Monolith to Microservices Migration

Project Overview

Led the migration of a monolithic e-commerce platform serving 100K+ daily active users to a microservices architecture. The project took 6 months and involved breaking down the monolith into 8 independent services.

Initial State

The legacy system had several critical issues:

• Monolithic Architecture: Single codebase with 500K+ lines of code
• Deployment Risk: Any update required full system deployment
• Scaling Limitations: Couldn't scale individual features independently
• Technology Lock-in: Stuck with outdated PHP framework

Migration Strategy

Phase 1: Service Identification

Used Domain-Driven Design to identify bounded contexts:

// Example: Order Service
@Module({
  imports: [
    MongooseModule.forFeature([{ name: Order.name, schema: OrderSchema }]),
    ClientsModule.register([
      {
        name: 'PAYMENT_SERVICE',
        transport: Transport.RMQ,
        options: {
          urls: [process.env.RABBITMQ_URL],
          queue: 'payment_queue',
        },
      },
    ]),
  ],
  controllers: [OrderController],
  providers: [OrderService],
})
export class OrderModule {}

Phase 2: Event-Driven Communication

Implemented async messaging for inter-service communication:

@Injectable()
export class OrderService {
  constructor(
    @InjectModel(Order.name) private orderModel: Model<Order>,
    @Inject('PAYMENT_SERVICE') private paymentClient: ClientProxy,
  ) {}

  async createOrder(createOrderDto: CreateOrderDto) {
    const order = await this.orderModel.create(createOrderDto);
    
    // Emit event to payment service
    this.paymentClient.emit('order_created', {
      orderId: order.id,
      amount: order.totalAmount,
      userId: order.userId,
    });
    
    return order;
  }
}

Phase 3: API Gateway

Created a unified entry point:

@Controller('api')
export class GatewayController {
  constructor(
    @Inject('ORDER_SERVICE') private orderService: ClientProxy,
    @Inject('USER_SERVICE') private userService: ClientProxy,
    @Inject('PRODUCT_SERVICE') private productService: ClientProxy,
  ) {}

  @Get('orders/:id')
  async getOrder(@Param('id') id: string) {
    return this.orderService.send({ cmd: 'get_order' }, id);
  }
}

Microservices Architecture

Final architecture with 8 independent services:

1. API Gateway - Entry point, authentication, routing
2. User Service - User management, profiles
3. Product Service - Product catalog, inventory
4. Order Service - Order processing, fulfillment
5. Payment Service - Payment processing, transactions
6. Notification Service - Email, SMS, push notifications
7. Analytics Service - Metrics, reporting
8. Search Service - Product search, Elasticsearch integration

Results

• ✅ Deployment time: 45 minutes → 5 minutes (89% improvement)
• ✅ System uptime: 99.2% → 99.8%
• ✅ Feature velocity: 2x faster development cycles
• ✅ Independent scaling: Each service scales based on demand
• ✅ Technology flexibility: Teams can choose best tools per service

Challenges & Solutions

Challenge 1: Data Consistency

Problem: Maintaining consistency across distributed databases Solution: Implemented Saga pattern for distributed transactions

// Saga coordinator for order processing
export class OrderSaga {
  async execute(order: Order) {
    try {
      await this.reserveInventory(order);
      await this.processPayment(order);
      await this.confirmOrder(order);
    } catch (error) {
      await this.compensate(order, error);
    }
  }
}

Challenge 2: Service Discovery

Problem: Services need to find each other dynamically Solution: Used Kubernetes service discovery with DNS

Challenge 3: Monitoring

Problem: Distributed logging and tracing Solution: Implemented ELK stack + Jaeger for distributed tracing

Key Learnings

1. Start Small: Migrate one service at a time, not big bang
2. Event-Driven is Key: Async messaging reduces coupling
3. API Gateway Pattern: Essential for client communication
4. Observability First: Set up monitoring before migration
5. Team Autonomy: Each team owns their service end-to-end

Technology Decisions

Why NestJS?

• TypeScript support for type safety
• Built-in microservices support
• Modular architecture
• Easy testing and documentation

Why RabbitMQ?

• Reliable message delivery
• Multiple messaging patterns
• Easy to monitor and manage
• Good performance for our scale

Why MongoDB?

• Flexible schema per service
• Horizontal scalability
• Good performance for read-heavy workloads