CGI Programming with Perl, 2nd Edition

Chapter 11. Maintaining State

HTTP is a stateless protocol. As we discussed in Chapter 2, the HTTP protocol defines how web clients and servers communicate with each other to provide documents and resources to the user. Unfortunately, as we noted in our discussion of HTTP (see Section 2.5.1), HTTP does not provide a direct way of identifying clients in order to keep track of them across multiple page requests. There are ways to track users through indirect methods, however, and we’ll explore these methods in this chapter. Web developers refer to the practice of tracking users as maintaining state . The series of interactions that a particular user has with our site is a session . The information that we collect for a user is session information.

Why would we want to maintain state? If you value privacy, the idea of tracking users may raise concerns. It is true that tracking users can be used for questionable purposes. However, there are legitimate instances when you must track users. Take an online store: in order to allow a customer to browse products, add some to a shopping cart, and then check out by purchasing the selected items, the server must maintain a separate shopping cart for each user. In this case, collecting selected items in a user’s session information is not only acceptable, but expected.

Before we discuss methods for maintaining state, let’s briefly review what we learned earlier about the HTTP transaction model. This will provide a context to understand the ...