Spring WebFlux in Practice

Traditional Spring applications hit a wall the moment traffic gets heavy. Threads pile up waiting on databases and slow network calls, and your server runs out of room long before your CPU does. Spring WebFlux takes a different approach. Built on Project Reactor and Reactor Netty, it lets a small number of threads handle thousands of concurrent requests by never blocking while waiting for I/O. The result is an application that scales further on the same hardware, without needing an army of threads to do it.

Spring WebFlux in Practice takes you from the first principles of reactive programming to running a production-ready application. You'll learn why blocking I/O breaks down under load, how Mono and Flux actually work, and how to build REST APIs, talk to databases and other services, stream real-time data, test everything properly, and deploy with confidence. Every concept is explained in plain language first, then shown in working code.

• Why the thread-per-request model breaks down, and what reactive programming actually changes
• Mono and Flux explained clearly, with the operators you'll use every day
• Building REST APIs with annotated controllers and functional endpoints
• Genuinely non-blocking data access with R2DBC and reactive MongoDB
• Calling other services with WebClient, including timeouts, retries, and running calls concurrently
• Streaming real-time data with Server-Sent Events and WebSockets
• Testing reactive code properly with StepVerifier and WebTestClient
• Running WebFlux in production: observability, tuning, monitoring, and deployment
• A practical appendix covering an operator reference, a migration checklist, and common errors explained

This book is written for Java developers who already know Spring, most likely Spring MVC, and want to understand reactive programming properly rather than by trial and error. Whether you're evaluating WebFlux for a new project, migrating an existing application, or simply trying to make sense of Mono and Flux for the first time, this book meets you where you are and builds from there, one clear explanation at a time.

Your users are waiting. Your threads are not. Every day you keep building on a model that runs out of room is a day your application stays one traffic spike away from falling over. The choice in front of you is simple: keep guessing at why your server struggles under load, or finally understand exactly what's happening and how to fix it for good.

Turn the page, and stop wondering why your application slows down under pressure. Start building the kind that doesn't.