The previous article in this series began an exploration of the first and most fundamental technology used in Web-based applications: HTML, the Hypertext Markup Language. This article continues the introduction to HTML in Web applications with a discussion of the core elements of both HTML and Web applications in general.
Web servers, clients, and the software that they run all play an important role in any Web application. Web applications are client/server software systems that use the Internet and standard Web protocols for communication. A client/server architecture is a network-based system in which software running on one computer (the client) makes a request for services from software running on a different computer (the server), which fulfills the request.
Servers and Clients
In its simplest form, a Web application consists of one or more documents published on a server that can be requested and viewed by users of different computers (see Figure 1). The server machine runs Web server software that constantly detects, evaluates, and then accepts or rejects connections and file transfer requests from other computers on the Internet. The most common Web server software includes Apache HTTP Server, Microsoft Internet Information Server, and Netscape Enterprise Server.
When a request is accepted, the Web server software is responsible for finding the document, evaluating it, carrying out any required server-side processing, and sending the requested documents to the other computer using the Hypertext Transfer Protocol (HTTP), a standard format for the transfer of HTML documents between client and server computers on the Internet.
Figure 1 Simplest Web application architecture.
On the other end of the communication, the user runs Web browser software on a client computer. The client is connected to the Internet either full-time or on demand. The Web browser software is responsible for accepting input from the user, connecting to the specified server computer across the Internet, and making an HTTP request for the desired document. When the document is received, the Web browser displays it.