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name: design-patterns description: Use when designing software architecture, refactoring code structure, solving recurring design problems, or when code exhibits symptoms like tight coupling, rigid hierarchies, scattered responsibilities, or difficult-to-test components. Also use when choosing between architectural approaches or reviewing code for structural improvements.

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:

Principle	Meaning	Violation Symptom
Encapsulate What Varies	Isolate changing parts from stable parts	Changes ripple through codebase
Program to Interfaces	Depend on abstractions, not concretions	Can't swap implementations
Composition Over Inheritance	Build behavior by composing objects	Deep rigid class hierarchies
Loose Coupling	Minimize interdependency between objects	Can't change one thing without breaking another
Open-Closed	Open for extension, closed for modification	Must edit existing code for new features
Single Responsibility	One reason to change per class	Classes doing too many things
Dependency Inversion	High-level modules don't depend on low-level	Business logic coupled to infrastructure

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

Symptom	Consider
Giant switch/if-else on type	Strategy, State, or polymorphism
Duplicate code across classes	Template Method, Strategy
Need to notify many objects of changes	Observer
Complex object creation logic	Factory, Builder
Adding features bloats class	Decorator
Third-party API doesn't fit your code	Adapter
Too many dependencies between components	Mediator, Facade
Can't test without database/network	Repository, Dependency Injection
Need undo/redo	Command
Object behavior depends on state	State
Request needs processing by multiple handlers	Chain of Responsibility

Domain Logic: Transaction Script vs Domain Model

Factor	Transaction Script	Domain Model
Logic complexity	Simple (< 500 lines)	Complex, many rules
Business rules	Few, straightforward	Many, interacting
Operations	CRUD-heavy	Rich behavior
Team/timeline	Small team, quick delivery	Long-term maintenance
Testing	Integration tests	Unit tests on domain

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)

Pattern	One-Line	When to Use	Reference
Strategy	Encapsulate interchangeable algorithms	Multiple ways to do something, swap at runtime	strategy.md
Observer	Notify dependents of state changes	Event systems, reactive updates	observer.md
Factory	Encapsulate object creation	Complex/conditional instantiation	factory.md
Decorator	Add behavior dynamically	Extend without inheritance	decorator.md
Command	Encapsulate requests as objects	Undo/redo, queuing, logging	command.md

Tier 2: Structural Patterns

Pattern	One-Line	When to Use	Reference
Adapter	Convert interfaces	Integrate incompatible code	adapter.md
Facade	Simplify complex subsystems	Hide complexity behind simple API	facade.md
Composite	Uniform tree structures	Part-whole hierarchies	composite.md
Proxy	Control access to objects	Lazy load, access control, caching	proxy.md

Tier 3: Enterprise/Architectural Patterns

Pattern	One-Line	When to Use	Reference
Repository	Collection-like data access	Decouple domain from data layer	repository.md
Unit of Work	Coordinate atomic changes	Transaction management	unit-of-work.md
Service Layer	Orchestrate business operations	Define application boundary	service-layer.md
DTO	Shape data for transfer	API contracts, prevent over-exposure	dto.md

Additional Important Patterns

Pattern	One-Line	When to Use	Reference
Builder	Step-by-step object construction	Complex objects, fluent APIs	builder.md
State	Behavior changes with state	State machines, workflow	state.md
Template Method	Algorithm skeleton with hooks	Framework extension points	template-method.md
Chain of Responsibility	Pass request along handlers	Middleware, pipelines	chain-of-responsibility.md
Mediator	Centralize complex communication	Reduce component coupling	mediator.md
Lazy Load	Defer expensive loading	Performance, large object graphs	lazy-load.md
Identity Map	Ensure object identity	ORM, prevent duplicates	identity-map.md

Common Mistakes

Mistake	Symptom	Fix
Pattern Overuse	Simple operations require navigating many classes	Only use when solving real problem
Wrong Pattern	Code feels forced, awkward	Re-examine actual problem
Inheritance Abuse	Deep hierarchies, fragile base class	Favor composition (Strategy, Decorator)
Singleton Abuse	Global state, hidden dependencies, hard to test	Use dependency injection instead
Premature Abstraction	Interfaces with single implementation	Wait for real need to vary

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

Modern Pattern	Based On	Description
Dependency Injection	Strategy + Factory	Container creates and injects dependencies
Middleware	Decorator + Chain of Responsibility	Request/response pipeline
Event Sourcing	Command	Store state changes as events
CQRS	Command/Query separation	Separate read/write models
Hooks (React/Vue)	Observer + Strategy	Functional lifecycle subscriptions

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