For a long time, Java didn't have a satisfactory way to represent money. Early versions exhibited issues with currency, precision, rounding, and formatting, and subsequent versions only partially improved these issues. Now, however, we have the best Java specification for handling money to date: JSR 354.
A common problem with the traditional blocking API style is what we call backpressure. Simply put, backpressure is when clients or consumers overwhelm the server's capacity to process requests. This creates the need for efficient hardware and software scaling. Fortunately, Spring WebFlux offers a solution.
Reactive has definitely become a trending word in software development over the past few years, in part because of the high growth of internet users and the corresponding need to create more scalable applications. Some people might say that reactive programming is a hard subject to learn, but certain frameworks and libraries are making it easier to implement even without a deep understanding of how it works.