Command and Query Responsibility Segregation (CQRS) Pattern

Comments

Segregate operations that read data from operations that update data by using separate interfaces. This pattern can maximize performance, scalability, and security; support evolution of the system over time through higher flexibility; and prevent update commands from causing merge conflicts at the domain level.

Code

Read Model

// Query interface
namespace ReadModel
{
  public interface ProductsDao
  {
    ProductDisplay FindById(int productId);
    IEnumerable FindByName(string name);
    IEnumerable FindOutOfStockProducts();
    IEnumerable FindRelatedProducts(int productId);
  }

  public class ProductDisplay
  {
    public int ID { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public decimal UnitPrice { get; set; }
    public bool IsOutOfStock { get; set; }
    public double UserRating { get; set; }
  }

  public class ProductInventory
  {
    public int ID { get; set; }
    public string Name { get; set; }
    public int CurrentStock { get; set; }
  }
}

The system allows users to rate products. The application code does this by using the RateProduct command shown in the following code.

public interface Icommand
{
  Guid Id { get; }
}

public class RateProduct : Icommand
{
  public RateProduct()
  {
    this.Id = Guid.NewGuid();
  }
  public Guid Id { get; set; }
  public int ProductId { get; set; }
  public int rating { get; set; }
  public int UserId {get; set; }
}

The system uses the ProductsCommandHandler class to handle commands sent by the application. Clients typically send commands to the domain through a messaging system such as a queue. The command handler accepts these commands and invokes methods of the domain interface. The granularity of each command is designed to mitigate the chance of conflicting requests. The following code shows an outline of theProductsCommandHandler class.

public class ProductsCommandHandler : 
    ICommandHandler,
    ICommandHandler,
    ICommandHandler,
    ICommandHandler,
    ICommandHandler    
{
  private readonly IRepository repository;

  public ProductsCommandHandler (IRepository repository)
  {
    this.repository = repository;
  }

  void Handle (AddNewProduct command)
  {
    ...
  }

  void Handle (RateProduct command)
  {
    var product = repository.Find(command.ProductId);
    if (product != null)
    {
      product.RateProuct(command.UserId, command.rating);
      repository.Save(product);
    }
  }

  void Handle (AddToInventory command)
  {
    ...
  }

  void Handle (ConfirmItemsShipped command)
  {
    ...
  }

  void Handle (UpdateStockFromInventoryRecount command)
  {
    ...
  }
}

Write Model

The following code shows the ProductsDoman interface from the write model.

public interface ProductsDomain
{
  void AddNewProduct(int id, string name, string description, decimal price);
  void RateProduct(int userId int rating);
  void AddToInventory(int productId, int quantity);
  void ConfirmItemsShipped(int productId, int quantity);
  void UpdateStockFromInventoryRecount(int productId, int updatedQuantity);
}

Also notice how the ProductsDomain interface contains methods that have a meaning in the domain. Typically, in a CRUD environment these methods would have generic names such as Save or Update, and have a DTO as the only argument. The CQRS approach can be better tailored to suit the way that this organization carries out business and inventory management.