The project AngularAndSpring has been refactored from Spring Reactor to Spring blocking APIs using JDK 25 and virtual threads. The Old ArchitectureThe reason AngularAndSpring was built using Spring Reactor was primarily the efficient use of operating system threads and memory under high load. Spring Reactor had a limited thread pool (usually the number of available CPU cores) and used OS threads only when the CPU had work to do. It did not let the operating system threads sleep. This used CPU resources much better and required far fewer OS threads, each of which needs around a megabyte of memory.