LEARN DESIGN PATTERNS DEEP DIVE

Learn Design Patterns: From Zero to Software Architecture Master

Goal: Deeply understand design patterns—from fundamental object-oriented principles to implementing all 23 Gang of Four patterns, architectural patterns, and building real-world applications that showcase elegant, maintainable software design.

Why Learn Design Patterns?

Design patterns are reusable solutions to common problems in software design. They represent decades of collective wisdom from experienced developers who faced the same challenges you will. Understanding design patterns is essential for:

Code Quality: Writing maintainable, flexible, and extensible code
Communication: Speaking a common language with other developers
Interview Success: Design patterns are a staple of technical interviews
Architecture Skills: Understanding how large systems are structured
Problem Solving: Recognizing when and how to apply proven solutions
Framework Understanding: Most frameworks (Spring, React, Django) use patterns extensively

After completing these projects, you will:

Understand the philosophy behind object-oriented design
Implement all 23 Gang of Four patterns from scratch
Know when to apply each pattern (and when NOT to)
Recognize patterns in existing codebases and frameworks
Design systems using SOLID principles
Build real-world applications demonstrating pattern mastery

Core Concept Analysis

The Pattern Categories

┌─────────────────────────────────────────────────────────────────────────────┐
│                          DESIGN PATTERNS                                     │
│                                                                              │
│   ┌──────────────────┐  ┌──────────────────┐  ┌──────────────────┐         │
│   │   CREATIONAL     │  │   STRUCTURAL     │  │   BEHAVIORAL     │         │
│   │                  │  │                  │  │                  │         │
│   │ • Singleton      │  │ • Adapter        │  │ • Chain of Resp. │         │
│   │ • Factory Method │  │ • Bridge         │  │ • Command        │         │
│   │ • Abstract Fact. │  │ • Composite      │  │ • Iterator       │         │
│   │ • Builder        │  │ • Decorator      │  │ • Mediator       │         │
│   │ • Prototype      │  │ • Facade         │  │ • Memento        │         │
│   │                  │  │ • Flyweight      │  │ • Observer       │         │
│   │                  │  │ • Proxy          │  │ • State          │         │
│   │                  │  │                  │  │ • Strategy       │         │
│   │                  │  │                  │  │ • Template Method│         │
│   │                  │  │                  │  │ • Visitor        │         │
│   └──────────────────┘  └──────────────────┘  └──────────────────┘         │
│                                                                              │
│   HOW objects are     HOW objects are        HOW objects                    │
│   created             composed               communicate                    │
└─────────────────────────────────────────────────────────────────────────────┘

Key Concepts Explained

1. SOLID Principles (Foundation)

Before patterns, you must understand SOLID:

┌─────────────────────────────────────────────────────────────────────────────┐
│  S - Single Responsibility Principle                                        │
│      A class should have only one reason to change                          │
│                                                                              │
│  O - Open/Closed Principle                                                  │
│      Open for extension, closed for modification                            │
│                                                                              │
│  L - Liskov Substitution Principle                                          │
│      Subtypes must be substitutable for their base types                    │
│                                                                              │
│  I - Interface Segregation Principle                                        │
│      Many specific interfaces are better than one general interface         │
│                                                                              │
│  D - Dependency Inversion Principle                                         │
│      Depend on abstractions, not concretions                                │
└─────────────────────────────────────────────────────────────────────────────┘

2. Creational Patterns

Purpose: Abstract the instantiation process, making a system independent of how objects are created.

Pattern	Intent	When to Use
Singleton	Ensure one instance, global access	Configuration, logging, connection pools
Factory Method	Defer instantiation to subclasses	Unknown types at compile time
Abstract Factory	Create families of related objects	Cross-platform UI, themed components
Builder	Separate construction from representation	Complex objects with many options
Prototype	Clone existing objects	Expensive object creation, configuration

3. Structural Patterns

Purpose: Compose objects into larger structures while keeping structures flexible and efficient.

Pattern	Intent	When to Use
Adapter	Convert interface to expected interface	Legacy integration, third-party libs
Bridge	Separate abstraction from implementation	Multiple dimensions of variation
Composite	Treat groups and individuals uniformly	Tree structures, hierarchies
Decorator	Add responsibilities dynamically	Streams, UI components
Facade	Simplified interface to subsystem	Complex libraries, APIs
Flyweight	Share state to support many objects	Text editors, game objects
Proxy	Control access to an object	Lazy loading, access control, caching

4. Behavioral Patterns

Purpose: Define how objects communicate and distribute responsibilities.

Pattern	Intent	When to Use
Chain of Responsibility	Pass request along chain	Handlers, middleware
Command	Encapsulate request as object	Undo/redo, queuing, logging
Iterator	Sequential access without exposing structure	Collections
Mediator	Centralize complex communications	Chat rooms, air traffic control
Memento	Capture and restore state	Undo, snapshots
Observer	Notify dependents of state changes	Events, MVC, reactive
State	Alter behavior when state changes	State machines, workflows
Strategy	Interchangeable algorithms	Sorting, validation, pricing
Template Method	Define skeleton, defer steps	Frameworks, hooks
Visitor	Add operations without modifying classes	Compilers, document processing

5. When NOT to Use Patterns

┌─────────────────────────────────────────────────────────────────────────────┐
│                    PATTERN ANTI-PATTERNS                                     │
├─────────────────────────────────────────────────────────────────────────────┤
│ ❌ Pattern Fever: Using patterns everywhere "just because"                   │
│ ❌ Wrong Pattern: Forcing a pattern that doesn't fit                         │
│ ❌ Over-Engineering: Simple problems don't need complex patterns             │
│ ❌ Premature Abstraction: Adding flexibility you don't need yet              │
│ ❌ Pattern Name Dropping: Using patterns to sound smart                      │
├─────────────────────────────────────────────────────────────────────────────┤
│ ✓ Use patterns when they solve real problems                                │
│ ✓ Start simple, refactor to patterns when needed                            │
│ ✓ Understand the problem before applying a solution                         │
└─────────────────────────────────────────────────────────────────────────────┘

Project List

The following 15 projects will teach you design patterns from fundamentals to mastery.

Project 1: SOLID Principles Violation Detector

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Python
Alternative Programming Languages: Java, TypeScript, C#
Coolness Level: Level 3: Genuinely Clever
Business Potential: 2. The “Micro-SaaS / Pro Tool”
Difficulty: Level 2: Intermediate
Knowledge Area: Code Analysis / OOP Principles
Software or Tool: AST parsers, linters
Main Book: “Clean Code: A Handbook of Agile Software Craftsmanship” by Robert C. Martin

What you’ll build: A static analysis tool that scans code and detects violations of each SOLID principle, providing specific feedback about what’s wrong and how to fix it.

Why it teaches design patterns: SOLID principles are the foundation of good object-oriented design. By building a detector, you must deeply understand what constitutes a violation—class responsibilities, interface design, inheritance hierarchies, and dependency management.

Core challenges you’ll face:

Detecting Single Responsibility violations → maps to counting responsibilities, analyzing method cohesion
Finding Open/Closed violations → maps to identifying modification points vs extension points
Spotting Liskov Substitution violations → maps to analyzing inheritance hierarchies
Identifying bloated interfaces → maps to Interface Segregation analysis
Finding concrete dependencies → maps to Dependency Inversion detection

Resources for key challenges:

“Clean Code” Chapters 10-11 - Robert C. Martin
“Agile Software Development: Principles, Patterns, and Practices” - Robert C. Martin

Key Concepts:

Single Responsibility: “Clean Code” Ch. 10 - Classes should have one reason to change
Open/Closed Principle: “Head First Design Patterns” Ch. 1 - Extension without modification
Dependency Inversion: “Clean Architecture” Ch. 11 - Robert C. Martin

Difficulty: Intermediate Time estimate: 2-3 weeks Prerequisites: Object-oriented programming basics, Python AST or similar

Real world outcome:

$ solid-detector analyze ./src

SOLID Principle Analysis Report
================================

src/user_service.py
-------------------
❌ SRP Violation (line 15-45):
   UserService class has 3 responsibilities:
   - User CRUD operations
   - Email sending
   - Password hashing
   
   Suggestion: Extract EmailService and PasswordService classes

❌ DIP Violation (line 23):
   UserService directly instantiates MySQLDatabase
   
   Suggestion: Inject database dependency via constructor

src/payment_processor.py
------------------------
❌ OCP Violation (line 34-67):
   PaymentProcessor uses switch/case for payment types
   
   Suggestion: Use Strategy pattern with PaymentMethod interface

❌ LSP Violation (line 89):
   SquareRectangle extends Rectangle but overrides setWidth/setHeight
   breaking Rectangle's behavior contract
   
   Suggestion: Use composition instead of inheritance

Summary: 4 violations found across 2 files

Implementation Hints:

Start by parsing code into an Abstract Syntax Tree (AST). For Python, use the ast module. For Java, consider JavaParser.

Questions to guide your implementation:

How do you measure “responsibility”? (Hint: look at method clusters, data access patterns)
How do you detect if a class is modified vs extended?
What makes an interface “too fat”?
How do you trace dependencies through constructor, method parameters, and field access?

