1. Introduction — Why Does WebFlux Exist? If you've built REST APIs with Spring MVC, you know the model: one thread handles one request, start to finish. That thread blocks while waiting for a database call, a downstream HTTP call, or a file read. This is called the thread-per-request model, and it works fine — until you have thousands of concurrent, slow, I/O-heavy requests. At that point, you run out of threads, not CPU. Spring WebFlux solves this by using a reactive, non-blocking m...