Explain what the abbreviation CQRS means and how it is used in system architecture.

CQRS (Command Query Responsibility Segregation) is an architectural pattern that separates operations that change state (commands) from operations that read data (queries).

Main idea:

• Commands are operations that modify the system's state (e.g., create, update, delete data).
• Queries are operations that only read data without changing it.

Using CQRS allows:

• Optimizing read and write operations separately, using different data models or even different databases.
• Simplifying scaling, as read and write operations can scale independently.
• Improving security and control by segregating permissions for commands and queries.

Example in C#:

// Command
public class CreateOrderCommand {
    public int ProductId { get; set; }
    public int Quantity { get; set; }
}

// Command handler
public void Handle(CreateOrderCommand command) {
    // Logic for creating an order
}

// Query
public class GetOrderQuery {
    public int OrderId { get; set; }
}

// Query handler
public OrderDto Handle(GetOrderQuery query) {
    // Logic for retrieving order data
}

Thus, CQRS helps to separate responsibilities and increase architectural flexibility.