For each principle, define heuristics:

SRP: Classes with methods that don’t share instance variables, high method count
OCP: Switch statements on type, frequent modifications to add new cases
LSP: Overridden methods that throw exceptions or change contracts
ISP: Classes implementing interfaces but leaving methods empty
DIP: Direct instantiation of concrete classes in constructors

Learning milestones:

Parse code to AST → Understand code structure programmatically
Detect SRP violations → Count and categorize responsibilities
Detect DIP violations → Trace dependency graphs
Generate actionable reports → Suggest specific refactorings

Project 2: Plugin Architecture Framework

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Python
Alternative Programming Languages: Java, C#, TypeScript
Coolness Level: Level 4: Hardcore Tech Flex
Business Potential: 4. The “Open Core” Infrastructure
Difficulty: Level 3: Advanced
Knowledge Area: Software Architecture / Extensibility
Software or Tool: Dynamic loading, reflection
Main Book: “Design Patterns: Elements of Reusable Object-Oriented Software” by Gang of Four

What you’ll build: A complete plugin framework that allows third-party extensions to be loaded at runtime, complete with dependency injection, lifecycle management, and a plugin marketplace concept.

Why it teaches design patterns: Plugin architectures are the ultimate test of SOLID and multiple patterns. You’ll implement Factory, Strategy, Observer, Facade, and more—all working together.

Core challenges you’ll face:

Dynamic class loading → maps to Factory pattern, reflection
Plugin lifecycle → maps to Template Method, State patterns
Plugin dependencies → maps to Dependency Injection, Composite
Event system → maps to Observer pattern
Extension points → maps to Strategy, Decorator patterns

Resources for key challenges:

“Plugin Architectures” - Martin Fowler’s blog
Eclipse Plugin Development Guide

Key Concepts:

Factory Pattern: “Design Patterns” Ch. 3 - Creating plugin instances
Observer Pattern: “Design Patterns” Ch. 5 - Plugin event communication
Dependency Injection: “Dependency Injection Principles” - Mark Seemann

Difficulty: Advanced Time estimate: 3-4 weeks Prerequisites: Project 1, reflection/metaprogramming knowledge

Real world outcome:

# Main application
from pluginframework import PluginManager, Extension

class Application:
    def __init__(self):
        self.plugin_manager = PluginManager()
        self.plugin_manager.load_plugins('./plugins')
        
    def run(self):
        # Plugins automatically registered
        for processor in self.plugin_manager.get_extensions('file_processor'):
            processor.process(self.current_file)

# Plugin (in separate file, loaded at runtime)
from pluginframework import Plugin, extension

@Plugin(name="ImageOptimizer", version="1.0")
class ImageOptimizerPlugin:
    
    @extension('file_processor')
    def process_image(self, file):
        if file.endswith(('.png', '.jpg')):
            return self.optimize(file)
    
    def on_load(self):
        print("Image optimizer loaded!")
        
    def on_unload(self):
        self.cleanup()

# Runtime output
$ python app.py
[PluginManager] Scanning ./plugins...
[PluginManager] Found: ImageOptimizer v1.0
[PluginManager] Found: VideoConverter v2.1
[PluginManager] Found: PDFProcessor v1.3
[PluginManager] 3 plugins loaded

Image optimizer loaded!
Processing image.png with ImageOptimizer...

Implementation Hints:

Core components to build:

Plugin Interface: Abstract base class all plugins implement
Plugin Manager: Discovers, loads, manages plugins
Extension Points: Named hooks where plugins can contribute
Event Bus: Plugins communicate without coupling

Dynamic loading in Python:

Use importlib to load modules at runtime
Use inspect to find plugin classes
Store metadata in module-level variables or decorators

Questions to consider:

How do you handle plugin dependencies on each other?
How do you version plugins and check compatibility?
How do you isolate faulty plugins from crashing the host?
How do you order plugin initialization?

Learning milestones:

Load plugins dynamically → Master runtime class loading
Implement extension points → Strategy pattern in action
Build event system → Observer pattern
Handle lifecycle → Template Method pattern

Project 3: Game Entity Component System

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: C++
Alternative Programming Languages: Rust, C#, Java
Coolness Level: Level 5: Pure Magic (Super Cool)
Business Potential: 1. The “Resume Gold”
Difficulty: Level 3: Advanced
Knowledge Area: Game Development / Composition
Software or Tool: Game engine concepts
Main Book: “Game Programming Patterns” by Robert Nystrom

What you’ll build: An Entity Component System (ECS) where game objects are composed of reusable components instead of using deep inheritance hierarchies—demonstrating composition over inheritance.

Why it teaches design patterns: ECS is the ultimate demonstration of Composite, Strategy, and Component patterns. You’ll see why “favor composition over inheritance” is so powerful.

Core challenges you’ll face:

Entity management → maps to Flyweight, Object Pool patterns
Component composition → maps to Composite, Strategy patterns
System iteration → maps to Iterator, Visitor patterns
Message passing → maps to Observer, Mediator patterns

Resources for key challenges:

“Game Programming Patterns” Ch. 14-15 - Robert Nystrom
Unity ECS documentation

Key Concepts:

Composition over Inheritance: “Effective Java” Item 18 - Bloch
Component Pattern: “Game Programming Patterns” Ch. 14
Data-Oriented Design: “Data-Oriented Design” - Richard Fabian

Difficulty: Advanced Time estimate: 3-4 weeks Prerequisites: C++ or systems language, basic game loop understanding

Real world outcome:

// Creating a player entity from components
Entity player = world.createEntity();
player.addComponent<Position>(100.0f, 200.0f);
player.addComponent<Velocity>(0.0f, 0.0f);
player.addComponent<Sprite>("player.png");
player.addComponent<Health>(100);
player.addComponent<PlayerInput>();
player.addComponent<Collider>(32, 32);

// Creating an enemy - reuses same components differently
Entity enemy = world.createEntity();
enemy.addComponent<Position>(500.0f, 200.0f);
enemy.addComponent<Velocity>(-2.0f, 0.0f);
enemy.addComponent<Sprite>("enemy.png");
enemy.addComponent<Health>(50);
enemy.addComponent<AIController>();
enemy.addComponent<Collider>(32, 32);

// Systems process entities with matching components
class MovementSystem : public System {
    void update(float dt) {
        for (auto entity : getEntitiesWithComponents<Position, Velocity>()) {
            auto& pos = entity.get<Position>();
            auto& vel = entity.get<Velocity>();
            pos.x += vel.x * dt;
            pos.y += vel.y * dt;
        }
    }
};

// Game output:
// [Frame 1] 150 entities processed
//   - MovementSystem: 120 entities (0.2ms)
//   - RenderSystem: 150 entities (1.2ms)  
//   - CollisionSystem: 45 pairs checked (0.5ms)
//   - AISystem: 30 entities (0.3ms)

Implementation Hints:

Core ECS architecture:

Entity: Just an ID (integer)
Component: Pure data (Position, Velocity, Health)
System: Logic that processes entities with specific components
World: Manages entities, components, and systems

Key data structures:

Component Storage: Use contiguous arrays per component type
Entity-Component Mapping: Bitmask per entity showing which components it has
System Filtering: Query entities by component requirements

Questions to consider:

How do you efficiently iterate entities with specific component combinations?
How do you handle component dependencies (Health needs Position for damage numbers)?
How do you add/remove components at runtime?
How do you handle inter-system communication?

Compare to inheritance:

BAD: Player -> MovableEntity -> Entity (rigid, deep hierarchy)
GOOD: Entity + Position + Velocity + PlayerInput (flexible composition)

Learning milestones:

Implement basic ECS → Entities and components work
Add systems → Processing logic separated from data
Efficient queries → Fast component combination lookup
Build small game → Prove the architecture works

Project 4: Undo/Redo Command System

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: TypeScript
Alternative Programming Languages: Java, Python, C#
Coolness Level: Level 3: Genuinely Clever
Business Potential: 2. The “Micro-SaaS / Pro Tool”
Difficulty: Level 2: Intermediate
Knowledge Area: UI Architecture / State Management
Software or Tool: Text editor, drawing app
Main Book: “Design Patterns: Elements of Reusable Object-Oriented Software” by Gang of Four

What you’ll build: A text/drawing editor with full undo/redo support using the Command pattern, including command grouping, macro recording, and persistent undo history.

Why it teaches design patterns: The Command pattern is one of the most practical patterns. Building undo/redo forces you to encapsulate operations as objects and understand Memento for state snapshots.

Core challenges you’ll face:

Encapsulating operations → maps to Command pattern
Storing state for undo → maps to Memento pattern
Command history → maps to stack management, serialization
Composite commands → maps to Composite pattern (macros)

Resources for key challenges:

“Design Patterns” Ch. 5.2 (Command) - Gang of Four
“Design Patterns” Ch. 5.6 (Memento) - Gang of Four

Key Concepts:

Command Pattern: “Design Patterns” Ch. 5.2 - Encapsulate operations
Memento Pattern: “Design Patterns” Ch. 5.6 - State snapshots
Composite Pattern: “Design Patterns” Ch. 4.3 - Macro commands

Difficulty: Intermediate Time estimate: 2-3 weeks Prerequisites: Basic OOP, TypeScript/JavaScript

Real world outcome:

// Using the command system
const editor = new TextEditor();
const history = new CommandHistory();

// Execute commands
history.execute(new InsertTextCommand(editor, 0, "Hello "));
history.execute(new InsertTextCommand(editor, 6, "World"));
// Document: "Hello World"

// Undo
history.undo();
// Document: "Hello "

// Redo  
history.redo();
// Document: "Hello World"

// Macro recording
history.startMacro("format-header");
history.execute(new SelectAllCommand(editor));
history.execute(new BoldCommand(editor));
history.execute(new IncreaseFontSizeCommand(editor, 4));
history.stopMacro();

// Play macro on any selection
history.execute(new PlayMacroCommand("format-header"));

// Persist history
history.save("document-history.json");
// Later...
history.load("document-history.json");
history.undo(); // Works across sessions!

