The latest trend in information portals and Web applications has been to build complex Web pages. To present large amounts of information and functionality without compromising usability, designers have imposed a clear structure by grouping related elements together. Such cohesive, visually distinct constructs, or compartments, often with their own presentation logic, have become an essential feature of complex Web applications.

The first half of this article introduces the notion of compartments as a fundamental concept that pervades the layers of many Web applications and stages in their lifecycle. It defines compartments and analyzes their structure and key characteristics. Then it will examine the existing presentation layer technologies for building compartmentalized appli-cations and demonstrate their weaknesses.

The second half of the article proposes an architecture and framework that directly support design and development of compartmentalized applications and improve their performance. It demonstrates how the use of this framework improves developer productivity, facilitates reuse and yields more flexible and maintainable applications.

Introducing Compartments
Figure 1 includes screenshots from some well-known Web sites. These sites all have a different look-and-feel from one another, serve different purposes and address different audiences. However, they use the same approach to organizing and presenting a wealth of information in regular, visually distinct blocks or compartments. Such compartmentalization is common throughout the Web; a quick survey of a number of prominent Web sites shows that many of them use easily identifiable compartments as primary structural elements.

Definition
Despite visual and structural diversity, all compartments share fundamental elements. We define a compartment as a rectangular area on the page that presents dynamic content and has the following characteristics:

• It has regular structure and layout
• It contains related content elements
• It's visually and functionally distinct from the rest of the page
• It has internal presentation logic (i.e., layout depends on the content displayed)
• It behaves atomically within the page.

To illustrate this definition, these are some of the compartments that can be seen on the screenshots in Figure 1:

• On Travelocity - Great Getaways, Fare Watcher, Cruises & Vacations and Travel Tools.
• On FirstGov - Agencies, Information by Topic and In Focus.

The following elements don't meet the definition and wouldn't be considered compartments:

• The grey left navigation bar on the BBC site because it's static
• "Find me the best priced trip" on Travelocity because it's static and doesn't have regular structure or presentation logic
• "E-mail This Page" on FirstGov and the round navigation buttons at the top of the Yahoo page (i.e., Personalize and Finance) because they lack regular structure or presentation logic.

Structure and Behavior
The Programs & Campaigns (P&C) Compartment presented in 0 came from one of the systems developed by the authors. It will be used as an example in this article.

Typically a compartment consists of a header, footer and a body. The header and footer together define the visual elements, including frame, background, title and global links such as "All Programs" and "All Campaigns" and the other elements that belong to the entire block.

The body consists of a series of contentlets, elementary units of dynamic content presentable on a Web page, separated by optional spacers or other structural elements. A contentlet has a number of core attributes: Name, Description, Image and Display Order. Additional attributes, such as Date, Type and Source, may be represented as needed for a particular problem domain or implementation. All these attributes are optional.

One important type of contentlet is a Content Reference, which adds another core attribute, a URL. Any dynamic content targeted to a compartment can be represented as a collection of contentlets.

In contrast to static page elements, a compartment has intrinsic behavior, which is governed by its presentation logic. Presentation logic is a set of rules determining how to render each contentlet, which contentlets are considered renderable and how to render the entire compartment, depending on the collection of contentlets it receives. Such rules may include:

• Lay out contentlet attributes in four columns in order - Image, Name and Description, Type, Date
• Consider a contentlet renderable if one of the core attributes is defined
• Remove the Date column if none of the rendered contentlet defines it
• Change the compartment title to "Programs" if all contentlets are the Type program;
• Remove the entire compartment if it doesn't have any renderable contentlets.

It's often difficult to demarcate the exact boundary between business logic and presentation logic; this is a subject of hot discussion in the development community1. A key distinction is that business rules exist in the problem domain, while presentation logic is pertinent only to a computer application. Nevertheless, presentation logic usually comes from business requirements and is equally important in building successful software. However, in contrast to the amount of attention paid to application business logic, there is considerably less support for implementing presentation logic in Web-based applications.

Role in Application Lifecycle
The role of compartments isn't limited to system design and construction. Normally, compartments are identified early in the development process and used to construct wireframes, capture functional requirements and design prototypes. They also play an important role in testing as a basis for individual test cases.

Presentation Technologies
There are a number of JSP-based technologies currently available to develop the presentation layer of compartmentalized Web applications. Portlets, different flavors of templates and tag libraries are commonly used. Each has inherent strengths and weaknesses.

Portlets are the most direct realization of compartments, but they can only be used in a portal platform. Using a portal platform is not always appropriate because of the cost, performance or technical limitations in the URL structure, as well as problems with bookmarkability, navigation, the lack of visibility by Web search engines, layout restrictions and other issues.

Template approaches like SSI, ColdFusion and Tiles² make page layouts flexible. They help to reduce the complexity of JSPs by breaking them into smaller, more manageable fragments that can be reused between pages. This decomposition can be taken to the level of individual compartments. However, such a mechanism can only be applied to identical JSP fragments; it can't be reused for similar compartments. Although templates can simplify the maintenance of small JSP applications, advantages disappear as the number of pages and compartments on each page grow and developers have to manage thousands of individual JSPs.

Neither of these approaches addresses the rendering of dynamic content or the implementation of presentation logic inside the compartments, leaving the developer to rely on tag libraries.

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• Table 1