Developing an Oracle Application.

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Developing an Oracle Application.

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When writing a program to interact with the Oracle database, an application developer has the following choices:

• Client/Server Model:
  
  In a traditional client/server program, the code of an application runs on a computer other than the Oracle database server. Database calls are transmitted from the client computer to the database server. Data is transmitted from the client to the server for insert and update operations and returned from the server to the client for query operations. The data is processed on the client computer. Client/server programs are typically written by using precompilers, where SQL statements are embedded within the code of another language such as C, C++, or COBOL.



• Server-Side Coding:
  
  An application developer can develop an application logic that resides entirely inside the database. This can be accomplished by using triggers that are executed automatically when changes occur in the database. This can also be accomplished by using stored procedures that are called explicitly. Offloading the work from the application lets the developer reuse the code that performs verification and cleanup and thereby allows the developer to control database operations from a variety of clients. For example, by making stored procedures callable through a Web server, an application developer can construct a Web-based user interface that performs the same functions as a client/server application.



• Two-Tier Versus Three-Tier Models:
  
  Client/server computing is often referred to as a two-tier model. This is because, in a client/server model, the application communicates directly with the database server. In the three-tier model, another server (known as the application server) works as the middle tier and processes the requests. The application server may be a basic Web server, or it may perform advanced functions such as caching and load balancing. Increasing the processing power of the middle tier allows users to lessen the resources needed by client computers. This results in a thin client configuration where the client computer may need only a Web browser or other means of sending requests over the TCP/IP or HTTP protocols.



• User Interface:
  
  The interface that an application displays to end-users depends on the technology behind the application as well as the needs of the users. Experienced users may enter SQL commands that are passed on to the database. Novice users may be shown a graphical user interface that uses the graphics libraries of the client computer (such as Windows or X-Windows). Any of these traditional user interfaces can also be provided in a Web browser by using HTML and Java.



• Stateful Versus Stateless User Interfaces:
  
  In traditional client/server applications, an application can keep a record of user actions and use this information over the course of one or multiple sessions. For example, past choices can be presented in a menu so that they do not have to be entered again. When an application is capable of saving this kind of information, it is referred to as a stateful application.
  
  On the other hand, the stateless applications gather all the required information, process it using the database, and then start over from the beginning with the next user. This is a popular way to process single-screen requests such as customer registration. Stateless applications are the easiest types of Web or thin-client applications to develop.
  
  There are several ways to add stateful behavior to Web applications that are stateless by default. For example, an entry form on a Web page can pass information on to subsequent Web pages, thereby allowing a developer to construct a wizard-like interface that remembers the user's choices through several different steps. Cookies can be used to store small items of information on the client computer and retrieve them when the user returns to a Website. Servlets can be used to keep a database session open and store variables between requests from the same client.

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