Implementation Hints:

Command interface:

interface Command {
    execute(): void;
    undo(): void;
    // Optional for serialization
    serialize(): object;
    static deserialize(data: object): Command;
}

Each command stores enough information to undo:

class InsertTextCommand implements Command {
    constructor(
        private editor: Editor,
        private position: number,
        private text: string
    ) {}
    
    execute() {
        this.editor.insertAt(this.position, this.text);
    }
    
    undo() {
        this.editor.deleteRange(this.position, this.text.length);
    }
}

Questions to consider:

How do you handle commands that can’t be undone?
How do you merge adjacent similar commands (typing “abc” vs three insert commands)?
How do you limit history size without losing too much?
How do you serialize commands for persistent undo?

Learning milestones:

Implement basic Command pattern → Execute and undo work
Build command history stack → Navigate undo/redo
Add composite commands → Group operations as macros
Serialize history → Persist across sessions

Project 5: Event-Driven Architecture Simulator

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Java
Alternative Programming Languages: C#, Kotlin, TypeScript
Coolness Level: Level 4: Hardcore Tech Flex
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 3: Advanced
Knowledge Area: Distributed Systems / Event Sourcing
Software or Tool: Message queues, event stores
Main Book: “Designing Data-Intensive Applications” by Martin Kleppmann

What you’ll build: A complete event-driven system with event sourcing, CQRS (Command Query Responsibility Segregation), and projections—demonstrating Observer, Mediator, and Command patterns at scale.

Why it teaches design patterns: Event-driven architectures are the Observer pattern at scale. You’ll implement event buses, handlers, projections, and understand how patterns compose into architectural styles.

Core challenges you’ll face:

Event publishing/subscribing → maps to Observer pattern
Command handling → maps to Command pattern, Mediator
Event storage → maps to Memento, event sourcing
Read model projections → maps to CQRS separation

Resources for key challenges:

“Designing Data-Intensive Applications” Ch. 11 - Kleppmann
“Implementing Domain-Driven Design” - Vaughn Vernon

Key Concepts:

Observer Pattern at Scale: “Enterprise Integration Patterns” - Hohpe
Event Sourcing: “Implementing DDD” Ch. 8 - Vernon
CQRS: “CQRS Journey” - Microsoft Patterns & Practices

Difficulty: Advanced Time estimate: 4-5 weeks Prerequisites: Projects 1-4, understanding of distributed systems basics

Real world outcome:

// Define events
public record OrderPlaced(String orderId, String customerId, 
                          List<LineItem> items, Instant timestamp) 
    implements DomainEvent {}

public record OrderShipped(String orderId, String trackingNumber,
                           Instant timestamp) 
    implements DomainEvent {}

// Event handler (Observer)
@EventHandler
public class InventoryHandler {
    @Subscribe
    public void on(OrderPlaced event) {
        for (LineItem item : event.items()) {
            inventoryService.reserve(item.productId(), item.quantity());
        }
    }
}

// Command handler
@CommandHandler  
public class OrderCommandHandler {
    public void handle(PlaceOrderCommand cmd) {
        Order order = new Order(cmd.customerId(), cmd.items());
        eventStore.append(order.getUncommittedEvents());
        eventBus.publish(order.getUncommittedEvents());
    }
}

// Projection (read model)
@Projection
public class OrderSummaryProjection {
    private Map<String, OrderSummary> summaries = new HashMap<>();
    
    @Subscribe
    public void on(OrderPlaced event) {
        summaries.put(event.orderId(), 
            new OrderSummary(event.orderId(), "PLACED", event.items().size()));
    }
    
    @Subscribe
    public void on(OrderShipped event) {
        summaries.get(event.orderId()).setStatus("SHIPPED");
    }
}

// Runtime output:
$ java -jar event-simulator.jar

[EventStore] Initialized with 0 events
[EventBus] 3 handlers registered

> place-order customer-123 item-A:2 item-B:1
[Command] PlaceOrderCommand received
[EventStore] Appended: OrderPlaced(order-001)
[EventBus] Publishing: OrderPlaced
  → InventoryHandler: Reserved 2x item-A, 1x item-B
  → OrderSummaryProjection: Created summary
  → NotificationHandler: Email queued

> ship-order order-001 TRACK123
[Command] ShipOrderCommand received
[EventStore] Appended: OrderShipped(order-001)
[EventBus] Publishing: OrderShipped
  → OrderSummaryProjection: Updated status to SHIPPED
  → NotificationHandler: SMS queued

> query orders
ORDER-001: SHIPPED (3 items) - Customer-123

Implementation Hints:

Core components:

Event Bus: Publish/subscribe system (Observer pattern)
Event Store: Append-only storage of all events (Memento-like)
Command Bus: Routes commands to handlers (Mediator)
Projections: Build read models from events

Event sourcing principle:

Never delete or update events
Rebuild state by replaying events
Events are the source of truth

Questions to consider:

How do you handle event ordering?
How do you rebuild projections from scratch?
How do you version events when schema changes?
How do you handle eventual consistency?

Learning milestones:

Implement event bus → Pub/sub working
Build event store → Persist and replay events
Add command handlers → CQRS command side
Create projections → CQRS query side

Project 6: HTTP Middleware Pipeline

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Go
Alternative Programming Languages: TypeScript (Node.js), Python, Rust
Coolness Level: Level 3: Genuinely Clever
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 2: Intermediate
Knowledge Area: Web Development / Request Processing
Software or Tool: HTTP servers
Main Book: “Head First Design Patterns” by Freeman & Robson

What you’ll build: An HTTP middleware system where request processing flows through a chain of handlers—authentication, logging, rate limiting, caching—demonstrating Chain of Responsibility and Decorator patterns.

Why it teaches design patterns: Middleware pipelines are the Chain of Responsibility pattern in action. Every major web framework (Express, ASP.NET Core, Koa) uses this pattern.

Core challenges you’ll face:

Chaining handlers → maps to Chain of Responsibility pattern
Wrapping functionality → maps to Decorator pattern
Request/response modification → maps to pipeline architecture
Early termination → maps to chain breaking conditions

Resources for key challenges:

Express.js middleware documentation
ASP.NET Core middleware documentation
“Design Patterns” Ch. 5.1 - Chain of Responsibility

Key Concepts:

Chain of Responsibility: “Design Patterns” Ch. 5.1 - Pass along chain
Decorator Pattern: “Design Patterns” Ch. 4.4 - Wrap and extend
Pipeline Pattern: “Enterprise Integration Patterns” - Pipes and Filters

Difficulty: Intermediate Time estimate: 2 weeks Prerequisites: HTTP basics, web server understanding

Real world outcome:

// Define middleware pipeline
func main() {
    pipeline := NewPipeline()
    
    // Add middleware (order matters!)
    pipeline.Use(LoggingMiddleware())
    pipeline.Use(RecoveryMiddleware())      // Catch panics
    pipeline.Use(RateLimitMiddleware(100))  // 100 req/min
    pipeline.Use(AuthMiddleware())          // Check JWT
    pipeline.Use(CacheMiddleware(60))       // 60 second cache
    pipeline.Use(CompressionMiddleware())   // Gzip response
    
    // Final handler
    pipeline.Handle("/api/users", usersHandler)
    
    http.ListenAndServe(":8080", pipeline)
}

// Custom middleware
func LoggingMiddleware() Middleware {
    return func(next Handler) Handler {
        return func(ctx *Context) {
            start := time.Now()
            
            // Before request
            log.Printf("→ %s %s", ctx.Method, ctx.Path)
            
            // Call next in chain
            next(ctx)
            
            // After request
            log.Printf("← %s %s [%d] %v", 
                ctx.Method, ctx.Path, ctx.StatusCode, time.Since(start))
        }
    }
}

// Runtime output:
$ curl http://localhost:8080/api/users

[Server Log]
→ GET /api/users
  [RateLimit] 42/100 requests used
  [Auth] Token valid: user-123
  [Cache] MISS - fetching fresh data
  [Handler] Fetching users from database
← GET /api/users [200] 45ms

$ curl http://localhost:8080/api/users  # Second request
→ GET /api/users
  [RateLimit] 43/100 requests used
  [Auth] Token valid: user-123
  [Cache] HIT - returning cached response
← GET /api/users [200] 2ms

Implementation Hints:

Core types:

type Handler func(ctx *Context)
type Middleware func(next Handler) Handler

type Pipeline struct {
    middlewares []Middleware
}

func (p *Pipeline) Use(m Middleware) {
    p.middlewares = append(p.middlewares, m)
}

func (p *Pipeline) Build(final Handler) Handler {
    // Chain middlewares in reverse order
    handler := final
    for i := len(p.middlewares) - 1; i >= 0; i-- {
        handler = p.middlewares[i](handler)
    }
    return handler
}

Questions to consider:

How do you short-circuit the chain (e.g., auth failure)?
How do you share data between middleware?
How do you handle errors in the chain?
How do you order middleware correctly?

