Architecture Decision Guide

Technology Selection Matrix

Backend Framework Selection

Decision criteria:

• Team expertise: Choose familiar stack
• Performance needs: Go/Fastify for high throughput
• Type safety: NestJS/FastAPI for TypeScript/Python
• Flexibility: Express for custom architectures

Frontend Framework Selection

Decision criteria:

• SEO requirements: Next.js/Nuxt for SSR
• Team size: Angular for large teams, Vue for small
• Ecosystem: React for maximum third-party support
• Performance: Svelte for minimal bundle size

Database Selection

Decision criteria:

• ACID requirements: PostgreSQL/MySQL
• Flexible schemas: MongoDB
• Caching layer: Redis (always)
• AWS serverless: DynamoDB
• Default choice: PostgreSQL (most versatile)

State Management (Frontend)

Decision criteria:

• Simple app: Context or Zustand
• Complex state logic: Redux Toolkit
• Server state: TanStack Query (don't use global state)
• Real-time apps: Zustand + WebSocket

Monolith vs Microservices

Decision Matrix

When to Use Monolith

✓ Starting new product (validate idea first)
✓ Small team (< 10 developers)
✓ Simple domain with few bounded contexts
✓ Need rapid development
✓ Limited infrastructure budget
✓ Straightforward deployment requirements

When to Use Microservices

✓ Large team (> 10 developers)
✓ Clear bounded contexts in domain
✓ Different services have different scaling needs
✓ Need independent deployment cycles
✓ Multiple teams working independently
✓ Polyglot requirements (different languages)
✓ Have DevOps expertise and infrastructure

Modular Monolith (Recommended Middle Ground)

// Structure monolith with clear boundaries
project/
├── src/
│   ├── modules/
│   │   ├── users/
│   │   │   ├── users.module.ts
│   │   │   ├── users.service.ts
│   │   │   ├── users.controller.ts
│   │   │   └── users.repository.ts
│   │   ├── orders/
│   │   │   ├── orders.module.ts
│   │   │   └── ...
│   │   └── payments/
│   │       └── ...
│   └── shared/
│       ├── database/
│       └── auth/

// Clear module boundaries, can split later if needed

API Architecture Patterns

REST vs GraphQL

Choose REST when:

• Building simple CRUD APIs
• Need HTTP caching
• Public API with many consumers
• Team unfamiliar with GraphQL

Choose GraphQL when:

• Mobile apps need flexible queries
• Complex data requirements
• Rapid frontend iteration
• Real-time subscriptions needed

BFF Pattern (Backend for Frontend)

// Use when frontend needs differ from backend APIs
// Mobile BFF: Returns minimal data, optimized responses
@Controller('mobile-bff')

  @Get('dashboard')
  async getMobileDashboard(@CurrentUser() user: User) {
    const [profile, notifications] = await Promise.all([
      this.userService.getProfile(user.id),
      this.notificationService.getUnread(user.id, 5), // Only 5 for mobile
    ]);
    return { profile, notifications }; // Minimal payload
  }
}

// Web BFF: Returns richer data
@Controller('web-bff')

  @Get('dashboard')
  async getWebDashboard(@CurrentUser() user: User) {
    const [profile, notifications, analytics, recentActivity] = await Promise.all([
      this.userService.getProfile(user.id),
      this.notificationService.getUnread(user.id, 20), // More for web
      this.analyticsService.getUserStats(user.id),
      this.activityService.getRecent(user.id),
    ]);
    return { profile, notifications, analytics, recentActivity };
  }
}

Authentication Strategy

JWT vs Session-based

Hybrid approach (Recommended):

// Short-lived access token (15min) + refresh token (7 days)
interface AuthTokens {
  accessToken: string;   // JWT, 15 minutes, stored in memory
  refreshToken: string;  // Opaque token, 7 days, httpOnly cookie
}

// Access token: Stateless, fast validation
// Refresh token: Stored in DB, can be revoked

SSO Integration Options

Caching Strategy

Layered Caching Approach

// Layer 1: CDN caching (static assets)
// CloudFront, Cloudflare

// Layer 2: API response caching (Redis)
const cacheKey = `user:${userId}:profile`;
let profile = await redis.get(cacheKey);

if (!profile) {
  profile = await db.users.findById(userId);
  await redis.setex(cacheKey, 300, JSON.stringify(profile)); // 5 min TTL
}

// Layer 3: Database query caching
// PostgreSQL prepared statements, query plan caching

// Layer 4: Application-level caching
const userCache = new LRU({ max: 1000 });

Cache Invalidation Patterns

// Write-through: Update cache on write
async updateUser(id: string, data: UpdateUserDto) {
  const user = await db.users.update(id, data);
  await redis.set(`user:${id}`, JSON.stringify(user), 'EX', 300);
  return user;
}

// Write-behind: Invalidate cache, lazy load
async updateUser(id: string, data: UpdateUserDto) {
  const user = await db.users.update(id, data);
  await redis.del(`user:${id}`); // Delete, will reload on next read
  return user;
}

// Event-based: Invalidate related caches
eventBus.on('user.updated', async ({ userId }) => {
  await Promise.all([
    redis.del(`user:${userId}`),
    redis.del(`user:${userId}:posts`),
    redis.del(`user:${userId}:followers`),
  ]);
});

Deployment Strategy

Environment Progression

Development → Staging → Production

Development:
- Local dev servers
- Docker Compose for dependencies
- Hot reload enabled
- Debug logging
- Relaxed security

Staging:
- Production-like environment
- Real integrations (test mode)
- E2E tests run here
- Performance testing
- Security scanning

Production:
- High availability setup
- Blue-green deployment
- Monitoring & alerting
- Automated rollback
- Strict security

Deployment Patterns

Quick Decision Trees

"Which database should I use?"

Need ACID transactions? → PostgreSQL
NoSQL with flexible schema? → MongoDB
Caching/sessions/queues? → Redis
AWS serverless? → DynamoDB
High read throughput? → PostgreSQL + read replicas

"Monolith or microservices?"

New product? → Modular monolith
Team < 10 people? → Modular monolith
Clear bounded contexts? → Consider microservices
Different scaling needs? → Microservices
Limited DevOps resources? → Monolith

"REST or GraphQL?"

Simple CRUD? → REST
Mobile app with flexible queries? → GraphQL
Public API? → REST
Complex data requirements? → GraphQL
Team knows GraphQL? → GraphQL, otherwise REST

"Which state management?"

Simple app, few global state? → React Context
Server state (API data)? → TanStack Query
Medium complexity? → Zustand
Complex state logic? → Redux Toolkit
Real-time updates? → Zustand + WebSocket