• Server-centric (thin client);
• Open (nonproprietary), standards based;
• Platform independent;
• Externally secured;
• Scalable;
• Manageable;
• Constructed of reusable components;
• Delivered via a Web browser.

Certain developers might argue one or two specific points on the above list or cite unique circumstances where one or more items could be considered less than optimal, but generally speaking, the above list represents the best approach in today's environment. Additionally, quality systems can be easily decomposed into their logical components, or layers, which should be loosely coupled to promote reuse and interoperability. These layers are:

• Presentation Layer
• Business Logic
• Data Access Layer

The layers are illustrated in Figure A.

FIGURE A


These are the basic information system application elements. Roll over picture for a larger image.

Another highly acclaimed best practice is the Model-View-Controller architecture, which couples the presentation layer to the business logic with a control layer, providing the fundamental framework for all applications. This framework is depicted in Figure B.

FIGURE B


The Model-View-Controller Architecture is depicted here. Roll over picture for a larger image.

Start small, deliver quickly
Armed with your technical vision, a few simple tools, and a well-prepared and trained staff of developers, you can replace an outdated and insufficient host-based system piece by piece, adding value and functionality every step of the way. The trick is to visualize the entire elephant, but only attack one bite-sized portion at a time. Your vision, which you must constantly keep in the minds of all stakeholders, should include a general strategy (such as one bite at a time) and a specific strategy, such as which bites to go after first.

One highly effective strategy is to employ a tool such as IBM's WebSphere Host Publisher to replace the presentation layer while keeping the business logic and data access layer intact (see Figure C).

FIGURE C


Get a brand new face with WebSphere host publisher. Roll over picture for a larger image.

This can be done in as small a unit as one screen at a time. Unlike the big bang theory in which the production user never sees any evidence of the developers' labors until all the work is completed in total, using this method you can put single screens or logical sets of screens in the users' hands almost immediately. This provides incredibly useful feedback while both engaging the system users and generating enthusiasm for the project.

Altering the presentation layer puts a new face on the old system, but it doesn't really add functionality beyond new navigation and a new "look and feel." However, once that has been accomplished, either for the entire legacy system or just for selected business functions, developers can begin to slide new features and functions underneath the new face. This can be in the form of new business logic that invokes existing legacy business functions, new business logic that manipulates the old legacy data directly (bypassing the old code entirely), or new business logic that is supported by an entirely new supplemental data source (see Figure D).