Common middleware to implement:

Logging: Request/response timing
Recovery: Catch panics, return 500
Auth: Validate tokens, attach user
Rate Limit: Track requests per client
Cache: Return cached responses
Compression: Gzip responses

Learning milestones:

Chain handlers → Requests flow through pipeline
Modify request/response → Add headers, change body
Short-circuit chain → Auth blocks unauthorized
Handle errors → Recovery catches panics

Project 7: UI Component Library with Decorators

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: TypeScript (React)
Alternative Programming Languages: Java (Swing), C# (WPF), Python (Qt)
Coolness Level: Level 3: Genuinely Clever
Business Potential: 4. The “Open Core” Infrastructure
Difficulty: Level 2: Intermediate
Knowledge Area: UI Development / Component Architecture
Software or Tool: React, Component libraries
Main Book: “Head First Design Patterns” by Freeman & Robson

What you’ll build: A UI component library where components can be dynamically enhanced with features like tooltips, loading states, error boundaries, and accessibility—all using the Decorator pattern.

Why it teaches design patterns: UI frameworks heavily use Decorator pattern. React’s Higher-Order Components (HOCs), Vue’s mixins, and Angular’s directives are all decorator implementations.

Core challenges you’ll face:

Wrapping components → maps to Decorator pattern
Composing decorators → maps to multiple wrappers
Props forwarding → maps to transparent decoration
Performance → maps to decorator overhead

Resources for key challenges:

“Head First Design Patterns” Ch. 3 - Decorator
React Higher-Order Components documentation

Key Concepts:

Decorator Pattern: “Head First Design Patterns” Ch. 3
Higher-Order Components: React documentation
Composition vs Inheritance: “Effective Java” Item 18

Difficulty: Intermediate Time estimate: 2-3 weeks Prerequisites: React/TypeScript basics

Real world outcome:

// Base component
const Button: React.FC<ButtonProps> = ({ children, onClick }) => (
    <button onClick={onClick}>{children}</button>
);

// Decorators
const withTooltip = (tip: string) => (Component) => (props) => (
    <Tooltip text={tip}>
        <Component {...props} />
    </Tooltip>
);

const withLoading = (Component) => ({ loading, ...props }) => (
    loading ? <Spinner /> : <Component {...props} />
);

const withErrorBoundary = (Component) => (props) => (
    <ErrorBoundary fallback={<ErrorMessage />}>
        <Component {...props} />
    </ErrorBoundary>
);

const withTracking = (eventName: string) => (Component) => (props) => {
    const handleClick = (e) => {
        analytics.track(eventName);
        props.onClick?.(e);
    };
    return <Component {...props} onClick={handleClick} />;
};

// Compose decorators
const EnhancedButton = compose(
    withErrorBoundary,
    withLoading,
    withTooltip("Click to submit"),
    withTracking("submit-button-clicked")
)(Button);

// Usage
<EnhancedButton loading={isSubmitting} onClick={handleSubmit}>
    Submit Order
</EnhancedButton>

// Renders as:
<ErrorBoundary>
    <Tooltip text="Click to submit">
        <button onClick={trackedHandleSubmit}>
            Submit Order
        </button>
    </Tooltip>
</ErrorBoundary>

Implementation Hints:

Create a compose utility:

const compose = (...decorators) => (Component) =>
    decorators.reduceRight(
        (decorated, decorator) => decorator(decorated),
        Component
    );

Common decorators to implement:

withTooltip: Show hint on hover
withLoading: Show spinner when loading
withErrorBoundary: Catch and display errors
withTracking: Analytics events
withAccessibility: ARIA attributes
withTheme: Style injection
withMemo: Performance optimization

Questions to consider:

How do you preserve the component’s display name for debugging?
How do you handle ref forwarding through decorators?
How do you type decorators correctly in TypeScript?
When is decoration overkill vs just adding props?

Learning milestones:

Create basic HOCs → Wrap and enhance components
Compose decorators → Stack multiple enhancements
Handle edge cases → Refs, display names, types
Build component library → Reusable decorated components

Project 8: State Machine Workflow Engine

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Python
Alternative Programming Languages: Java, TypeScript, Go
Coolness Level: Level 4: Hardcore Tech Flex
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 3: Advanced
Knowledge Area: Business Logic / Workflow Automation
Software or Tool: State machine libraries, workflow engines
Main Book: “Design Patterns: Elements of Reusable Object-Oriented Software” by Gang of Four

What you’ll build: A workflow engine that models business processes as state machines—order processing, approval workflows, subscription lifecycles—with visual state diagrams and persistence.

Why it teaches design patterns: The State pattern is perfect for modeling entities with complex lifecycles. Combined with Command (transitions) and Observer (notifications), you’ll build a powerful abstraction.

Core challenges you’ll face:

State transitions → maps to State pattern
Transition actions → maps to Command pattern
State change notifications → maps to Observer pattern
Persistence → maps to Memento pattern

Resources for key challenges:

“Design Patterns” Ch. 5.8 (State) - Gang of Four
“Refactoring to Patterns” Ch. 7 - Joshua Kerievsky

Key Concepts:

State Pattern: “Design Patterns” Ch. 5.8 - Behavior changes with state
Finite State Machines: “Introduction to Automata Theory” - Hopcroft
Workflow Patterns: Workflow Patterns Initiative (workflowpatterns.com)

Difficulty: Advanced Time estimate: 3-4 weeks Prerequisites: Projects 1-4, understanding of state machines

Real world outcome:

# Define order workflow
class OrderWorkflow(StateMachine):
    # States
    pending = State(initial=True)
    confirmed = State()
    processing = State()
    shipped = State()
    delivered = State(final=True)
    cancelled = State(final=True)
    
    # Transitions
    confirm = pending.to(confirmed)
    start_processing = confirmed.to(processing)
    ship = processing.to(shipped)
    deliver = shipped.to(delivered)
    cancel = pending.to(cancelled) | confirmed.to(cancelled)
    
    # Transition actions
    @confirm.on_transition
    def on_confirm(self, event):
        self.send_confirmation_email()
        self.reserve_inventory()
    
    @ship.on_transition  
    def on_ship(self, event):
        self.generate_tracking_number()
        self.notify_customer(f"Shipped! Track: {self.tracking}")

# Usage
order = OrderWorkflow(order_id="ORD-123")
print(order.current_state)  # pending

order.confirm()
print(order.current_state)  # confirmed
# → Email sent: "Order ORD-123 confirmed!"
# → Inventory reserved

order.start_processing()
order.ship(carrier="FedEx")
print(order.current_state)  # shipped
# → Tracking: FEDEX-789456123

# Invalid transition raises error
order.confirm()  # StateTransitionError: Cannot confirm from 'shipped'

# Visualize workflow
order.render_diagram("order_workflow.png")

# Query workflow
print(order.available_transitions)  # ['deliver']
print(order.is_final)  # False
print(order.history)
# [
#   (pending → confirmed, 2024-01-15 10:00),
#   (confirmed → processing, 2024-01-15 10:05),
#   (processing → shipped, 2024-01-15 14:30)
# ]

Implementation Hints:

Core State pattern structure:

class State:
    def __init__(self, name, initial=False, final=False):
        self.name = name
        self.initial = initial
        self.final = final
        self.transitions = {}
    
    def to(self, target_state):
        return Transition(self, target_state)

class StateMachine:
    def __init__(self):
        self._current_state = self._get_initial_state()
        self._observers = []
    
    def trigger(self, transition_name, **kwargs):
        transition = self._current_state.transitions.get(transition_name)
        if not transition:
            raise StateTransitionError(...)
        
        # Execute before hooks
        transition.execute_before_hooks()
        
        # Change state
        old_state = self._current_state
        self._current_state = transition.target
        
        # Execute after hooks
        transition.execute_after_hooks()
        
        # Notify observers
        self._notify_observers(old_state, self._current_state)

Questions to consider:

How do you persist state machine instances?
How do you handle concurrent transitions?
How do you version workflows (old orders on old workflow)?
How do you visualize the state diagram?

Learning milestones:

Basic state transitions → Move between states
Transition guards → Conditional transitions
Actions and hooks → Side effects on transitions
Persistence → Save and restore workflows

Project 9: Object Pool Resource Manager

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Java
Alternative Programming Languages: C#, Go, Rust
Coolness Level: Level 3: Genuinely Clever
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 2: Intermediate
Knowledge Area: Performance / Resource Management
Software or Tool: Connection pools, thread pools
Main Book: “Effective Java” by Joshua Bloch

What you’ll build: A generic object pool that manages expensive resources (database connections, threads, network sockets) with configurable limits, health checks, and automatic cleanup.

Why it teaches design patterns: Object Pool combines Flyweight (sharing), Factory (creation), and Singleton (pool instance). It’s essential for performance-critical applications.

