Design Concepts and Principles

1
Design Concepts and Principles
2
Design Principles

• The design process should not suffer from tunnel
  vision.
• The design should be traceable to the analysis
  model.
• The design should not reinvent the wheel.
• The design should minimize the intellectual
  distance DAV95 between the software and the
  problem as it exists in the real world.
• The design should exhibit uniformity and
  integration.
• The design should be structured to accommodate
  change.
• The design should be structured to degrade
  gently, even when aberrant data, events, or
  operating conditions are encountered.
• Design is not coding, coding is not design.
• The design should be assessed for quality as it
  is being created, not after the fact.
• The design should be reviewed to minimize
  conceptual (semantic) errors.

3
Fundamental Concepts

• abstractiondata, procedure, control
• refinementelaboration of detail for all
  abstractions
• modularitycompartmentalization of data and
  function
• architectureoverall structure of the software
• procedurethe algorithms that achieve function
• hidingcontrolled interfaces

4
Data Abstraction
5
Procedural Abstraction
6
Stepwise Refinement
7
Modular Design
8
Modularity Trade-offs
9
Information Hiding
10
Why Information Hiding?

• reduces the likelihood of side effects
• limits the global impact of local design
  decisions
• emphasizes communication through controlled
  interfaces
• discourages the use of global data
• leads to encapsulationan attribute of high
  quality design results in higher quality software

11
Architecture

• The overall structure of the software and the
  ways in which that structure provides conceptual
  integrity for a system. SHA95a

12
Top-Down Design

• In principle, top-down design involves starting
  at the uppermost components in the hierarchy and
  working down the hierarchy level by level
• In practice, large systems design is never truly
  top-down. Some branches are designed before
  others. Designers reuse experience (and sometimes
  components) during the design process

13
Top-Down Design
14
Design Quality

• Cohesion
• Coupling
• Understandability
• Adaptability
• Design traceability
• Adaptability and inheritance

15
Understandability

• Related to several component characteristics
• Cohesion. Can the component be understood on its
  own?
• Naming. Are meaningful names used?
• Documentation. Is the design well-documented?
• Complexity. Are complex algorithms used?
• Informally, high complexity means many
  relationships between different parts of the
  design. Hence it is hard to understand.
• Most design quality metrics are oriented towards
  complexity measurement. They are of limited use.

16
Adaptability

• A design is adaptable if
• Its components are loosely coupled
• It is well-documented and the documentation is up
  to date
• There is an obvious correspondence between design
  levels (design visibility)
• Each component is a self-contained entity
  (tightly cohesive)
• To adapt a design, it must be possible to trace
  the links between design components so that
  change consequences can be analyzed

17
Design Traceability
18
Adaptability and Inheritance

• Inheritance dramatically improves adaptability.
• Components may be adapted without change by
  deriving a sub-class and modifying that derived
  class
• However, as the depth of the inheritance
  hierarchy increases, it becomes increasingly
  complex. It must be periodically reviewed and
  restructured