architecture-patterns

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# Architecture Patterns

Master proven backend architecture patterns including Clean Architecture, Hexagonal Architecture, and Domain-Driven Design to build maintainable, testable, and scalable systems.

## When to Use This Skill

- Designing new backend systems from scratch
- Refactoring monolithic applications for better maintainability
- Establishing architecture standards for your team
- Migrating from tightly coupled to loosely coupled architectures
- Implementing domain-driven design principles
- Creating testable and mockable codebases
- Planning microservices decomposition

## Core Concepts

### 1. Clean Architecture (Uncle Bob)

**Layers (dependency flows inward):**

- **Entities**: Core business models
- **Use Cases**: Application business rules
- **Interface Adapters**: Controllers, presenters, gateways
- **Frameworks & Drivers**: UI, database, external services

**Key Principles:**

- Dependencies point inward
- Inner layers know nothing about outer layers
- Business logic independent of frameworks
- Testable without UI, database, or external services

### 2. Hexagonal Architecture (Ports and Adapters)

**Components:**

- **Domain Core**: Business logic
- **Ports**: Interfaces defining interactions
- **Adapters**: Implementations of ports (database, REST, message queue)

**Benefits:**

- Swap implementations easily (mock for testing)
- Technology-agnostic core
- Clear separation of concerns

### 3. Domain-Driven Design (DDD)

**Strategic Patterns:**

- **Bounded Contexts**: Separate models for different domains
- **Context Mapping**: How contexts relate
- **Ubiquitous Language**: Shared terminology

**Tactical Patterns:**

- **Entities**: Objects with identity
- **Value Objects**: Immutable objects defined by attributes
- **Aggregates**: Consistency boundaries
- **Repositories**: Data access abstraction
- **Domain Events**: Things that happened

## Clean Architecture Pattern

### Directory Structure

```text
app/
├── domain/           # Entities & business rules
│   ├── entities/
│   │   ├── user.py
│   │   └── order.py
│   ├── value_objects/
│   │   ├── email.py
│   │   └── money.py
│   └── interfaces/   # Abstract interfaces
│       ├── user_repository.py
│       └── payment_gateway.py
├── use_cases/        # Application business rules
│   ├── create_user.py
│   ├── process_order.py
│   └── send_notification.py
├── adapters/         # Interface implementations
│   ├── repositories/
│   │   ├── postgres_user_repository.py
│   │   └── redis_cache_repository.py
│   ├── controllers/
│   │   └── user_controller.py
│   └── gateways/
│       ├── stripe_payment_gateway.py
│       └── sendgrid_email_gateway.py
└── infrastructure/   # Framework & external concerns
    ├── database.py
    ├── config.py
    └── logging.py
```

### Implementation Example

```python
# domain/entities/user.py
from dataclasses import dataclass
from datetime import datetime
from typing import Optional

@dataclass
class User:
    """Core user entity - no framework dependencies."""
    id: str
    email: str
    name: str
    created_at: datetime
    is_active: bool = True

    def deactivate(self):
        """Business rule: deactivating user."""
        self.is_active = False

    def can_place_order(self) -> bool:

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