Core challenges you’ll face:

Pool management → maps to Object Pool, Flyweight patterns
Object creation → maps to Factory pattern
Thread safety → maps to concurrent access
Resource cleanup → maps to lifecycle management

Resources for key challenges:

“Effective Java” Item 6 - Avoid creating unnecessary objects
HikariCP source code (Java connection pool)
Apache Commons Pool documentation

Key Concepts:

Object Pool Pattern: “Game Programming Patterns” Ch. 19
Flyweight Pattern: “Design Patterns” Ch. 4.6 - Share objects
Factory Pattern: “Design Patterns” Ch. 3 - Object creation

Difficulty: Intermediate Time estimate: 2-3 weeks Prerequisites: Concurrency basics, Java

Real world outcome:

// Configure pool
PoolConfig<DatabaseConnection> config = PoolConfig.<DatabaseConnection>builder()
    .factory(() -> new DatabaseConnection(DB_URL))
    .validator(conn -> conn.isValid(1))
    .minSize(5)
    .maxSize(20)
    .maxWaitTime(Duration.ofSeconds(30))
    .idleTimeout(Duration.ofMinutes(10))
    .healthCheckInterval(Duration.ofMinutes(1))
    .build();

// Create pool
ObjectPool<DatabaseConnection> pool = new ObjectPool<>(config);

// Use pooled objects
try (PooledObject<DatabaseConnection> pooled = pool.acquire()) {
    DatabaseConnection conn = pooled.getObject();
    ResultSet rs = conn.executeQuery("SELECT * FROM users");
    // ... use connection
} // Automatically returned to pool

// Pool statistics
System.out.println(pool.getStats());
// PoolStats{
//   total=20, 
//   available=15, 
//   inUse=5, 
//   waitingThreads=0,
//   totalCreated=25,
//   totalDestroyed=5,
//   avgAcquireTime=2ms
// }

// Monitor pool health
pool.addListener(event -> {
    switch(event.getType()) {
        case OBJECT_CREATED -> log.info("New connection created");
        case OBJECT_DESTROYED -> log.warn("Connection destroyed: " + event.getReason());
        case POOL_EXHAUSTED -> alert("Pool exhausted! Consider increasing max size");
        case VALIDATION_FAILED -> log.error("Connection validation failed");
    }
});

Implementation Hints:

Core pool structure:

public class ObjectPool<T> {
    private final BlockingQueue<T> available;
    private final Set<T> inUse;
    private final PoolConfig<T> config;
    private final Semaphore permits;
    
    public PooledObject<T> acquire() throws TimeoutException {
        // Wait for permit (respects maxSize)
        if (!permits.tryAcquire(config.maxWaitTime)) {
            throw new TimeoutException("Pool exhausted");
        }
        
        T object = available.poll();
        if (object == null) {
            object = config.factory.create();
        }
        
        // Validate before returning
        if (!config.validator.validate(object)) {
            destroy(object);
            object = config.factory.create();
        }
        
        inUse.add(object);
        return new PooledObject<>(object, this::release);
    }
    
    private void release(T object) {
        inUse.remove(object);
        available.offer(object);
        permits.release();
    }
}

Questions to consider:

How do you handle object validation failure?
How do you evict idle objects?
How do you prevent resource leaks (unreturned objects)?
How do you handle sudden demand spikes?

Learning milestones:

Basic pooling → Acquire and release objects
Size limits → Min/max pool size
Validation → Health check before return
Monitoring → Stats and events

Project 10: Abstract Document Model

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Java
Alternative Programming Languages: Kotlin, TypeScript, Python
Coolness Level: Level 3: Genuinely Clever
Business Potential: 2. The “Micro-SaaS / Pro Tool”
Difficulty: Level 2: Intermediate
Knowledge Area: Data Modeling / Flexibility
Software or Tool: Document databases, configuration systems
Main Book: “Patterns of Enterprise Application Architecture” by Martin Fowler

What you’ll build: A flexible document model where objects can have dynamic properties while still maintaining type safety—like MongoDB documents but in code, combining Abstract Document with Builder patterns.

Why it teaches design patterns: This project demonstrates Abstract Document, Builder, and Composite patterns working together to create flexible yet type-safe data structures.

Core challenges you’ll face:

Dynamic properties → maps to Abstract Document pattern
Type-safe access → maps to interface segregation
Hierarchical data → maps to Composite pattern
Fluent construction → maps to Builder pattern

Resources for key challenges:

“Patterns of Enterprise Application Architecture” - Fowler
“Abstract Document Pattern” - java-design-patterns.com

Key Concepts:

Abstract Document: Dynamic properties with typed access
Builder Pattern: “Effective Java” Item 2 - Complex construction
Composite Pattern: “Design Patterns” Ch. 4.3 - Tree structures

Difficulty: Intermediate Time estimate: 2 weeks Prerequisites: Java generics, OOP basics

Real world outcome:

// Define property interfaces
interface HasPrice extends Document {
    default Optional<Double> getPrice() {
        return get("price", Double.class);
    }
}

interface HasName extends Document {
    default Optional<String> getName() {
        return get("name", String.class);
    }
}

interface HasParts extends Document {
    default Stream<Part> getParts() {
        return children("parts", Part::new);
    }
}

// Compose interfaces for domain types
class Car extends AbstractDocument implements HasName, HasPrice, HasParts {
    public Car(Map<String, Object> properties) {
        super(properties);
    }
}

class Part extends AbstractDocument implements HasName, HasPrice {
    public Part(Map<String, Object> properties) {
        super(properties);
    }
}

// Build documents fluently
Car car = Document.builder(Car::new)
    .put("name", "Tesla Model 3")
    .put("price", 45000.0)
    .put("color", "red")  // Dynamic property!
    .children("parts",
        Document.builder(Part::new)
            .put("name", "Battery Pack")
            .put("price", 15000.0)
            .build(),
        Document.builder(Part::new)
            .put("name", "Electric Motor")
            .put("price", 5000.0)
            .build()
    )
    .build();

// Type-safe access via interfaces
System.out.println(car.getName().orElse("Unknown"));  // Tesla Model 3
System.out.println(car.getPrice().orElse(0.0));        // 45000.0

// Dynamic access
System.out.println(car.get("color", String.class));    // Optional[red]

// Navigate children
car.getParts().forEach(part -> {
    System.out.println(part.getName().orElse("?") + ": $" + part.getPrice().orElse(0.0));
});
// Battery Pack: $15000.0
// Electric Motor: $5000.0

// Serialize to JSON naturally
String json = car.toJson();

Implementation Hints:

Core abstract document:

public abstract class AbstractDocument implements Document {
    private final Map<String, Object> properties;
    
    protected AbstractDocument(Map<String, Object> properties) {
        this.properties = new HashMap<>(properties);
    }
    
    @Override
    public void put(String key, Object value) {
        properties.put(key, value);
    }
    
    @Override
    public <T> Optional<T> get(String key, Class<T> type) {
        return Optional.ofNullable(properties.get(key))
                       .filter(type::isInstance)
                       .map(type::cast);
    }
    
    @Override
    public <T extends Document> Stream<T> children(String key, 
            Function<Map<String, Object>, T> constructor) {
        return Optional.ofNullable(properties.get(key))
            .filter(List.class::isInstance)
            .map(list -> ((List<Map<String, Object>>) list).stream()
                         .map(constructor))
            .orElse(Stream.empty());
    }
}

Questions to consider:

How do you handle missing vs null values?
How do you validate required properties?
How do you handle nested documents?
How do you serialize/deserialize documents?

Learning milestones:

Implement abstract document → Dynamic properties work
Add typed interfaces → Type-safe access methods
Handle children → Nested documents
Build fluently → Builder pattern integration

Project 11: Interpreter for Domain-Specific Language

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Python
Alternative Programming Languages: Java, Kotlin, TypeScript
Coolness Level: Level 5: Pure Magic (Super Cool)
Business Potential: 4. The “Open Core” Infrastructure
Difficulty: Level 4: Expert
Knowledge Area: Language Design / Parsing
Software or Tool: Parser generators, AST
Main Book: “Language Implementation Patterns” by Terence Parr

What you’ll build: A complete interpreter for a domain-specific language (DSL) for business rules, query building, or configuration—demonstrating Interpreter, Composite, and Visitor patterns.

Why it teaches design patterns: The Interpreter pattern is about building a grammar and evaluating expressions. Combined with Composite (expression trees) and Visitor (operations on trees), you’ll understand how languages work.

