Asynchronous programming and networking

GUIs are a great example of asynchronous programming. Once you open a web or a desktop application, it simply sits there, idle, waiting for user input.

This state is often called the event or main event loop. It is simply waiting for external stimuli, such as a key press, a mouse button click, new data from the network, or even a simple timer.

Each of these stimuli is associated with an event handler that gets called when one of these events happen, hence the asynchronous nature of GUI systems.

This is a style of programming that we have been used to for many years, but as business and user needs grow, these applications grow in complexity as well, and better abstractions are needed to handle the dependencies between all the components of an application.

Another great example that deals with managing complexity around network traffic is Netflix, which uses CES to provide a reactive API for their backend services.