CQRS Architectural Pattern: Command Query Responsibility Segregation Explained.

// 1. COMMAND SIDE (Write Operations)
// Focuses on business rules, validation, and state modification
package com.example.cqrs.command;

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

@Service
public class OrderCommandService {

    private final OrderRepository orderRepository;
    private final EventPublisher eventPublisher;

    public OrderCommandService(OrderRepository orderRepository, EventPublisher eventPublisher) {
        this.orderRepository = orderRepository;
        this.eventPublisher = eventPublisher;
    }

    @Transactional
    public Long handle(CreateOrderCommand command) {
        // 1. Validate business rules & stock availability
        if (command.items().isEmpty()) {
            throw new IllegalArgumentException("Order must have at least one item");
        }

        // 2. Persist state changes to Write Database
        Order order = new Order(command.customerId(), command.items());
        Order savedOrder = orderRepository.save(order);

        // 3. Publish event for Query synchronization
        eventPublisher.publish(new OrderCreatedEvent(savedOrder.getId(), savedOrder.getTotal()));

        return savedOrder.getId();
    }
}

// 2. QUERY SIDE (Read Operations)
// Optimized for lightning-fast reads, denormalized views, and zero join overhead
package com.example.cqrs.query;

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

@Service
@Transactional(readOnly = true)
public class OrderQueryService {

    private final OrderReadModelRepository readRepo;

    public OrderQueryService(OrderReadModelRepository readRepo) {
        this.readRepo = readRepo;
    }

    // Fetches pre-aggregated, denormalized summary directly (e.g. from Read-Replica or Elasticsearch)
    public OrderSummaryView getOrderSummary(Long orderId) {
        return readRepo.findSummaryById(orderId)
                .orElseThrow(() -> new OrderNotFoundException(orderId));
    }

    public List<OrderDashboardDTO> getCustomerDashboard(Long customerId) {
        return readRepo.findCustomerDashboard(customerId);
    }
}