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 to the payment page. You need to write code to parse the URL and the request, and then route the request to the relevant handler.

You might now want to customize the HTML returned dynamically, based on specific elements of the request. The page for every product on amazon.com follows the same outline, but specific elements are different. It would be wasteful to store the entire HTML content for every product. A better way is to store the details for each product in a database and inject them into an HTML template when a client requests information on that product. You can do this with a template processor. Of course, writing a good template processor is difficult.

You might deploy your web framework and realize that it cannot handle the traffic directed to it. You decide that handlers responding to client requests should run asynchronously. You now have to deal with concurrency.

A web framework essentially provides the wires to bind everything together. Besides bundling an HTTP server, most frameworks will have a router that automatically routes a request, based on the URL, to the correct handler. In most cases, the handler will run asynchronously, giving you much better scalability. Many frameworks have a template processor that lets you write HTML (or sometimes JSON or XML) templates intuitively. Some web frameworks also provide functionality for accessing a database, for parsing JSON or XML, for formulating HTTP requests and for localization and internationalization.