Architecture Patterns

This skill provides knowledge about common architectural patterns to help design feature implementations. Apply these patterns based on the project's existing architecture and the feature's requirements.

Diagram Convention

Architecture visualizations use Mermaid syntax with classDef styling (color:#000 for text readability). When creating architecture visualizations based on these patterns, follow the technical-diagrams skill conventions.

Pattern Selection Guide

Choose patterns based on:

1. Existing architecture - Match what's already in use
2. Team familiarity - Use patterns the team knows
3. Feature requirements - Some patterns fit better for certain features
4. Scale requirements - Consider current and future scale

---

Layered Architecture (N-Tier)

When to use: Most web applications, CRUD operations, clear separation of concerns needed

Layers:

flowchart TD
    A["Presentation Layer — UI, API endpoints, controllers"]:::primary
    B["Application Layer — Use cases, orchestration, DTOs"]:::secondary
    C["Domain Layer — Business logic, entities, rules"]:::success
    D["Infrastructure Layer — Database, external services, I/O"]:::neutral
    A --> B --> C --> D

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
    classDef success fill:#dcfce7,stroke:#16a34a,color:#000
    classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

Key rules:

• Dependencies flow downward only
• Each layer only talks to the layer directly below
• Domain layer has no external dependencies

Implementation tips:

• Use interfaces at layer boundaries
• Keep domain logic in the domain layer, not controllers
• Use DTOs to transfer data between layers

---

MVC (Model-View-Controller)

When to use: Web applications with server-rendered views, simple CRUD apps

Components:

flowchart LR
    U1[User]:::neutral -->|input| CT[Controller]:::primary
    CT -->|updates| M[Model]:::secondary
    M -->|reads| CT
    CT -->|renders| V[View]:::success
    V -->|response| U2[User]:::neutral

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
    classDef success fill:#dcfce7,stroke:#16a34a,color:#000
    classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

Model: Data and business logic View: Presentation/UI Controller: Handles input, coordinates model and view

Implementation tips:

• Keep controllers thin - delegate to services
• Models should be framework-agnostic when possible
• Views should have minimal logic

---

Repository Pattern

When to use: Data access abstraction, testability, multiple data sources

Structure:

interface UserRepository {
  findById(id: string): Promise<User | null>;
  findByEmail(email: string): Promise<User | null>;
  save(user: User): Promise<User>;
  delete(id: string): Promise<void>;
}

class PostgresUserRepository implements UserRepository {
  // Implementation using PostgreSQL
}

class InMemoryUserRepository implements UserRepository {
  // Implementation for testing
}

Benefits:

• Abstracts data access details
• Easy to swap implementations
• Simplifies testing with in-memory implementations

---

Service Layer Pattern

When to use: Complex business logic, multiple entry points (API, CLI, queue)

Structure:

class UserService {
  constructor(
    private userRepo: UserRepository,
    private emailService: EmailService,
    private logger: Logger
  ) {}

  async registerUser(data: RegisterDTO): Promise<User> {
    // Validation
    // Business logic
    // Coordination of multiple repositories/services
    // Return result
  }
}

Implementation tips:

• Services contain business logic, not controllers
• One service per domain concept
• Services can call other services (but avoid cycles)

---

Event-Driven Architecture

When to use: Decoupled components, async processing, audit trails, notifications

Patterns:

Event Emitter (Simple)

// Emit events for side effects
userService.on('userCreated', async (user) => {
  await emailService.sendWelcome(user);
  await analyticsService.trackSignup(user);
});

Message Queue (Distributed)

flowchart LR
    P[Producer]:::primary --> Q[Queue]:::warning
    Q --> C1[Consumer 1]:::secondary
    Q --> C2[Consumer 2]:::secondary

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
    classDef warning fill:#fef3c7,stroke:#d97706,color:#000

Event structure:

interface DomainEvent {
  type: string;
  timestamp: Date;
  payload: unknown;
  metadata: {
    correlationId: string;
    causationId: string;
  };
}

Implementation tips:

• Events should be immutable
• Include enough context to process without additional queries
• Handle idempotency for at-least-once delivery

---

CQRS (Command Query Responsibility Segregation)

When to use: Complex domains, different read/write patterns, high-performance reads needed

