If we remember from Chapter 1, Why Reactive Spring?, the essence of reactive systems is in message-driven communication. Moreover, the previous chapters made it clear that by applying reactive programming techniques, we can write async interactions for interprocess/cross-service communication. In addition, by using the Reactive Streams specification, we are equipped to manage backpressure as well as failures in an asynchronous manner. Gathering all these features, we are capable of building a high-grained reactive application within one computer. Unfortunately, a one-node application has its constraints, which are expressed in hardware limitations. First of all, it is impossible to provide new computation resources such as additional CPU, RAM, and hard drive/SSD without shutting down the whole system. No benefit is...