Working with Stateful Applications

The previous exercise demonstrates the challenge of working with stateful applications in a distributed context. As a brief overview, a stateless app is an application program that does not save client data generated in one session for use in the next session with that client. This means that in general, a stateless application depends entirely on the input to derive its output. Imagine a server displaying a static web page that does not need to change for any reason. In the real world, stateless applications typically need to be combined with stateful applications in order to create a useful experience for clients or consumers of the application. There are, of course, exceptions to this.

A stateful application is one whose output depends on multiple factors, such as user input, input from other applications, and past saved events. These factors are called the "state" of the application, which determines its behavior. One of the most...