Example: A blueprint for a house is the classic example of an architectural design.

Why do we need design? Imagine trying to build a house without a blueprint.

• Create a quality product.
• Reduce confusion and errors. 
• Manage complexity
• Improve estimation
• Provide a robust framework
• Facilitate maintenance / enhancement
• Organize the construction effort

Unfortunately, often software development proceeds without adequate design, using a "house of cards" approach. The result is fragile, unstable, inflexible, error prone software.

What you need to know about design:

• General design approaches and history
• The four components of a design
• Three design principles: Decomposition, Abstraction, Information Hiding
  
• Design Methods (in a nutshell): Functional and Object Oriented
• Know how to choose between OO approach and Functional approach
• Design Quality: How to know if you have a good design
Quality Factors and Tradeoffs
Assessing Decomposition
Quality Metrics and an OO Quality Metrics Tool
Example QA Checklists: High Level Design and Detailed Design
• Examples and Case Studies: Traffic Jam and ATM Simulator
• Practice Problems

• How to apply a specific design method: Functional Design
• How to apply a specific design method: Object Oriented Design
• How to express your Functional design: Structure Charts
• How to express your OO design: UML notation for class diagrams
• Hybrid design method

Design Rationale Documentation

• Template for writing Design Issues

General design approaches and history

Each has many specific methodologies, E.g.

• Structured Design (functional)
• UML (object oriented)

• Algorithms (functional)
• Data (object oriented)

Historically, functional approaches came first.

• Requirements are stated as functions, so functional design is an obvious transition from requirements.
• Computer Architectures are functional, so early languages were functional, and lend themselves to functional designs.

Four components of a design

Architectural design defines the structure and relationships among major elements of the software. The architecture is the framework of the solution, often comprised of "design patterns." Examples: Class Diagram, Module hierarchy chart (structure chart).

Interface design describes how the elements of the design will communicate with each other. It describes the "protocols" or "contracts" that allow elements of the design to request services of each other. Examples: C++ header file, Ada spec file, Java interface.

Procedural design describes how the functional aspects of the software will work. It describes the logic and control structures for the operations in the design. It's the algorithms for each element in the solution. Examples: pseudocode, flowcharts,  interaction diagrams.

Data design describes the data structures which will store the information used in the solution.

Usually each component has its own representation or notation, but sometimes they are combined.
 
 

Design Principles

Decomposition

Decomposition is the primary tool for attacking large problems, by using a "Divide and conquer" strategy.

It helps us several ways:

• Allows for distribution of effort via parallel development.
• Makes testing easier by isolating errors
• Promotes reuse.
• Localizes changes for easier maintenance.
• Makes the solution simpler to comprehend

Abstraction

Abstraction is a tool for coping with complexity. If you find yourself feeling like there's too many things to keep in your head at once, then invent an abstraction.

It helps us several ways:

• It simplifies a design by hiding low level details and keeping the emphasis on the main features of the solution.
• It promotes implementation independence, meaning we can change the implementation (low level) without affecting the entire system (at a high level).
• It simplifies our thinking by allowing us to focus on the problem domain and defer the details of the implementation until later.

Information Hiding