25. What is Session Tracking.

There are a number of problems that arise from the fact that HTTP is a “stateless” protocol. In particular, when you are doing on-line shopping, it is a real annoyance that the Web server can’t easily remember previous transactions. This makes applications like shopping carts very problematic: when you add an entry to your cart, how does the server know what’s already in your cart? Even if servers did retain contextual information, you’d still have problems with e-commerce. When you move from the page where you specify what you want to buy (hosted on the regular Web server) to the page that takes your credit card number and shipping address (hosted on the secure server that uses SSL), how does the server remember what you were buying?

There are three typical solutions to this problem.

  1. Cookies. You can use HTTP cookies to store information about a shopping session, and each subsequent connection can look up the current session and then extract information about that session from some location on the server machine. This is an excellent alternative, and is the most widely used approach. However, even though servlets have a high-level and easy-to-use interface to cookies, there are still a number of relatively tedious details that need to be handled:
    • Extracting the cookie that stores the session identifier from the other cookies (there may be many, after all),
    • Setting an appropriate expiration time for the cookie (sessions interrupted by 24 hours probably should be reset), and
    • Associating information on the server with the session identifier (there may be far too much information to actually store it in the cookie, plus sensitive data like credit card numbers should never go in cookies).
  2. URL Rewriting. You can append some extra data on the end of each URL that identifies the session, and the server can associate that session identifier with data it has stored about that session. This is also an excellent solution, and even has the advantage that it works with browsers that don’t support cookies or where the user has disabled cookies. However, it has most of the same problems as cookies, namely that the server-side program has a lot of straightforward but tedious processing to do. In addition, you have to be very careful that every URL returned to the user (even via indirect means like Location fields in server redirects) has the extra information appended. And, if the user leaves the session and comes back via a bookmark or link, the session information can be lost.
  3. Hidden form fields. HTML forms have an entry that looks like the following: <INPUT TYPE="HIDDEN" NAME="session" VALUE="...">. This means that, when the form is submitted, the specified name and value are included in the GET or POST data. This can be used to store information about the session. However, it has the major disadvantage that it only works if every page is dynamically generated, since the whole point is that each session has a unique identifier.
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