Example of CQRS pattern

The following code shows some extracts from an example of a CQRS implementation that uses different definitions for the read and the write models. The model interfaces don't dictate any features of the underlying data stores, and they can evolve and be fine-tuned independently because these interfaces are separated. The following code shows the read model definition.

// Query interface
namespace ReadModel
{
  public interface ProductsDao
  {
    ProductDisplay FindById(int productId);
    ICollection FindByName(string name);
    ICollection FindOutOfStockProducts();
    ICollection 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 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 reduce the chance of conflicting requests. The following code shows an outline of the ProductsCommandHandler 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.RateProduct(command.UserId, command.Rating);
      repository.Save(product);
    }
  }

  void Handle (AddToInventory command)
  {
    ...
  }

  void Handle (ConfirmItemsShipped command)
  {
    ...
  }

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