Next Generation Internet: The "Fourth Tier" Is Born - Internet/Web/Online Service Information

Computer Technology Review, Jan, 2001 by Reza B'Far

Solving the problem of disparate content types

Web content began as static HTML p ages and evolved to include client-side scripting, proprietary content technologies, and application programming interfaces. HTML has remained the basis of all Web content-until now. We are about to witness the revolutionary move of content from HTML to XML (Extensible Markup Language).

XML is a set of rules for defining a document using tags in a self-described vendor- and platform-neutral manner. XML has numerous advantages over HTML. It is easily transformable and can describe any type of content.

HTML is a rendered presentation of data for a specific set of clients (namely HTML-based browsers), while XML can be data, its presentation, or a combination of both. Metaphorically speaking, HTML is a picture of a 3D object (Data, Presentation, and Flow Logic) while XML is the 3D object itself. Viewing an HTML object from a different perspective will produce a fuzzy picture at best because the object's entire data set is unavailable. Cell phones, PDAs, or embedded devices may have problems with HTML, which often has extraneous or missing data.

Content in XML can be transformed into a wide range of other content (like voice based content) and made available to a wider range of devices (like digital cell phones). XML content can be rendered in one way for cell phones (like WML for WAP) and in another way for PC-based browsers (like XHTML).

The Complexities Of Content

Content, more complex than ever before, is currently provided by a variety of servers. It exists in three distinct formats: data, audio (including voice), and visual (including video). Each content type requires different mechanisms and systems for storage, processing, and serving. Content combinations depend on user specifications, device capabilities, and available content. Configuring the combination correctly is a complex process and is not accounted for in the current development model. Today's systems solve only one problem: data. Audio and visual components must be integrated and content mixtures served to clients must be synchronized.

The variety of Internet clients has multiplied considerably. Five years ago, users only expected to access content from different PC browsers. Today users expect to tap into the same content from multiple devices with vastly different capabilities. These devices range from cell phones to PDAs to web browsers. Each client can process different amounts and combinations of each type of data. Web browsers, for example, typically have a keyboard/mouse/monitor interface while cell phones have a phonepad/voice interface. Hybrid devices combine the capabilities of devices: PDAs have cell phones integrated into them and cell phones possess many PDA capabilities.

When building today's systems, future requirements must be considered, including content form.

Most audio and video systems have switched from analog to digital. Most digital, visual, and aural based content is served in proprietary formats. There has been no equivalent to non-proprietary HTTP until recently, with the development of XML.

Audio, data, and video content can be described by metadata in XML. Clients can easily process data if video or audio streams are wrapped inside XML with some meta data about the stream. VoiceXML, for example, allows voice-based content to be described by XML, encapsulating the data concerning the content. This data can then be used to "introduce" the content to any client wanting to use it. Content can be customized at run-time using an XML description of the client, user settings, and content structure.

Approaching content in this way gives birth to a new software layer separating the user interface and the application server (the middle-tier). This layer enables us to author content once using the new presentation layer: then the formats and logical flows required for various clients can be created. This new system will separate the data, presentation, and logic of the user interface.

The Move From Three-Tier To Four-Tier Architecture

Three-tier architecture has been the prevailing design for Internet systems during the past few years. In this design, as seen in Figure 1, there are three primary components: database, application server, and client.

Three-tier architecture was considered an evolutionary step over the client server model. It removed business logic from the client and the database and placed it into an application server. It became responsible for the processing work that implemented the business logic in the system. It enabled systems to scale larger and become more reliable by implementing various forms of fail-over at a lower cost. By clearly defining an API (Application Programming Interface) at each tier in the architecture, it allowed for a more sophisticated development cycle. System development could now be broken down and distributed among three groups: database engineers, software engineers, and user interface developers. New features could be added in a faster, more methodical way.