Again, asynchronous programming is fundamentally different from synchronous programming because of its task-switching nature. In synchronous programming, the instructions of a program are executed sequentially: a task has to have finished executing before the next task in the program starts processing. With asynchronous programming, if the current task takes significant time to finish, you have the option to specify a time during the task at which the execution is switched to another task. As we have observed, doing this would result in potential improvements in the execution time of the whole program.

One common example of asynchronous programming is the interaction between a server and a client during an HTTP request. If HTTP requests were synchronous, clients would have to wait after making a request until receiving the response from the server. Imagine a scenario in which your browser would hang every time you go to a new link or start playing a video until the actual data returns from the server. This would be extremely inconvenient and inefficient for HTTP communication.

A better approach is asynchronous communication, where the client is free to continue working, and when data from the requests made returns from the server is when the client will be notified and proceed to process that data. Asynchronous programming is so common in web development that a whole programming model called AJAX (short for Asynchronous JavaScript and XML) is now used in almost every website. Additionally, if you have used common libraries in JavaScript such as jQuery or Node.js, chances are you have worked with, or at least heard the term callback, which simply means a function that can be passed to another function to execute later in the future. Switching back and forth between the execution of functions is the main idea of asynchronous programming, and we will actually analyze an advanced example of callback usage in Chapter 18, Building a Server from Scratch.

The following diagram further illustrates the difference between synchronous and asynchronous client-server communication:

Asynchronous programming is, of course, not limited to HTTP requests. Tasks that involve general network communication, software data processing, interaction with databases, and so on all take advantage of asynchronous programming. Contrary to synchronous programming, asynchronous programming provides responsiveness for users by preventing the program from hanging while waiting for data. Therefore, it is a great tool to implement in programs that deal with a large amount of data.