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.
In a stateless environment, an application may need to repeat an expensive operation. An example might be a financial calculation that requires many SQL statements and calls to mathematics libraries before displaying the results on several web pages. An application that uses a session variable to remember the result exposes the user, and the server, to the cost of the calculation only once.
Sequence of interaction
Often a database application-or indeed any application-needs to present a series of screens in a controlled order. One style of application-known as a wizard-guides a user through what would otherwise be a complex task with a series of screens. Wizards are sometimes used for complex configurations, such as some software installations, and often alter the flow of screens based on user input. Some applications require that a user enter via a known page. Applications, such as online banking, often force a user to enter via a login page rather than allow access directly to a function such as funds transfer.
Many database applications validate data before creating or updating a record in the database, preventing erroneous data from being saved. Sessions can keep the intermediate data, so that incomplete data can be edited-rather that rekeyed-when errors are detected. Earlier in this chapter we used sessions to improve the interaction between the client entry
<form> and validation scripts of the winestore application. In the case study, the fields entered by the user are held in an array as a session variable until the validation is successful. Another example where intermediate results can be used is when a database application collects and validates data for a single record over a number of fill-in forms. A shopping cart is an example where complete data may not be created until a user requests a purchase. The winestore application doesn't implement the shopping cart this way; rather, a shopping cart is implemented by creating a row in the orders table and adding rows to the items table as items are selected. The winestore application then needs to store only the
cust_id and the
order_no-the combination is the primary key of the orders table-as session variables while a shopping cart is being used. We develop the shopping cart in Chapter 11.