# Project Reactor — Reactive Streams Library for the JVM

> Project Reactor is a reactive programming library for building non-blocking, backpressure-aware applications on the JVM, providing Mono and Flux types that power Spring WebFlux.

## Install

Save as a script file and run:

# Project Reactor — Reactive Streams Library for the JVM

## Quick Use
```xml
<dependency>
  <groupId>io.projectreactor</groupId>
  <artifactId>reactor-core</artifactId>
  <version>3.6.11</version>
</dependency>
```
```java
Flux.just("Hello", "Reactor")
    .map(String::toUpperCase)
    .subscribe(System.out::println);

Mono.fromCallable(() -> fetchData())
    .subscribeOn(Schedulers.boundedElastic())
    .subscribe(data -> process(data));
```

## Introduction
Project Reactor is a Reactive Streams implementation for Java, developed by the Spring team at VMware. It provides two core publisher types: Mono (0 or 1 element) and Flux (0 to N elements), enabling composable, non-blocking data pipelines with built-in backpressure. Reactor is the foundation of Spring WebFlux and Spring Data Reactive.

## What Project Reactor Does
- Models asynchronous sequences with Flux (many elements) and Mono (single element)
- Provides a rich operator library for transforming, filtering, combining, and error-handling streams
- Implements the Reactive Streams specification with built-in backpressure propagation
- Supports multiple threading strategies via Schedulers (parallel, elastic, single, immediate)
- Integrates with Netty, Spring WebFlux, R2DBC, and other reactive libraries

## Architecture Overview
Reactor is built around the Publisher-Subscriber pattern defined by the Reactive Streams specification. Operators are implemented as decorating publishers that form a chain. When a subscriber subscribes, a request signal propagates upstream, activating the data flow with backpressure. Reactor uses Netty's event loop for I/O-bound work and configurable thread pools (Schedulers) for CPU-bound or blocking tasks.

## Self-Hosting & Configuration
- Add reactor-core as a Maven or Gradle dependency
- For testing, add reactor-test and use StepVerifier to assert stream behavior
- Use Schedulers.boundedElastic() to wrap blocking calls without starving the event loop
- Enable Reactor's debug mode with Hooks.onOperatorDebug() during development
- For production tracing, use reactor-core-micrometer for metrics and context propagation

## Key Features
- Mono and Flux types with 200+ composable operators
- Backpressure-aware with configurable overflow strategies (buffer, drop, error, latest)
- Context propagation for carrying metadata (trace IDs, auth tokens) across async boundaries
- StepVerifier test utility for deterministic verification of reactive sequences
- Built-in retry and timeout operators with exponential backoff support

## Comparison with Similar Tools
- **RxJava** — earlier reactive library for Java; Reactor is lighter and aligned with Spring
- **Mutiny (Quarkus)** — simpler reactive API with Uni/Multi; Reactor has richer operators
- **Kotlin Coroutines Flow** — Kotlin-native reactive streams; Reactor targets Java-first
- **Java 9 Flow** — JDK built-in reactive interfaces; Reactor adds the operator library on top
- **Akka Streams** — Scala-centric with actor model integration; Reactor is Spring-ecosystem native

## FAQ
**Q: When should I use Mono vs Flux?**
A: Use Mono for operations that return zero or one result (database lookups, HTTP calls). Use Flux for streams of multiple items (event feeds, query result sets).

**Q: How do I handle blocking code in Reactor?**
A: Wrap blocking calls with Mono.fromCallable().subscribeOn(Schedulers.boundedElastic()) to offload them from the event loop.

**Q: Is Reactor required for Spring WebFlux?**
A: Yes. Spring WebFlux uses Reactor as its reactive foundation. Controllers return Mono or Flux types.

**Q: How do I debug reactive pipelines?**
A: Enable Hooks.onOperatorDebug() to get assembly-time stack traces, or use checkpoint() operators to add diagnostic markers.

## Sources
- https://github.com/reactor/reactor-core
- https://projectreactor.io/docs/core/release/reference/

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Source: https://tokrepo.com/en/workflows/asset-945d8a96
Author: Script Depot