Design

You design to mitigate risks. Design is the creative process of transforming a problem into a solution. It's the process of defining the architecture, components, interfaces and other characteristics of a system or component and the result of that process.

Objectives of Design

• Create design representations
• Evaluate construction alternatives
• Assess requirements satisfaction
• Assess feasibility

Key risks

• understanding problems
• large systems
• team work
• ensuring qualities
• evolution of systems

Evolution

• cost per line of code increases over time
• productivity per release goes down over time

Software Design Process

• Solution Postulation
  • problem (requirements, constraints)
    • diversification (enumeration of alternatives)
    • convergence (choice of the alternative that better satisfies requirements)
• Solution representation
• Solution Elaboration

Design Strategies

• Decompositional
  • break the problem down
• Compositional
  • look at the problem as a whole
• Template-based
  • look for a common pattern

Representation of Design Decisions

• code
• text
• modelling language

Why Model?

• Capture Ideas
• Share
• Validate
• Track

Architectural Design

• addresses architectural significant requirements

How much design?

Big Design Up Front (BDUF)

• issues found before implementation
• commits too early
• produces heavy documentation

Emergent Design

• simpler, just enough design
• adapts to changing requirements
• technical debt (cost)

Rough Up-Front Design (RUFD)

• avoids issues with BDUF and Emergent Design
• based on good prioritization

Principles

• Frameworks
• Patterns
• Principles

Abstraction Principle

• Create abstractions for repeated things
  • Factor in commonalities
  • Abstract out varying parts
• Don't Repeat Yourself (DRY) Principle

Modularity

• Division in components
  • Cohesion
  • Coupling

Separation of Concerns

• Allocate different concerns to different modules
  • separation of policy and mechanism
  • separation of interface and implementation