Core challenges you’ll face:

Parsing grammar → maps to Interpreter pattern
Expression trees → maps to Composite pattern
Tree operations → maps to Visitor pattern
Evaluation → maps to recursive interpretation

Resources for key challenges:

“Language Implementation Patterns” - Terence Parr
“Crafting Interpreters” - Robert Nystrom (free online)

Key Concepts:

Interpreter Pattern: “Design Patterns” Ch. 5.3 - Grammar evaluation
Composite Pattern: “Design Patterns” Ch. 4.3 - Expression trees
Visitor Pattern: “Design Patterns” Ch. 5.11 - Tree operations

Difficulty: Expert Time estimate: 4-5 weeks Prerequisites: Recursion, tree structures, parsing basics

Real world outcome:

# Business Rule DSL
rules = """
RULE HighValueCustomer
    WHEN customer.total_purchases > 10000
     AND customer.membership_years >= 2
     AND NOT customer.has_outstanding_debt
    THEN
        APPLY discount OF 15%
        GRANT badge "VIP"
        SEND email "vip_welcome"
END

RULE NewCustomerDiscount  
    WHEN customer.is_new
     AND order.total > 100
    THEN
        APPLY discount OF 10%
END
"""

# Parse and create rule engine
engine = RuleEngine.from_dsl(rules)

# Evaluate rules
customer = Customer(total_purchases=15000, membership_years=3, is_new=False)
order = Order(total=500)

results = engine.evaluate(customer=customer, order=order)
# [
#   RuleResult(rule="HighValueCustomer", applied=True, actions=[
#     ApplyDiscount(15%),
#     GrantBadge("VIP"),
#     SendEmail("vip_welcome")
#   ])
# ]

# Explain rule evaluation (Visitor pattern)
explanation = engine.explain(customer=customer)
# HighValueCustomer:
#   ✓ customer.total_purchases > 10000 (15000 > 10000)
#   ✓ customer.membership_years >= 2 (3 >= 2)
#   ✓ NOT customer.has_outstanding_debt (NOT false)
#   → All conditions met, rule applies

# Validate rules (another Visitor)
errors = engine.validate()
# [ValidationError: Rule 'BrokenRule' references undefined field 'customer.invalid']

# Serialize rules (Visitor for code generation)
sql = engine.to_sql()
# SELECT * FROM customers 
# WHERE total_purchases > 10000 
#   AND membership_years >= 2 
#   AND NOT has_outstanding_debt

Implementation Hints:

Expression tree (Composite):

class Expression(ABC):
    @abstractmethod
    def evaluate(self, context: dict) -> Any:
        pass
    
    @abstractmethod
    def accept(self, visitor: ExpressionVisitor) -> Any:
        pass

class BinaryExpression(Expression):
    def __init__(self, left: Expression, operator: str, right: Expression):
        self.left = left
        self.operator = operator
        self.right = right
    
    def evaluate(self, context):
        left_val = self.left.evaluate(context)
        right_val = self.right.evaluate(context)
        return self.operators[self.operator](left_val, right_val)
    
    def accept(self, visitor):
        return visitor.visit_binary(self)

class PropertyAccess(Expression):
    def __init__(self, object_name: str, property_name: str):
        self.object_name = object_name
        self.property_name = property_name
    
    def evaluate(self, context):
        obj = context[self.object_name]
        return getattr(obj, self.property_name)

Visitor for operations:

class ExpressionVisitor(ABC):
    @abstractmethod
    def visit_binary(self, expr: BinaryExpression): pass
    
    @abstractmethod
    def visit_property(self, expr: PropertyAccess): pass

class ExplainVisitor(ExpressionVisitor):
    def visit_binary(self, expr):
        left = expr.left.accept(self)
        right = expr.right.accept(self)
        return f"({left} {expr.operator} {right})"

Questions to consider:

How do you handle syntax errors gracefully?
How do you optimize evaluation (caching, short-circuit)?
How do you add new operations without modifying expressions?
How do you support custom functions?

Learning milestones:

Parse simple expressions → Tokenize and build AST
Evaluate expressions → Recursive interpretation
Add Visitor → Multiple operations on same tree
Build complete DSL → Full rule language

Project 12: Smart Home Automation with Observer

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Python
Alternative Programming Languages: TypeScript, Java, C#
Coolness Level: Level 4: Hardcore Tech Flex
Business Potential: 2. The “Micro-SaaS / Pro Tool”
Difficulty: Level 2: Intermediate
Knowledge Area: IoT / Event-Driven Systems
Software or Tool: Home Assistant concepts, MQTT
Main Book: “Head First Design Patterns” by Freeman & Robson

What you’ll build: A smart home simulation where sensors publish events and devices react—motion sensors trigger lights, temperature sensors control AC, all demonstrating Observer and Mediator patterns.

Why it teaches design patterns: Observer is everywhere in event-driven systems. Combined with Mediator (central hub) and Command (actions), you’ll build a reactive system.

Core challenges you’ll face:

Event publishing → maps to Observer pattern
Device coordination → maps to Mediator pattern
Action execution → maps to Command pattern
State management → maps to State pattern

Resources for key challenges:

“Head First Design Patterns” Ch. 2 - Observer
Home Assistant architecture documentation

Key Concepts:

Observer Pattern: “Head First Design Patterns” Ch. 2 - Pub/sub
Mediator Pattern: “Design Patterns” Ch. 5.5 - Central coordination
Command Pattern: “Design Patterns” Ch. 5.2 - Encapsulated actions

Difficulty: Intermediate Time estimate: 2-3 weeks Prerequisites: Event-driven programming basics

Real world outcome:

# Define devices and sensors
hub = SmartHomeHub()

# Sensors (Subjects)
motion_sensor = MotionSensor("living_room")
temp_sensor = TemperatureSensor("bedroom")
door_sensor = DoorSensor("front_door")
light_sensor = LightSensor("outdoor")

# Devices (Observers)
living_room_light = SmartLight("living_room_light")
bedroom_ac = SmartAC("bedroom_ac")
security_camera = SecurityCamera("front_camera")
outdoor_lights = SmartLight("outdoor_lights")

# Register with hub
hub.add_sensor(motion_sensor, temp_sensor, door_sensor, light_sensor)
hub.add_device(living_room_light, bedroom_ac, security_camera, outdoor_lights)

# Define automation rules
hub.add_rule(
    Rule("Motion activates lights")
        .when(motion_sensor).detects(MotionEvent)
        .then(living_room_light).do(TurnOn())
        .expires_after(minutes=5)
)

hub.add_rule(
    Rule("Temperature control")
        .when(temp_sensor).exceeds(75)
        .then(bedroom_ac).do(SetTemperature(72))
        .until(temp_sensor).below(70)
)

hub.add_rule(
    Rule("Security mode")
        .when(door_sensor).detects(OpenEvent)
        .and_if(hub.mode == "away")
        .then(security_camera).do(StartRecording())
        .and_then(Notification("Alert: Door opened!"))
)

hub.add_rule(
    Rule("Sunset lights")
        .when(light_sensor).below(100)  # lux
        .then(outdoor_lights).do(TurnOn())
)

# Simulation output:
$ python smart_home.py

[18:45:02] LightSensor(outdoor): 95 lux
[18:45:02] → Rule "Sunset lights" triggered
[18:45:02] → SmartLight(outdoor_lights): ON

[19:30:15] MotionSensor(living_room): motion detected
[19:30:15] → Rule "Motion activates lights" triggered  
[19:30:15] → SmartLight(living_room_light): ON
[19:30:15] → Timer set: 5 minutes

[19:35:15] → Timer expired
[19:35:15] → SmartLight(living_room_light): OFF

[21:00:00] TemperatureSensor(bedroom): 76°F
[21:00:00] → Rule "Temperature control" triggered
[21:00:00] → SmartAC(bedroom_ac): Set to 72°F

Implementation Hints:

Observer pattern core:

class Sensor(ABC):  # Subject
    def __init__(self, name):
        self.name = name
        self._observers = []
    
    def attach(self, observer):
        self._observers.append(observer)
    
    def notify(self, event):
        for observer in self._observers:
            observer.update(self, event)

class Device(ABC):  # Observer
    @abstractmethod
    def update(self, sensor, event):
        pass

class SmartLight(Device):
    def update(self, sensor, event):
        if isinstance(event, MotionEvent):
            self.turn_on()

Mediator for complex coordination:

class SmartHomeHub:  # Mediator
    def __init__(self):
        self.sensors = []
        self.devices = []
        self.rules = []
    
    def sensor_event(self, sensor, event):
        # Central coordination
        for rule in self.rules:
            if rule.matches(sensor, event):
                rule.execute(self.devices)

Questions to consider:

How do you avoid circular dependencies between devices?
How do you handle conflicting rules?
How do you implement rule priorities?
How do you test automation without real hardware?

Learning milestones:

Implement sensor/device communication → Basic Observer
Add central hub → Mediator coordination
Create rule engine → Complex automations
Add scheduling → Time-based triggers

Project 13: Report Generator with Template Method

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Python
Alternative Programming Languages: Java, C#, TypeScript
Coolness Level: Level 2: Practical but Forgettable
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 1: Beginner
Knowledge Area: Document Generation / Frameworks
Software or Tool: PDF libraries, templating engines
Main Book: “Head First Design Patterns” by Freeman & Robson

What you’ll build: A report generation framework where the skeleton algorithm is fixed but specific steps (data fetching, formatting, rendering) can be customized—demonstrating Template Method and Strategy patterns.

Why it teaches design patterns: Template Method defines “how” something is done while letting subclasses customize “what.” This is the foundation of frameworks.

