As we mentioned at the start of the chapter, most modern websites allow registration through external authentication services, such as Facebook, Google, or LinkedIn. What this means is that these external authentication services validate a user's credentials, and then forward our application some details about the user. These details generally include the username, email address, and possibly even a link to a picture that we can use on our site. This is a fairly easy replacement for the standard registration process, where we gather and store information about a user and their password. The use of external authentication services also makes our sites more appealing and accessible to a broader range of users.

In this section of the chapter, we will integrate with Google to allow anyone who has a valid Google account to access our site. The libraries...