For our chat application, we implemented our own http.Handler type in order to easily compile, execute, and deliver HTML content to browsers. Since this is a very simple but powerful interface, we are going to continue to use it wherever possible when adding functionality to our HTTP processing.

In order to determine whether a user is authenticated, we will create an authentication wrapper handler that performs the check, and passes execution on to the inner handler only if the user is authenticated.

Our wrapper handler will satisfy the same http.Handler interface as the object inside it, allowing us to wrap any valid handler. In fact, even the authentication handler we are about to write could be later encapsulated inside a similar wrapper if needed.

Diagram of a chaining pattern when applied to HTTP handlers

The preceding figure shows how this pattern could be applied in a more complicated HTTP handler scenario. Each object implements the http.Handler interface, which...