Service Decomposition Strategies

// Business capabilities → services

// Capability: Order Management
// Service: order-service
// Owns: orders DB table, order lifecycle state machine

// Capability: Customer Management
// Service: customer-service
// Owns: customers DB, profile management

// Capability: Billing
// Service: billing-service
// Owns: invoices, payment records

// Anti-pattern — shared database across services:
// order-service → JOIN customers → violates service independence
// ✗ SELECT o.*, c.email FROM orders o JOIN customers c ON o.customer_id = c.id

// Correct — order-service calls customer-service API:
// ✓ GET /customers/{id} → returns CustomerDTO to order-service

// Bounded contexts in an e-commerce platform

// "Order" context — order-service
// Ubiquitous language: Order, LineItem, Fulfilment, ShipmentStatus

// "Catalogue" context — catalogue-service
// Ubiquitous language: Product, Variant, Inventory, PriceList
// Note: "Product" means something different in each context!

// Context Map — how they integrate
// ① Conformist: order-service conforms to catalogue-service's Product model
// ② Anti-Corruption Layer (ACL): order-service translates catalogue concepts
//    into its own domain model (LineItem, SKU) via an adapter

// ACL example
public class CatalogueAdapter {
    private final CatalogueClient catalogueClient;

    public LineItemDetails getLineItemDetails(String sku) {
        ProductDTO product = catalogueClient.getProduct(sku);  // external model
        return new LineItemDetails(                             // internal model
            product.getSku(),
            product.getDisplayName(),
            Money.of(product.getPriceCents(), "GBP")
        );
    }
}

# Step 1 — route new capability to microservice via API gateway (nginx example)
# Old path goes to monolith; new /api/v2/orders goes to order-service

location /api/v2/orders {
    proxy_pass http://order-service:8080;
}

location / {
    proxy_pass http://monolith:8080;   # all other traffic still hits the monolith
}

# Step 2 — strangler: migrate /api/v1/orders from monolith to order-service
# Deploy order-service, run dual-write, switch routing, then deprecate monolith endpoint

# Step 3 — extract customer capability
# Build customer-service, migrate data, update routing

# Eventually the monolith handles nothing and is decommissioned