Design Concepts

Summary

• Definitions
• Detail Design
• Design Method
• Issues with Validation
• Other Design Issues
• Design Documentation
• Design Rationale
• Conceptual Integrity
  • Abstraction Mechanisms
• Design Philosophy

Definitions

Design is deliberative, purposeful planning

Engineering is skillful or artful contrivance applying scientific and mathematical principles

Craft is a skilled occupation

Art is the use of skill, taste, and imagination in the production of aesthetic objects

Differences between Software Design and Programming include:

• Scale
• Emphasis on Non-Functional Requirements

Software Design := The process of building a program while satisfying a problem’s functional requirements and not violating its non-functional constraints

Architectural Design is the process of identifying and assigning the responsibility for aspects of behavior to various modules or components of a software

Detail Design is the process of specifying the behavior of each of the system components that you’ve identified during architectural design

Detail Design

The primary purpose of detail design is to determine which data structures and algorithms to use

Can be represented using design notation:

• Psuedocode
• Structured programming
• Flow charts/Call graphs/DAGs
• Decision Tables

All approaches to design include 3 attributes:

1. Design Method
2. Design Representation
3. Design Validation

Design Method

A method is a systematic sequence of steps that a design team uses to solve a problem

Always a trade off between long-term maintainability and short-term schedule

Issues with Validation

• Independence of validators
  • If design team does their own validation, they could miss some issues
• Dependence of design method
• On-going vs after-the-fact

Other Design Issues

• Architectural vs Detail design
• Function behavior versus non-functional constraints
• Specification (what) versus design (how)
• Application specificity

Design Documentation

Some things to document:

• Subcomponents
  • Processes / activities
  • data
• Control flow
• Performance
• Resources consumption
• Stakeholders
• Issue bases
  • What issues were raised
  • What solutions were offered
  • Why were there solutions offered
• Temporal relations
  • Versions
• Constraints
• Aggregates

IEEE 1016

Design Rationale

Design decisions are explicit choices of how to trade off two non-functional aspects of a design, such as speed versus size

Conceptual Integrity

Conceptual integrity is the most important part of design

Related:

• Coupling
• Cohesion

Coupling is the extent to which two components depend on each other for successful execution

• Low coupling is good

Cohesion is the extent to which one component has a single purpose or function

• High cohesion is good

Abstract Interfaces hide information and reduce coupling

Abstraction Mechanisms

• Declarative
  • what, not how
• Aggregation
  • container, not contents
• Generalization
  • class, not individuals
• Parameterization
  • binding details later
• Non-deterministic specification
  • leaving choices unspecified

Design Philosophy

• Descartes
  • analysis
  • use of models
• Marx
  • understanding the social context
  • user-centered design
• Heidegger
  • Tools for accomplishing goals
• Wittgenstein
  • language games desktop metaphor
    • inventing a vocabulary to help you think about a problem