CQRS (Command Query Responsibility Segregation) is a pattern that separates a system into two parts: one for handling commands and another for handling queries. This separation allows the system to optimize each part independently based on its specific needs.

CQRS builds on the Command Query Separation (CQS) principle, which states that methods should either be a command that changes the state of the system or a query that retrieves data from the system, but not both. CQRS extends this idea to separate the write and read models of a system.

In the context of DDD, CQRS can be applied to separate the commands that change the state of the domain model from the queries that retrieve data from the model. This separation allows the domain model to be optimized for each use case, as queries and commands have different performance and scalability requirements.

Here’s an example of how CQRS can be implemented in Java with DDD:

First, let’s define the domain model. Suppose we have an e-commerce application that needs to manage orders. We can define the following classes:

public class Order {
    private OrderId id;
    private List<OrderLine> lines;
    private Address shippingAddress;
    // ...
}

public class OrderLine {
    private ProductId productId;
    private int quantity;
    private Money price;
    // ...
}

public class Address {
    private String street;
    private String city;
    private String state;
    private String country;
    // ...
}

public class Money {
    private BigDecimal amount;
    private Currency currency;
    // ...
}

Next, we can define the write model that handles commands to change the state of the domain model:

<code>public class OrderService {
    private final OrderRepository orderRepository;

    public void createOrder(Order order) {
        orderRepository.save(order);
        // notify other services about the new order
    }

    public void cancelOrder(UUID orderId) {
        Order order = orderRepository.findById(orderId);
        order.cancel();
        orderRepository.save(order);
        // notify other services about the canceled order
    }
}
</code>

In this example, the OrderService handles commands to create and cancel orders. It uses the OrderRepository to persist the order in the database.

Finally, we can define the read model that handles queries to retrieve data from the domain model:

public class OrderQueryService {
    private final OrderRepository orderRepository;

    public List<OrderSummary> getOrderSummaries() {
        List<Order> orders = orderRepository.findAll();
        return orders.stream().map(OrderSummary::fromOrder).collect(Collectors.toList());
    }

    public OrderDetails getOrderDetails(UUID orderId) {
        Order order = orderRepository.findById(orderId);
        return OrderDetails.fromOrder(order);
    }
}

In this example, the OrderQueryService handles queries to retrieve order data. It uses the OrderRepository to retrieve the order from the database and maps the data to a different format suitable for the query.

Note that the write and read models use the same domain model classes, but they have different responsibilities and optimizations. The write model is optimized for consistency and data integrity, while the read model is optimized for performance and scalability.

Overall, CQRS can be a useful pattern to separate the concerns of commands and queries in a domain model, and it can be applied in conjunction with DDD to optimize the performance and scalability of the system.