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8.5. When to Use Sessions

So far in this chapter we have described how to implement stateful applications using sessions, but we have not discussed when they should or should not be used. Sessions allow some kinds of applications to be developed that otherwise would be difficult to implement on the Web. However, because HTTP is a stateless protocol, building a stateful application can present problems and restrictions. Avoiding the need to maintain state information is often a desirable goal. In this section we list some reasons sessions are used and some reasons to avoid them.

8.5.1. Reasons to Use Sessions

Sessions can be used in web database applications for several reasons. Many traditional database applications use sessions to help control user interaction, while other applications use sessions to reduce server processing.

8.5.2. Reasons to Avoid Sessions

The reasons to avoid sessions focus mainly on the stateless nature of HTTP. The features of HTTP that support browsing access to a disparate collection of resources don't support stateful applications. Stateful applications work over the Web often at the expense of HTTP features.

8.5.2.4. Bookmark restrictions

Because HTTP is stateless, browsers allow users to save URLs as a list of bookmarks or favorite sites. The user can return to a web site at a later date by simply selecting a bookmarked URL. Web sites that provide weather forecasts, stock prices, and even search results from a web search engine are examples of the sites a user might want to bookmark. Consider the URL for a fictional site that provides stock prices:

http://www.someexchange.com/stockprice.php?code=SIMCO

The URL encodes a query that identifies a particular stock, and presumably, the script stockprice.php uses the query to display the current stock price of the company. The URL can be bookmarked because it contains all that is needed to generate the stock price page for the given company code. An alternative site may collect the company code using a <form> and, when the form is submitted, use a session variable to hold the company code as a query. The script that generates the stock price page reads the session variable, looks up the current price, and generates the result for the entered company code. If a user bookmarks the session-based stock price page and comes back in a week, the session that stored the company code is unlikely to still exist, and the script fails to display the desired company's stock price.

Sometimes bookmarking a page makes no sense. Consider an online banking application that allows transfer of funds between two accounts. A user would log in to the application, then request the transfer page that collects the source and target account details in a <form>. When that <form> is submitted, a confirmation page is shown without actually performing the transaction. Revisiting this page through a bookmark has no meaning if the transaction was subsequently confirmed or canceled. Generally, the pages generated from applications such as online banking can't be bookmarked because of the reliance on session variables. Session management in such applications is often tied closely to authentication, a topic explored further in Chapter 9.



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