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4 Web Application Development

Web application development is the development of web applications used in the web services on the Internet. Since these presume use of the Internet, they may require technology differing from that of general software development.

4 - 1 Web Applications

Web application is a general name for software used in web services. In general, this often means software that runs on the web servers that compose web systems and provide services to web clients. Web systems are client/server systems in which web clients make a processing request to web servers that provide web services. However, in the case of web systems that use databases, 3-tier client/server systems are often used. The system configuration of 3-tier client/server systems uses web browsers at the presentation layer, application servers (web servers) at the function layer, and database servers at the database access layer. Using an application server as the web server facilitates adaption to system changes or enhancements.

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Fig. 4-9. Web client contains Browser; Web server contains Web applications; Database server contains DBMS and a Database cylinder. Processing request arrows go right from Browser to Web applications and then DBMS. Processing result arrows return left from DBMS to Web applications and then Browser. DBMS connects to Database.

A web server launches and executes web applications in response to processing requests from web clients (i.e., browsers). This mechanism is known as CGI (Common Gateway Interface), and the web applications that run on web servers are sometimes called CGI programs. Web applications receiving processing requests use database servers as required. At this time, a processing request from a web application is passed as SQL statements to a database server, which in turn returns the execution result of the SQL statements to the web application. Then, the web application returns HTML document, dynamically generated from the received result of executing the SQL statements, to the browser, which displays the HTML document. Web applications executed on the web server side, as in this example, are called servlets. Conversely, web applications downloaded to and executed by the web client are called applets, and the feature by which such applets are downloaded to and executed by web clients as necessary is called rich clients.