Monolith vs Service Extraction

Monolithic Architecture

Magento 2 Monolith:
├── Catalog Module
├── Checkout Module
├── Order Module
├── Customer Module
├── Admin Module
└── All deployed as single unit

Pros:
├── Simple deployment
├── Easy debugging
├── No network overhead
├── ACID transactions
└── Simple testing

Cons:
├── Scaling entire application
├── Tight coupling
├── Large codebase
├── Long build times
└── Single point of failure

Microservices Architecture

Extracted Services:
├── Catalog Service (Products, Categories)
├── Inventory Service (Stock, Reservations)
├── Order Service (Orders, Fulfillment)
├── Payment Service (Payments, Refunds)
├── Customer Service (Accounts, Addresses)
└── Notification Service (Email, SMS)

Pros:
├── Independent scaling
├── Technology flexibility
├── Fault isolation
├── Team autonomy
└── Smaller deployments

Cons:
├── Network complexity
├── Distributed transactions
├── Service discovery
├── Data consistency
└── Operational overhead

Keep Monolith When:

1. Early stage startup
   ├── Small team (< 10 developers)
   ├── Simple domain
   └── Limited traffic

2. Performance critical paths
   ├── Checkout flow
   ├── Payment processing
   └── Real-time inventory

3. Strong consistency needed
   ├── Order processing
   ├── Inventory updates
   └── Payment transactions

4. Simple deployment
   ├── Single server
   ├── Limited DevOps
   └── Fast iteration needed

Extract Services When:

1. Scaling independently
   ├── Search: High read, low write
   ├── Catalog: High read, medium write
   ├── Orders: Medium read, medium write
   └── Inventory: Low read, high write

2. Technology requirements
   ├── Search: Elasticsearch
   ├── Recommendations: ML/Python
   ├── Real-time: WebSockets
   └── Analytics: Big data

3. Team boundaries
   ├── Catalog team
   ├── Checkout team
   ├── Fulfillment team
   └── Each owns their service

4. Failure isolation
   ├── If search fails, checkout works
   ├── If notifications fail, orders work
   └── If analytics fails, core works

Strangler Fig Pattern

// Gradually replace monolith with services
public function getOrder($id)
{
    // Check if order should come from service
    if ($this->featureFlag->isEnabled('order_service')) {
        try {
            return $this->orderServiceClient->get($id);
        } catch (\Exception $e) {
            // Fallback to monolith
            $this->logger->warning('Order service failed, using monolith');
        }
    }
    
    // Monolith implementation
    return $this->orderRepository->getById($id);
}

Domain-Driven Design

Identify bounded contexts:

1. Catalog Context
   ├── Products
   ├── Categories
   ├── Attributes
   └── Search

2. Inventory Context
   ├── Stock levels
   ├── Reservations
   └── Sources

3. Order Context
   ├── Orders
   ├── Line items
   └── Status

4. Payment Context
   ├── Payments
   ├── Refunds
   └── Gateway

Each context → potential service boundary

API Gateway

Client Request Flow:

Client → API Gateway → Catalog Service
                     → Inventory Service
                     → Order Service
                     → Payment Service

API Gateway responsibilities:
├── Request routing
├── Authentication
├── Rate limiting
├── Load balancing
└── Response aggregation
``

Database per Service

Catalog Service:
├── catalog_product
├── catalog_category
└── catalog_product_index

Inventory Service:
├── inventory_stock
├── inventory_reservation
└── inventory_source

Order Service:
├── sales_order
├── sales_order_item
└── sales_order_status

Problem: Cross-service queries
Solution: API composition, event sourcing

Data Consistency

// Saga pattern for distributed transactions
class OrderSaga
{
    public function createOrder($orderData)
    {
        try {
            // 1. Reserve inventory
            $this->inventoryService->reserve($orderData);
            
            // 2. Process payment
            $this->paymentService->charge($orderData);
            
            // 3. Create order
            $this->orderService->create($orderData);
            
            // 4. Send confirmation
            $this->notificationService->send($orderData);
            
        } catch (\Exception $e) {
            // Compensating transactions
            $this->compensate($orderData, $e);
        }
    }
    
    private function compensate($orderData, $error)
    {
        // Reverse operations in reverse order
        $this->orderService->cancel($orderData);
        $this->paymentService->refund($orderData);
        $this->inventoryService->release($orderData);
    }
}

Event-Driven Communication

// Publish events for cross-service communication
class OrderService
{
    public function createOrder($orderData)
    {
        $order = $this->orderRepository->create($orderData);
        
        // Publish event
        $this->eventBus->publish('order.created', [
            'order_id' => $order->getId(),
            'items' => $order->getItems(),
            'total' => $order->getTotal()
        ]);
    }
}

// Subscribe to events
class InventorySubscriber
{
    public function onOrderCreated($event)
    {
        $this->inventoryService->reserve($event->getItems());
    }
}

// Notification subscriber
class NotificationSubscriber
{
    public function onOrderCreated($event)
    {
        $this->emailService->sendConfirmation($event->getOrderId());
    }
}