Introduction to web frameworks

This section is a brief introduction to how modern web applications are designed. Go ahead and skip it if you already feel comfortable writing backend code.

Loosely, a web framework is a set of tools and code libraries for building web applications. To understand what a web framework provides, let's take a step back and think about what you would need to do if you did not have one.

You want to write a program that listens on port 80 and sends HTML (or JSON or XML) back to clients that request it. This is simple if you are serving the same file back to every client: just load the HTML from file when you start the server, and send it to clients who request it.

So far, so good. But what if you now want to customize the HTML based on the client request? You might choose to respond differently based on part of the URL that the client put in his browser, or based on specific elements in the HTTP request. For instance, the product page on amazon.com is different...