Mono and Flux Basics

Reactive programming is about building non-blocking, asynchronous applications that handle streams of data.

Instead of returning a value directly, a reactive method returns a publisher that emits data over time. The caller subscribes to receive it.

• Threads are not blocked waiting for I/O
• Great for high-concurrency, low-latency services

Spring WebFlux is built on Project Reactor, which provides two core publisher types:

• Mono<T> — emits 0 or 1 item
• Flux<T> — emits 0 to N items

Both implement the Reactive Streams Publisher interface.