Core challenges you’ll face:

Defining the skeleton → maps to Template Method pattern
Customizing steps → maps to hook methods
Swapping renderers → maps to Strategy pattern
Composing reports → maps to Composite pattern

Resources for key challenges:

“Head First Design Patterns” Ch. 8 - Template Method
“Design Patterns” Ch. 5.10 - Template Method

Key Concepts:

Template Method: “Design Patterns” Ch. 5.10 - Skeleton algorithm
Strategy Pattern: “Design Patterns” Ch. 5.9 - Swappable algorithms
Hook Methods: “Head First Design Patterns” Ch. 8

Difficulty: Beginner Time estimate: 1-2 weeks Prerequisites: Basic OOP, inheritance

Real world outcome:

# Define report template
class SalesReport(ReportTemplate):
    def get_title(self) -> str:
        return f"Sales Report - {self.period}"
    
    def fetch_data(self) -> dict:
        return self.database.query("""
            SELECT product, SUM(quantity), SUM(revenue)
            FROM sales
            WHERE date BETWEEN %s AND %s
            GROUP BY product
        """, self.start_date, self.end_date)
    
    def format_data(self, data: dict) -> list:
        return [
            {"product": row[0], "units": row[1], "revenue": f"${row[2]:,.2f}"}
            for row in data
        ]
    
    # Hook: optional customization
    def add_summary(self, data: list) -> dict:
        total = sum(float(r["revenue"].replace("$", "").replace(",", "")) for r in data)
        return {"total_revenue": f"${total:,.2f}"}

# Different renderers (Strategy)
pdf_renderer = PDFRenderer(template="corporate.html")
excel_renderer = ExcelRenderer()
html_renderer = HTMLRenderer(template="dashboard.html")

# Generate reports
report = SalesReport(period="Q4 2024", start="2024-10-01", end="2024-12-31")

report.generate(pdf_renderer)   # → sales_report_Q4_2024.pdf
report.generate(excel_renderer) # → sales_report_Q4_2024.xlsx
report.generate(html_renderer)  # → sales_report_Q4_2024.html

# Template method in action (console output):
$ python generate_reports.py

[SalesReport] Generating report...
  1. Validating parameters ✓
  2. Fetching data from database... ✓ (150 rows)
  3. Formatting data... ✓
  4. Adding summary... ✓ (hook)
  5. Rendering to PDF... ✓
  6. Saving to sales_report_Q4_2024.pdf ✓

Report generated in 2.3 seconds

Implementation Hints:

Template Method skeleton:

class ReportTemplate(ABC):
    def generate(self, renderer: Renderer) -> str:
        # Template method - fixed algorithm
        self.validate()                  # Fixed step
        data = self.fetch_data()         # Abstract - must override
        formatted = self.format_data(data)  # Abstract
        
        # Hook - optional override
        summary = self.add_summary(formatted)
        
        output = renderer.render(
            title=self.get_title(),
            data=formatted,
            summary=summary
        )
        
        return self.save(output)         # Fixed step
    
    # Abstract methods - subclasses MUST implement
    @abstractmethod
    def get_title(self) -> str: pass
    
    @abstractmethod
    def fetch_data(self) -> dict: pass
    
    @abstractmethod
    def format_data(self, data: dict) -> list: pass
    
    # Hook - subclasses CAN override
    def add_summary(self, data: list) -> dict:
        return {}  # Default: no summary
    
    # Fixed methods
    def validate(self):
        if not self.start_date or not self.end_date:
            raise ValueError("Dates required")
    
    def save(self, output) -> str:
        filename = f"{self.__class__.__name__}_{self.period}"
        return output.save(filename)

Strategy for renderers:

class Renderer(ABC):
    @abstractmethod
    def render(self, title: str, data: list, summary: dict) -> Output:
        pass

class PDFRenderer(Renderer):
    def render(self, title, data, summary):
        # PDF generation logic
        pass

Learning milestones:

Create template class → Fixed skeleton algorithm
Add abstract methods → Required customization points
Add hooks → Optional customization points
Swap strategies → Different renderers

Project 14: Caching Proxy System

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: Go
Alternative Programming Languages: Java, Python, TypeScript
Coolness Level: Level 3: Genuinely Clever
Business Potential: 3. The “Service & Support” Model
Difficulty: Level 2: Intermediate
Knowledge Area: Performance / Caching
Software or Tool: Redis concepts, CDN concepts
Main Book: “Design Patterns: Elements of Reusable Object-Oriented Software” by Gang of Four

What you’ll build: A caching proxy that sits in front of expensive operations, providing transparent caching, lazy loading, access control, and metrics—demonstrating multiple Proxy variants.

Why it teaches design patterns: Proxy is one of the most versatile patterns. You’ll implement virtual proxy (lazy loading), protection proxy (access control), and caching proxy—all with the same interface.

Core challenges you’ll face:

Same interface → maps to Proxy pattern core
Caching logic → maps to caching proxy
Lazy loading → maps to virtual proxy
Access control → maps to protection proxy

Resources for key challenges:

“Design Patterns” Ch. 4.7 - Proxy
“Head First Design Patterns” Ch. 11

Key Concepts:

Proxy Pattern: “Design Patterns” Ch. 4.7 - Control access
Caching Strategies: LRU, TTL, write-through
Lazy Loading: Defer expensive operations

Difficulty: Intermediate Time estimate: 2-3 weeks Prerequisites: Caching concepts, concurrency basics

Real world outcome:

// Original service
type DatabaseService interface {
    GetUser(id string) (*User, error)
    UpdateUser(user *User) error
}

type RealDatabaseService struct {
    // Actual database connection
}

// Caching proxy
type CachingProxy struct {
    service DatabaseService
    cache   *Cache
    ttl     time.Duration
    metrics *Metrics
}

func (p *CachingProxy) GetUser(id string) (*User, error) {
    // Check cache first
    if cached, found := p.cache.Get(id); found {
        p.metrics.RecordHit()
        return cached.(*User), nil
    }
    
    p.metrics.RecordMiss()
    
    // Fetch from real service
    user, err := p.service.GetUser(id)
    if err != nil {
        return nil, err
    }
    
    // Cache for next time
    p.cache.Set(id, user, p.ttl)
    return user, nil
}

// Protection proxy
type AuthProxy struct {
    service DatabaseService
    auth    *AuthService
}

func (p *AuthProxy) GetUser(id string) (*User, error) {
    if !p.auth.HasPermission("read:users") {
        return nil, ErrUnauthorized
    }
    return p.service.GetUser(id)
}

// Compose proxies
service := NewRealDatabaseService(db)
cached := NewCachingProxy(service, cache, 5*time.Minute)
protected := NewAuthProxy(cached, auth)
logged := NewLoggingProxy(protected, logger)

// Use transparently
user, _ := logged.GetUser("user-123")

// Output:
[LOG] GetUser("user-123") started
[AUTH] Permission check: read:users ✓
[CACHE] MISS for user-123
[DB] SELECT * FROM users WHERE id = 'user-123'
[CACHE] Stored user-123 (TTL: 5m)
[LOG] GetUser("user-123") completed in 45ms

// Second call:
[LOG] GetUser("user-123") started
[AUTH] Permission check: read:users ✓
[CACHE] HIT for user-123
[LOG] GetUser("user-123") completed in 0.5ms

// Metrics:
$ curl /metrics
cache_hits_total: 1523
cache_misses_total: 342
cache_hit_ratio: 0.816
avg_cached_response_time_ms: 0.8
avg_uncached_response_time_ms: 42.3

Implementation Hints:

Proxy must implement same interface:

type Proxy struct {
    realService Service
}

func (p *Proxy) DoSomething() Result {
    // Pre-processing
    result := p.realService.DoSomething()
    // Post-processing
    return result
}

Cache implementation:

type Cache struct {
    items map[string]*cacheItem
    mu    sync.RWMutex
    maxSize int
}

type cacheItem struct {
    value      interface{}
    expiration time.Time
}

func (c *Cache) Get(key string) (interface{}, bool) {
    c.mu.RLock()
    defer c.mu.RUnlock()
    
    item, exists := c.items[key]
    if !exists || time.Now().After(item.expiration) {
        return nil, false
    }
    return item.value, true
}

Proxy variations:

Virtual Proxy: Lazy instantiation of expensive objects
Protection Proxy: Access control checks
Caching Proxy: Store and return cached results
Logging Proxy: Record all method calls
Remote Proxy: Handle network communication

Learning milestones:

Implement basic proxy → Same interface, wraps real service
Add caching → Cache results, check before calling
Add protection → Check permissions
Compose proxies → Layer multiple behaviors

Project 15: Design Pattern Catalog Application

File: LEARN_DESIGN_PATTERNS_DEEP_DIVE.md
Main Programming Language: TypeScript (Full-stack)
Alternative Programming Languages: Java/Spring, Python/Django
Coolness Level: Level 4: Hardcore Tech Flex
Business Potential: 2. The “Micro-SaaS / Pro Tool”
Difficulty: Level 3: Advanced
Knowledge Area: Full-Stack Development / Meta-Application
Software or Tool: React, Node.js, Database
Main Book: All previous books

What you’ll build: A full-stack application that catalogs design patterns, with code examples in multiple languages, interactive diagrams, and a recommendation engine—using the patterns you’ve learned throughout.

Why it teaches design patterns: Building a meta-application about patterns forces you to apply patterns everywhere. The catalog itself demonstrates Factory (pattern creation), Strategy (language selection), Observer (search updates), and more.

