At first, personal computers were stand-alone productivity tools. To share data with a colleague, one had to store it on a floppy disk or some other removable media and hand deliver it. Time sharing systems had enabled users to communicate with each other and conveniently share data, but early personal computers did not.
The solution to this problem was hardware and communication software for connecting the computers in an office or a building, forming a local area network or LAN. There were several competing LAN technologies at first, but they were proprietary. Because it was an open standard, Ethernet eventually became dominant.
Once connected to a LAN, users could share common databases and documents as well as hardware like printers. Each user had his or her own computer, and common computers were added to the network. These computers were called servers, since they were programmed to offer a service like printing or database management to the user’s computers, the clients.
The LAN became the next important software development platform. Programmers developed client-server applications in which they separated the user interface program, which ran on the user’s personal computer, from the application logic and databases, which ran on servers. Programmers also had to develop software for hardware services like sharing files, printers, and modems. As we see below, client-server applications are also prominent on the Internet. When you use the Web, you are running a client program like Internet Explorer or Firefox on your computer (the client) and retrieving information from Web servers.
Sidebar: Internet client options
The client-server model has moved from the LAN to the Internet. Several client options are found on the Internet. The most common is the Web browser. Early browser-based applications retrieved Web pages with HTML (hypertext markup language) tags added to control the formatting of the text and images. 1 Any computer, regardless of its operating system, could use these applications, and very little skill was required to add HTML tags.
However, the pages were largely static text and images. The next generation of Web client could execute simple programs included in Web pages, making dynamic behavior like cascading menus and buttons that changed appearance when clicked possible. These programs are generally written in the JavaScript programming language.
AJAX, asynchronous JavaScript and XML, uses JavaScript to download content without displaying it while the user looks at a page. Consider for example Google maps. While a user is looking at a portion of a map, adjacent portions are downloaded and cached. When the user scrolls the display, the adjacent portions are displayed immediately.
An applet is a program that is automatically downloaded and executed when the user links to an HTML page that contains it. Applets are useful when a fairly complex client program is needed for an application. They are most commonly written in the Java programming language.
A Web browser can be augmented using a plug-in like the Macromedia Flash player, which enables your Web browser to show animations and movies. Once you install this plug-in, your browser will use it to play Flash movies from any Web site.
For complex tasks rich user interfaces and specialized server software and databases, a Web browser may not suffice. In those cases, the user must install a custom client program. That program, not a Web browser, would be used to interact with the server. These applications can be fast and complex, but the user must be able to install them. Examples of Web sites that require custom clients include Apple's iTunes client, Google Earth and Microsoft's Virtual Earth 3D.
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