, because
it helps to express cardinality -- for example, whether a single or multiple asynchronous
values are expected, and that can be essential for making decisions (for example, when
encoding or decoding HTTP messages).

For annotated controllers, WebFlux transparently adapts to the reactive library chosen by
the application. This is done with the help of the
{spring-framework-api}/core/ReactiveAdapterRegistry.html[`ReactiveAdapterRegistry`], which
provides pluggable support for reactive library and other asynchronous types. The registry
has built-in support for RxJava 3, Kotlin coroutines and SmallRye Mutiny, but you can
register others, too.

For functional APIs (such as <<webflux-fn>>, the `WebClient`, and others), the general rules
for WebFlux APIs apply -- `Flux` and `Mono` as return values and a Reactive Streams
`Publisher` as input. When a `Publisher`, whether custom or from another reactive library,
is provided, it can be treated only as a stream with unknown semantics (0..N). If, however,
the semantics are known, you can wrap it with `Flux` or `Mono.from(Publisher)` instead
of passing the raw `Publisher`.

For example, given a `Publisher` that is not a `Mono`, the Jackson JSON message writer
expects multiple values. If the media type implies an infinite stream (for example,
`application/json+stream`), values are written and flushed individually. Otherwise,
values are buffered into a list and rendered as a JSON array.