What is Spring WebFlux and when to use it

Spring WebFlux is the web framework for reactive-stack. Spring WebFlux is added in Spring 5.0. It is fully non-blocking and supports reactive stream back pressure. It runs on the non-blocking I/O client-server framework such as Netty, Undertow, and Servlet 3.1+ containers. In an Spring application, both modules (spring-webmvc and spring-webflux) can exist side-by-side.

The term "reactive" in programming model refers to react to change, for example, network components reacting to I/O events, UI controllers reacting to mouse events. Here non-blocking is reactive because, instead of being blocked, we are now in the mode of reacting to notifications as operations complete or data becomes available.

Spring WebFlux is needed for a non-blocking web stack to handle concurrency with a small number of threads and scale with fewer hardware resources. Functional programming is good to write concurrent programs. Due to immutability as a default construct of functional paradigm, concurrent programming guarantees deterministic execution. After the introduction of lambda expression, it creates way to write functional programming. Hence to manage reactive programming for web application, Spring introduces WebFlux module.

https://docs.spring.io/spring/docs/current/spring-framework-reference/web-reactive.html#webflux