Design Patterns

Design Patterns

Overview

Design patterns are proven solutions to recurring software design problems. They provide a shared vocabulary for discussing design and capture collective wisdom refined through decades of real-world use.

Core Philosophy: Patterns are templates you adapt to your context, not blueprints to copy. Use the right pattern when it genuinely simplifies your design—not to impress or over-engineer.

Foundational Principles

These principles underpin all good design:

Pattern Selection Guide

By Problem Type

CREATING OBJECTS
├── Complex/conditional creation ──────────→ Factory Method
├── Families of related objects ───────────→ Abstract Factory
├── Step-by-step construction ─────────────→ Builder
├── Clone existing objects ────────────────→ Prototype
└── Single instance needed ────────────────→ Singleton (use sparingly!)

STRUCTURING/COMPOSING OBJECTS
├── Incompatible interface ────────────────→ Adapter
├── Simplify complex subsystem ────────────→ Facade
├── Tree/hierarchy structure ──────────────→ Composite
├── Add behavior dynamically ──────────────→ Decorator
└── Control access to object ──────────────→ Proxy

MANAGING COMMUNICATION/BEHAVIOR
├── One-to-many notification ──────────────→ Observer
├── Encapsulate requests as objects ───────→ Command
├── Behavior varies by internal state ─────→ State
├── Swap algorithms at runtime ────────────→ Strategy
├── Algorithm skeleton with hooks ─────────→ Template Method
├── Reduce N-to-N communication ───────────→ Mediator
└── Sequential handlers ───────────────────→ Chain of Responsibility

MANAGING DATA ACCESS
├── Abstract data source ──────────────────→ Repository
├── Track changes for atomic commit ───────→ Unit of Work
├── Ensure object identity ────────────────→ Identity Map
├── Defer expensive loading ───────────────→ Lazy Load
├── Map objects to database ───────────────→ Data Mapper
└── Shape data for transfer ───────────────→ DTO

By Symptom

Domain Logic: Transaction Script vs Domain Model

Rule of thumb: Start with Transaction Script. Refactor to Domain Model when procedural code becomes hard to maintain.

Quick Reference

Tier 1: Essential Patterns (Master First)

Tier 2: Structural Patterns

Tier 3: Enterprise/Architectural Patterns

Additional Important Patterns

Common Mistakes

Anti-Patterns to Recognize

• God Object: One class does everything → Split using SRP
• Anemic Domain Model: Objects are just data bags → Move behavior to objects
• Golden Hammer: Same pattern everywhere → Match pattern to problem
• Lava Flow: Dead code nobody removes → Delete it, VCS has your back

Modern Variations

Implementation Checklist

Before implementing a pattern:

• Pattern solves a real problem in this codebase
• Considered simpler alternatives
• Trade-offs acceptable for this context
• Team understands the pattern
• Won't over-engineer the solution