Essay 7 - Application Design

date:2005-09-09 15:27:13 category:War Stories and Advice

Architecture is the first step in creating software to meet the requirements. This is followed by the design effort to transform the high-level picture into components that can be realized in software. This line between assemblies of components, and the internal construction of components is a good dividing line between architecture and design.

It is generally best to use well-known and widely used design patterns when developing a design. This creates a design with elements that are reasonably well understood, and lead to early confidence that the design will meet the requirements.

A common component is a stand-alone application. This may be a fat-client application that runs on a desktop, or a web application accessed through the browser. An application typicall has five layers or tiers:

• View . The presentation of data to the users, or other interfaces.
• Control . The sequence of activities or state changes to manage interaction.
• Model . The underlying data model that the application manipulates. This is optimized for interaction, and may differ from the persistent data structure.
• Access . The mechanism for acquiring persistent data. This may be CORBA, JDBC or other access mechanism.
• Persistence . The mechanism of persistence. This may be the file system or the RDBMS.

Each of these has more narrowly defined responsibilities. The overall functionality of the application can then be partitioned according to these layers. Once the larger problem is decomposed, each can be tackled and built separately.

Generally, it is best to proceed up the hierarchy. The first phase of effort moves from persistent data, through access, to the model. This creates a simple framework for the following:

• Validating the persistent data model. The initial design may have ommitted classes or attributes, or mis-communicated those.
• Assuring that the access methods meet requirements. The initial design may have omitted useful navigation paths or other physical access mechanisms.
• Developing a usable application data model. The matching between the working model and the persistent model must be resolved and tested.

Once the Model-Access-Persistence is in place, then the View and Control can be added. It is critical that the model’s methods be completely tested before attempting to add view and control features. If the model is incomplete, inconsistent or incorrect, and these problems are fixed in View and Control, then reuse is compromised, and the essential functionality is spread across the application.

Since the essential feature of object-oriented design is correct assignment of responsibility, spreading application features through multiple layers defeats basic good design. For this reason, the essential functions must be completely developed and tested in the lower layer of the application design. Then the GUI can be fastened onto a working structure.