CHOOSING THE SYSTEM ARHITECTURE AND DESIGN

• Since choosing the architecture is the first design decision, it greatly affects all subsequent design decisions. The architecture should be suitable to the type of problem to be solved.


• Architecture is the essential to building a disciplined structured development, allowing for future extensions - similar products - with reduced effort and risk.


• Architecture is usually the first aspect to be examined by a new programmer (especially a maintenance programmer) when starting to work on the system


• There are several powerful architectures who have been refined and adapted to solve problems of particular types. It is preferable to choose from these one adapted to the customer's specific problem, since its usefulness, reliability and limitations have already been determined by extensive domain use.

The Repository:
In the Repository model: Shared data is stored in centralized location (the repository subsystem), and accessed by various (other) subsystems Shared data is passed from one subsystem to the next (“pipeline” architecture)
Advantages: Efficient way to share large amounts of data Data integrity localized to repository module		Disadvantages: Subsystems must agree on a repository data model. Schema evolution is difficult and expensive Distribution can be a problem
The Client-Server Model:
In the Client-Server model: Subsystems are viewed as independent processes The set of stand-alone servers provide specific services such as printing and data management The set of clients call on these services
Advantages: Makes effective use of networked systems and thus may require cheaper hardware It is easy to add new servers or upgrade existing servers Increased availability of information by redundancy		Disadvantages: Data interchange can be hampered by different data layouts Communication may be expensive Data integrity functionality must be implemented for each server
The Abstract Machine: In the Abstract Machine model, the system is organized into a set of layers (tiers) each of which provide a specific set of services.
The most popular architecture from this category is the Model-View-Controller architecture, which divides a business application into its three main components: the View (or Presentation) layer refers to the user interface and concentrates on making it as easy and intuitive to use as possible the Model layer represents the business logic the Controller handles the interaction between the first two
Advantages: Minimal coupling between components performing totally independent functions Specialized developers work only within their layer Flexibility and portability, layers can be changed completely without affecting other layers if the interface remains the same		Disadvantages: Not suitable for simple problems, for which it introduces additional artificial complexity
After choosing a system architecture, the detailed Software Design Specification document is created:
A good SDS: Contains system architecture description (the high-level design), with an overview of modules or components, structure and relationships and user interface issues Contains detailed descriptions of components (the detailed design), specifying type, purpose, function, dependencies, interfaces, subordinates, resources Analyzes reuse and relationship to other products Explains design decisions and examines trade offs