Core challenges you’ll face:

Pattern organization → maps to Composite, Catalog patterns
Multi-language examples → maps to Strategy, Template Method
Interactive diagrams → maps to Observer, State patterns
Search and filter → maps to Iterator, Specification patterns

Difficulty: Advanced Time estimate: 4-6 weeks Prerequisites: All previous projects, full-stack development

Real world outcome:

┌─────────────────────────────────────────────────────────────────────────────┐
│  🏛️ Design Pattern Catalog                                    [Search...] │
├──────────────────┬──────────────────────────────────────────────────────────┤
│ Categories       │  Observer Pattern                              ⭐⭐⭐⭐ │
│ ──────────────── │  ────────────────────────────────────────────────────── │
│ ▼ Creational     │                                                         │
│   □ Singleton    │  "Define a one-to-many dependency between objects so    │
│   □ Factory      │   that when one object changes state, all dependents    │
│   □ Builder      │   are notified automatically."                          │
│   □ Prototype    │                                                         │
│                  │  ┌─────────┐         ┌──────────┐                       │
│ ▼ Structural     │  │ Subject │────────▶│ Observer │                       │
│   □ Adapter      │  └────┬────┘         └────┬─────┘                       │
│   □ Decorator    │       │                   │                              │
│   □ Proxy        │       │    ┌──────────────┼──────────────┐              │
│                  │       ▼    ▼              ▼              ▼              │
│ ▼ Behavioral     │  ┌─────────────┐   ┌───────────┐   ┌───────────┐        │
│   ■ Observer     │  │ ConcreteObs │   │ ConcreteObs│   │ ConcreteObs│       │
│   □ Strategy     │  └─────────────┘   └───────────┘   └───────────┘        │
│   □ Command      │                                                         │
│   □ State        │  Language: [TypeScript ▼]   View: [Code | Diagram]      │
│                  │  ───────────────────────────────────────────────────    │
│ Related Patterns │  interface Observer {                                   │
│ ──────────────── │      update(subject: Subject): void;                    │
│ • Mediator       │  }                                                      │
│ • Singleton      │                                                         │
│ • Command        │  class Subject {                                        │
│                  │      private observers: Observer[] = [];                │
│ Use Cases        │                                                         │
│ ──────────────── │      attach(o: Observer) { this.observers.push(o); }   │
│ • Event systems  │      notify() {                                         │
│ • MVC/MVVM       │          this.observers.forEach(o => o.update(this));  │
│ • Reactive UIs   │      }                                                  │
│                  │  }                                                      │
│ [Try in Editor]  │                                                         │
└──────────────────┴──────────────────────────────────────────────────────────┘

Implementation Hints:

Patterns used in the application itself:

Repository Pattern: Store/retrieve patterns from database
Factory Pattern: Create pattern examples in different languages
Strategy Pattern: Switch between languages, diagram renderers
Observer Pattern: Update UI when filters change
Composite Pattern: Pattern categories and subcategories
Decorator Pattern: Enhance code examples with syntax highlighting
Facade Pattern: Simple API for complex search operations

Data model:

interface Pattern {
    id: string;
    name: string;
    category: Category;
    intent: string;
    applicability: string[];
    structure: Diagram;
    examples: Map<Language, CodeExample>;
    relatedPatterns: Pattern[];
    realWorldExamples: UseCase[];
}

Features to implement:

Pattern browser with filtering
Multi-language code examples
Interactive UML diagrams
Pattern comparison tool
Recommendation engine (“If you like X, try Y”)
Progress tracking (patterns learned)

Learning milestones:

Build data model → Represent patterns in code
Create CRUD API → Manage pattern data
Build interactive UI → Browse and search
Add interactivity → Diagrams, code editor
Apply patterns → Use patterns while building

Project Comparison Table

#	Project	Difficulty	Time	Key Patterns	Fun
1	SOLID Detector	⭐⭐	2-3 weeks	SOLID Principles	⭐⭐⭐
2	Plugin Framework	⭐⭐⭐	3-4 weeks	Factory, Observer, Strategy	⭐⭐⭐⭐
3	Game ECS	⭐⭐⭐	3-4 weeks	Composite, Flyweight	⭐⭐⭐⭐⭐
4	Undo/Redo System	⭐⭐	2-3 weeks	Command, Memento	⭐⭐⭐⭐
5	Event-Driven Architecture	⭐⭐⭐	4-5 weeks	Observer, Mediator, Command	⭐⭐⭐⭐
6	HTTP Middleware	⭐⭐	2 weeks	Chain of Responsibility, Decorator	⭐⭐⭐
7	UI Component Library	⭐⭐	2-3 weeks	Decorator, Composite	⭐⭐⭐⭐
8	State Machine Workflow	⭐⭐⭐	3-4 weeks	State, Command, Observer	⭐⭐⭐⭐
9	Object Pool	⭐⭐	2-3 weeks	Object Pool, Flyweight, Factory	⭐⭐⭐
10	Abstract Document	⭐⭐	2 weeks	Abstract Document, Builder	⭐⭐⭐
11	DSL Interpreter	⭐⭐⭐⭐	4-5 weeks	Interpreter, Composite, Visitor	⭐⭐⭐⭐⭐
12	Smart Home Automation	⭐⭐	2-3 weeks	Observer, Mediator, Command	⭐⭐⭐⭐⭐
13	Report Generator	⭐	1-2 weeks	Template Method, Strategy	⭐⭐⭐
14	Caching Proxy	⭐⭐	2-3 weeks	Proxy (all variants)	⭐⭐⭐
15	Pattern Catalog App	⭐⭐⭐	4-6 weeks	Multiple patterns	⭐⭐⭐⭐

Recommended Learning Path

Phase 1: Foundations (2-3 weeks)

Understand SOLID and basic patterns:

Project 1: SOLID Detector - Master the principles
Project 13: Report Generator - Simple Template Method

Phase 2: Structural Patterns (3-4 weeks)

Learn how objects compose:

Project 6: HTTP Middleware - Chain and Decorator
Project 14: Caching Proxy - Proxy variations
Project 7: UI Component Library - Decorator in UI

Phase 3: Behavioral Patterns (4-5 weeks)

Learn how objects communicate:

Project 4: Undo/Redo System - Command and Memento
Project 8: State Machine Workflow - State pattern
Project 12: Smart Home - Observer and Mediator

Phase 4: Creational and Advanced (4-6 weeks)

Master object creation and complex patterns:

Project 2: Plugin Framework - Factory, Strategy
Project 9: Object Pool - Resource management
Project 3: Game ECS - Composition mastery

Phase 5: Integration and Mastery (4-6 weeks)

Apply everything together:

Project 5: Event-Driven Architecture - Patterns at scale
Project 11: DSL Interpreter - Advanced patterns
Project 10: Abstract Document - Flexible data
Project 15: Pattern Catalog App - Complete application

Final Overall Project: Enterprise Application Framework

As your capstone, build a complete enterprise application framework that other developers can use to build applications. This framework should:

What you’ll build: A mini-framework (like a simplified Spring or Django) that provides:

Dependency injection container (Factory, Singleton)
ORM with repository pattern (Proxy, Template Method)
Event system (Observer, Mediator)
Plugin architecture (Strategy, Factory)
Configuration management (Builder, Abstract Document)
Middleware pipeline (Chain of Responsibility, Decorator)
State machine for workflows (State, Command)

Why it’s the ultimate test: Frameworks are designed around patterns. Building one proves you understand when and why to apply each pattern.

Time estimate: 2-3 months Difficulty: Expert

Summary

#	Project	Main Language
1	SOLID Principles Violation Detector	Python
2	Plugin Architecture Framework	Python
3	Game Entity Component System	C++
4	Undo/Redo Command System	TypeScript
5	Event-Driven Architecture Simulator	Java
6	HTTP Middleware Pipeline	Go
7	UI Component Library with Decorators	TypeScript (React)
8	State Machine Workflow Engine	Python
9	Object Pool Resource Manager	Java
10	Abstract Document Model	Java
11	Interpreter for Domain-Specific Language	Python
12	Smart Home Automation with Observer	Python
13	Report Generator with Template Method	Python
14	Caching Proxy System	Go
15	Design Pattern Catalog Application	TypeScript (Full-stack)

Resources

Essential Books

“Design Patterns: Elements of Reusable Object-Oriented Software” (Gang of Four) - The original patterns bible
“Head First Design Patterns” by Freeman & Robson - Most accessible introduction
“Clean Code” by Robert C. Martin - SOLID and clean design
“Clean Architecture” by Robert C. Martin - Architectural patterns
“Refactoring to Patterns” by Joshua Kerievsky - How to evolve toward patterns
“Game Programming Patterns” by Robert Nystrom - Patterns in practice (free online)
“Patterns of Enterprise Application Architecture” by Martin Fowler - Enterprise patterns

Online Resources

Refactoring.Guru - Excellent pattern catalog with examples
Source Making - Patterns with UML and code
Java Design Patterns - Comprehensive Java examples
Game Programming Patterns - Free book online

Practice Platforms

Design pattern coding challenges on LeetCode/HackerRank
Refactoring exercises (refactoring.guru)
Code review practice

Total Estimated Time: 6-9 months of dedicated study

After completion: You’ll be able to recognize patterns in any codebase, apply the right pattern at the right time, communicate effectively with other developers using pattern vocabulary, and design flexible, maintainable software architectures. Design patterns are a career-long investment that separates good developers from great ones.