How I Migrated from Monolith to Microservices: Lessons Learned

During my time as a senior developer, I had the opportunity to lead a complex migration from a monolithic application to a microservices architecture. In this post, I share the main lessons learned and strategies that worked (and some that didn't).

The Context

Our monolithic application had grown significantly over 3 years. With more than 100,000 lines of code and a team of 8 developers, we started facing some problems:

• Slow deployments (45+ minutes)
• Difficulty scaling specific features
• High coupling between modules
• Slow development cycles

The Migration Strategy

1. Analysis and Planning

Before starting the migration, we did a detailed analysis of the application, grouping the monolith's tables and routes into the domains they actually served:

• Authentication — users, sessions and permissions
• Products — catalog, inventory and pricing
• Orders — orders, payment and shipping
• Notifications — email, SMS and push

The criterion was not the shape of the code but what changed together: if two tables were always modified in the same release, they belonged to the same domain.

2. Strangler Fig Pattern

We chose the "Strangler Fig" pattern, extracting services gradually:

# API Gateway configured to route requests
def route_request(path, method):
    if path.startswith('/api/v2/auth'):
        return route_to_auth_service(path, method)
    elif path.startswith('/api/v2/products'):
        return route_to_product_service(path, method)
    else:
        return route_to_legacy_monolith(path, method)

Main Challenges

1. Data Management

One of the biggest challenges was dealing with shared data:

-- Before: shared tables
SELECT u.name, o.total, p.title
FROM users u
JOIN orders o ON u.id = o.user_id
JOIN products p ON o.product_id = p.id;

-- After: aggregation via API
// We need to make multiple calls
const user = await userService.getUser(userId);
const orders = await orderService.getUserOrders(userId);
const products = await productService.getProducts(productIds);

2. Distributed Transactions

We implemented the Saga pattern for transactions that crossed multiple services:

class OrderSaga {
  async execute(orderData) {
    try {
      const payment = await this.paymentService.processPayment(orderData.payment);
      const inventory = await this.inventoryService.reserveItems(orderData.items);
      const order = await this.orderService.createOrder(orderData);

      return { success: true, orderId: order.id };
    } catch (error) {
      await this.compensate(error.failedStep);
      throw error;
    }
  }

  async compensate(failedStep) {
    // Compensation logic based on the failed step
  }
}

Solutions That Worked

1. Observability from the Start

We implemented structured logging and distributed tracing:

// Example of log correlation
const correlationId = req.headers['x-correlation-id'] || uuid();

logger.info('Processing request', {
  correlationId,
  service: 'product-service',
  action: 'getProduct',
  productId: req.params.id
});

2. Contract Testing

We used Pact to ensure compatibility between services:

// Consumer test
it('should get product details', async () => {
  await provider
    .given('product exists')
    .uponReceiving('a request for product details')
    .withRequest({
      method: 'GET',
      path: '/products/123'
    })
    .willRespondWith({
      status: 200,
      body: {
        id: 123,
        name: 'Product Name',
        price: 99.99
      }
    });
});

Results

After 8 months of gradual migration, we achieved:

• Deployment time: 45min → 5min
• Time to market: 3 weeks → 1 week
• Availability: 99.5% → 99.9%
• Scalability: Critical services can scale independently

Lessons Learned

1. Start small: Extract simple services first
2. Invest in observability: It's impossible to debug without visibility
3. Automate everything: CI/CD is essential with multiple services
4. Communication patterns: Prefer async when possible
5. Data ownership: Each service should own its data

Conclusion

Migration to microservices is not a silver bullet, but when well executed, it can bring significant benefits for growing teams and products. The secret lies in careful planning and gradual execution.

If you're considering a similar migration, remember: focus on the problems you're trying to solve, not the technology itself.

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Have experience with microservices? Share your insights in the comments or contact me on LinkedIn.