Structure:

flowchart TD
    subgraph write["Commands (Write)"]
        CMD[Command]:::primary --> CH[Command Handler]:::secondary
        CH --> WM[Write Model]:::secondary
        WM --> WDB[Write Database]:::neutral
    end
    subgraph read["Queries (Read)"]
        QRY[Query]:::primary --> QH[Query Handler]:::secondary
        QH --> RM[Read Model]:::secondary
        RM --> RDB[Read Database]:::neutral
    end

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef secondary fill:#f3e8ff,stroke:#7c3aed,color:#000
    classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

    style write fill:#f8fafc,stroke:#94a3b8,color:#000
    style read fill:#f8fafc,stroke:#94a3b8,color:#000

Simplified CQRS:

// Commands modify state
class CreateUserCommand {
  execute(data: CreateUserDTO): Promise<void>
}

// Queries return data without modification
class GetUserQuery {
  execute(id: string): Promise<UserDTO>
}

Implementation tips:

• Start simple - same database, different models
• Use for complex domains where read/write models differ
• Consider eventual consistency implications

---

Ports and Adapters (Hexagonal)

When to use: High testability needs, multiple I/O channels, long-lived applications

Structure:

flowchart TD
    subgraph driving["Driving Adapters"]
        H[HTTP]:::primary
        CL[CLI]:::primary
        Q[Queue]:::primary
        T[Timer]:::primary
    end
    subgraph core["Application Core"]
        subgraph domain["Domain Logic"]
            DL[Business Rules]:::success
        end
    end
    subgraph driven["Driven Adapters"]
        DB[Database]:::neutral
        CA[Cache]:::neutral
        EM[Email]:::neutral
        API[External API]:::neutral
    end
    H -->|port| DL
    CL -->|port| DL
    Q -->|port| DL
    T -->|port| DL
    DL -->|port| DB
    DL -->|port| CA
    DL -->|port| EM
    DL -->|port| API

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef success fill:#dcfce7,stroke:#16a34a,color:#000
    classDef neutral fill:#f3f4f6,stroke:#6b7280,color:#000

    style driving fill:#f8fafc,stroke:#94a3b8,color:#000
    style core fill:#fefce8,stroke:#ca8a04,color:#000
    style domain fill:#dcfce7,stroke:#16a34a,color:#000
    style driven fill:#f8fafc,stroke:#94a3b8,color:#000

Key concept: Business logic at center, all I/O through ports/adapters

Implementation tips:

• Define ports (interfaces) for all external interactions
• Adapters implement ports for specific technologies
• Domain code never imports adapter code

---

Microservices Patterns

When to use: Large teams, independent deployability, different scaling needs

API Gateway

Single entry point that routes to services

Service Discovery

Services register themselves, clients look them up

Circuit Breaker

Prevent cascade failures when services are down

const breaker = new CircuitBreaker(remoteService.call, {
  timeout: 3000,
  errorThreshold: 50,
  resetTimeout: 30000
});

Saga Pattern

Coordinate transactions across services

flowchart LR
    SA[Service A]:::primary -->|step 1| SB[Service B]:::primary -->|step 2| SC[Service C]:::primary
    SC -.->|failure| CC[Compensate C]:::danger
    CC -.-> CB[Compensate B]:::danger
    CB -.-> CA[Compensate A]:::danger

    classDef primary fill:#dbeafe,stroke:#2563eb,color:#000
    classDef danger fill:#fee2e2,stroke:#dc2626,color:#000

---

Choosing the Right Pattern

Scenario	Recommended Pattern
Simple CRUD app	MVC + Repository
Complex business logic	Layered + Service Layer
Need audit trail	Event-Driven
High read/write disparity	CQRS
Maximum testability	Hexagonal
Multiple teams/services	Microservices patterns

Anti-Patterns to Avoid

1. Big Ball of Mud - No clear structure
2. God Object - One class does everything
3. Spaghetti Code - Tangled dependencies
4. Golden Hammer - Using one pattern for everything
5. Premature Optimization - Complex patterns for simple needs

Application Guidelines

1. Match existing architecture - Don't introduce new patterns unnecessarily
2. Start simple - Add complexity only when needed
3. Document decisions - Explain why a pattern was chosen
4. Consider team skills - A simpler pattern well-executed beats a complex one poorly understood