Microservices Architecture

Service Design Principles

Single Responsibility

Each service owns one business capability:

• User Service - Authentication, profiles
• Order Service - Orders, checkout
• Inventory Service - Stock management
• Payment Service - Payments, refunds

Database per Service

┌──────────────┐    ┌──────────────┐
│ User Service │    │ Order Service│
└──────┬───────┘    └──────┬───────┘
       │                   │
   ┌───┴───┐           ┌───┴───┐
   │User DB│           │Order DB│
   └───────┘           └────────┘

Communication Patterns

Synchronous (REST/gRPC)

// REST call
const user = await fetch(`${USER_SERVICE}/users/${userId}`);

// gRPC (faster, typed)
const user = await userClient.getUser({ userId });

Asynchronous (Events)

// Publisher
await messageQueue.publish('order.created', {
  orderId: order.id,
  userId: order.userId,
  items: order.items
});

// Subscriber (Inventory Service)
messageQueue.subscribe('order.created', async (event) => {
  await reserveInventory(event.items);
});

API Gateway

Client → API Gateway → Services
              │
              ├── Authentication
              ├── Rate Limiting
              ├── Load Balancing
              └── Request Routing

Service Discovery

// Kubernetes - DNS-based
const userService = 'http://user-service.default.svc.cluster.local';

// Consul/Eureka - Registry-based
const userService = await serviceRegistry.getService('user-service');

Resilience Patterns

Circuit Breaker

const breaker = new CircuitBreaker(fetchUser, {
  failureThreshold: 5,
  resetTimeout: 30000
});

const user = await breaker.fire(userId);

Retry with Backoff

async function withRetry<T>(fn: () => Promise<T>, maxRetries = 3): Promise<T> {
  for (let i = 0; i < maxRetries; i++) {
    try {
      return await fn();
    } catch (error) {
      if (i === maxRetries - 1) throw error;
      await sleep(Math.pow(2, i) * 1000);
    }
  }
}

Data Consistency

Saga Pattern

1. Create Order (Order Service)
2. Reserve Inventory (Inventory Service)
3. Process Payment (Payment Service)
4. Confirm Order (Order Service)

If step fails → Compensating transactions