This chapter is a little detour to reactive programming. It lets us handle the concurrency requirements of an application in some cases. It provides an abstraction to handle concurrency. Even though the concepts are old, it has gained interest in recent years due to the beginning of large inflow of data. In modern times, billions of devices generate data every day. Tapping into this data is essential for the growth of business; in some cases, processing the data to statistically analyze it or feeding it to some machine learning algorithm may be the entire business in itself. This makes it essential to support the processing of this large in-flow of data, provide a quick response, and be resilient to failures. Of course, one can do these things even using a traditional or imperative programming paradigm, just as one can, in theory, build any application using an assembly language. However, this makes the application extremely complex to maintain and impossible...