Skip to main content

Design Principles Overview

Design principles are fundamental guidelines that help developers create maintainable, scalable, and robust software systems. These principles form the foundation of software architecture and guide day-to-day coding decisions. Mastering these principles is essential for any software engineer aiming to write high-quality code.

Why Design Principles Matter​

  • Reduce complexity: Well-designed code is easier to understand and reason about
  • Improve maintainability: Changes and bug fixes become faster and safer
  • Enable scalability: Systems built on solid principles adapt better to growth
  • Facilitate collaboration: Teams work more effectively with shared design language
  • Prevent technical debt: Following principles early prevents costly rewrites later

Principles Covered in This Module​

1. SOLID Principles​

The five core object-oriented design principles that ensure code is flexible, maintainable, and testable:

  • Single Responsibility Principle
  • Open/Closed Principle
  • Liskov Substitution Principle
  • Interface Segregation Principle
  • Dependency Inversion Principle

When to use: Apply SOLID principles in all object-oriented codebases from day one.

2. DRY, KISS, YAGNI​

Three pragmatic principles that combat over-engineering:

  • DRY (Don't Repeat Yourself): Eliminate duplication
  • KISS (Keep It Simple, Stupid): Favor simplicity over complexity
  • YAGNI (You Aren't Gonna Need It): Don't build what you don't need now

When to use: Apply these principles continuously during development to prevent unnecessary complexity.

3. Conway's Law​

"Organizations design systems that mirror their communication structures." Understanding this principle helps align team structure with desired architecture.

When to use: Consider when designing architecture or restructuring teams.

4. Separation of Concerns​

Divide programs into distinct sections, each addressing a separate concern (UI, business logic, data access). This principle reduces coupling and increases cohesion.

When to use: Apply from the start of any project; it's the foundation of layered and clean architectures.

5. Principle of Least Astonishment (POLA)​

Code should behave exactly as users expect—no surprises. Functions, classes, and APIs should do what their names suggest.

When to use: Apply in every naming and design decision; POLA guides how code communicates intent.

How These Principles Work Together​

                    ┌─────────────────────────┐
                    │   Design Principles     │
                    └───────────┬─────────────┘
                                │
                ┌───────────────┼───────────────┐
                │               │               │
                ▼               ▼               ▼
        ┌───────────────┐ ┌──────────┐ ┌──────────────┐
        │ Structure     │ │ Behavior │ │ Organization │
        │ (How to build)│ │(What to  │ │(How to align)│
        │               │ │  avoid)  │ │              │
        │ • SOLID       │ │ • KISS   │ │ • Conway's   │
        │ • SoC         │ │ • YAGNI  │ │   Law        │
        │ • POLA        │ │ • DRY    │ │              │
        └───────────────┘ └──────────┘ └──────────────┘

Principle Relationships​

  • SOLID + SoC: SOLID principles (especially SRP) are concrete implementations of Separation of Concerns
  • DRY + SOLID: DRY aligns with Single Responsibility Principle—avoid duplicating logic across responsibilities
  • KISS + YAGNI: Work together to prevent over-engineering; build simple solutions for current needs
  • POLA + All Others: Good naming and predictable behavior make all other principles easier to apply
  • Conway's Law: Influences how you organize teams to successfully apply technical principles

Practical Application Guide​

Starting a New Project​

  1. Apply SoC(Separation of concerns) first: Structure your project into clear layers (presentation, business, data)
  2. Follow SOLID: Design classes and interfaces following SOLID principles
  3. Keep it simple: Use KISS and YAGNI to avoid unnecessary abstractions
  4. Name clearly: Apply POLA to all naming decisions

Reviewing Existing Code​

  1. Check for surprises: Does code behave as its name suggests? (POLA)
  2. Identify duplication: Look for DRY violations and refactor when appropriate
  3. Test SOLID compliance: Can you change one thing without breaking others?
  4. Assess complexity: Is the code simpler than it could be? (KISS)

Refactoring​

  1. Don't over-DRY: Sometimes duplication is better than wrong abstraction
  2. Refactor toward SOLID: Each refactoring should improve principle adherence
  3. Maintain simplicity: Don't add complexity in the name of following principles
  4. Preserve expectations: Refactoring shouldn't change external behavior (POLA)

Common Mistakes​

  • Dogmatic application: Principles are guidelines, not laws; apply pragmatically based on context
  • Premature abstraction: Following DRY too early can create wrong abstractions
  • Over-engineering: Applying SOLID everywhere can lead to unnecessary complexity; balance with KISS
  • Ignoring Conway's Law: Technical decisions that fight organizational structure will fail
  • Misleading names: Violating POLA creates more problems than most other principle violations

Key Takeaways​

  • Principles work together: They're not independent rules but interconnected guidelines
  • Context matters: Apply principles based on project size, team experience, and business needs
  • Start simple: Begin with KISS and YAGNI, then refactor toward SOLID as patterns emerge
  • Name things well: POLA is perhaps the most important daily practice
  • Align organization: Use Conway's Law proactively to structure teams for architectural success
  • Balance is key: Extreme adherence to any principle can be harmful; seek pragmatic balance