Monolith to Microservices - Migration Guide

Converting Monolith to Microservices

Complete migration guide for professional developers

Quick Summary (30 seconds):
Convert incrementally using the Strangler Fig Pattern. Never do big bang migration. Start with identifying bounded contexts, extract one service at a time, use anti-corruption layer, and gradually redirect traffic. The goal is zero downtime throughout migration.

Phase 1: Assessment and Planning

Duration: Weeks 1-4

1 Identify Bounded Contexts (DDD)
Look for:

Naturally isolated business capabilities
Teams already organized around functions
Database tables with clear ownership
Low cross-domain transaction frequency

2 Score Candidates for Extraction
Criteria:

Business value (high = extract early)
Technical complexity (low = extract early)
Independence from monolith (high = extract early)
Team ownership (clear = extract early)

3 Understand Dependencies
Tools:

Code analysis: JDepend, SonarQube
Runtime tracing: Distributed tracing tools
Database foreign key analysis
Team interviews for hidden dependencies

Phase 2: Strangler Fig Pattern Implementation

Step 1: Add Proxy Layer

// Route traffic through API Gateway
// /payments/* -> Payment Service
// /* -> Monolith (gradually migrate)

Step 2: Anti-Corruption Layer (ACL)

// Anti-Corruption Layer - Protects monolith from service changes
class PaymentAntiCorruptionLayer {
    
    function processPayment(monolithRequest) {
        // Transform from monolith model to service model
        serviceRequest = transformer.toServiceModel(monolithRequest);
        
        // Call service
        serviceResponse = paymentClient.process(serviceRequest);
        
        // Transform back to monolith model
        return transformer.toMonolithModel(serviceResponse);
    }
    
    // Fallback when service is down
    function fallbackPayment(monolithRequest) {
        return new PaymentResponse("PENDING", "Payment queued for processing");
    }
}

Phase 3: Data Migration Strategy

Dual Write Pattern

// Phase 1: Dual Write (Write to both)
function createOrder(request) {
    // Write to monolith database (legacy)
    order = monolithOrderRepository.save(createOrder(request));
    
    // Also write to new service via API
    asyncCall(() => {
        orderServiceClient.createOrder(order);
    });
    return order;
}

// Phase 2: Backfill Historical Data
function backfillHistoricalData() {
    oldOrders = monolithOrderRepository.findNotMigrated();
    for (order in oldOrders) {
        try {
            orderServiceClient.createOrder(order);
            order.setMigrated(true);
            monolithOrderRepository.save(order);
        } catch(error) {
            log.error("Failed to migrate order", order.id);
        }
    }
}

// Phase 3: Switch to New Service as Source of Truth
function createOrder(request) {
    // Write only to new service
    response = orderServiceClient.createOrder(request);
    return convert(response);
}

Example: E-Commerce Migration

Before - Monolith Structure

// MONOLITH: Single application with all features
class ECommerceMonolith {
    // Contains: Order, Payment, Inventory, Shipping, User, Review
}

// Single database with all tables
// orders, payments, inventory, shipments, users, reviews

// Tightly coupled code
class OrderService {
    
    function createOrder(request) {
        // All in one transaction
        inventoryService.reserveStock(request.getItem());
        paymentService.processPayment(request.getItem());
        order = orderRepository.save(createOrder(request));
        shippingService.createShipment(order);
        return order;
    }
}

After - Step 1: Extract Payment Service

// NEW PAYMENT SERVICE (Independent Microservice)
class PaymentServiceApplication {
    // Own database: payment_db
}

class PaymentController {
    function process(request) {
        return paymentService.process(request);
    }
}

// MODIFIED MONOLITH - Call Payment Service via HTTP
class OrderService {
    private paymentClient;
    
    function createOrder(request) {
        // Call Payment Service via API
        payment = paymentClient.process(request.getPayment());
        
        // Still in monolith
        inventoryService.reserveStock(request.getItems());
        order = orderRepository.save(createOrder(request));
        shippingService.createShipment(order);
        return order;
    }
}

Common Pitfalls and Solutions

Problem: Distributed Transactions
Symptom: Data inconsistency across services
Solution: Use Saga pattern with compensating transactions

Problem: Shared Database
Symptom: Tight coupling continues
Solution: Database per service + event-driven sync

Problem: Chatty APIs
Symptom: Too many network calls
Solution: API composition + GraphQL/BFF

Solution: Fallback Failures
Use Circuit Breaker + Timeout + Retry to prevent cascade failures

Solution: Migration Never Ends
Set hard deadline, incentivize completion

Migration Checklist

Phase	Key Activities	Success Criteria
Assessment	Identify bounded contexts, score candidates	Priority list of services to extract
Proxy Layer	Add API Gateway, route traffic	Zero downtime routing
Extract Service	Create new microservice, dual write	Service handles 100% traffic
Data Migration	Backfill data, switch source of truth	Legacy writes stopped
Decommission	Remove old code, shutdown monolith part	Monolith shrunk

Expert Insight:
"The Strangler Fig Pattern isn't just about technical migration - it's about business continuity. Each extracted service must provide value independently, so stakeholders see progress. The Anti-Corruption Layer is your most important safety net. Database migration is harder than code migration - use the Dual Write pattern."

Key Takeaways

Never do big bang migration - incremental is the only safe way
Strangler Fig Pattern is the industry standard approach
Anti-Corruption Layer protects your system during transition
Dual Write pattern ensures zero data loss
Set deadlines - without them, migration never completes
Extract high-value, low-complexity services first

Final Advice:
"Start with a monolith. Seriously. Extract services only when you have proven need - different scaling requirements, different teams, or different tech stacks. The goal isn't microservices; the goal is business agility."

Monolith To Microservices