Overview of C# CQRS (Command Query Responsibility Segregation)

CQRS (Command Query Responsibility Segregation) is a software architectural pattern that separates the handling of read (query) and write (command) operations for a data store, leading to optimised and decoupled solutions. In the context of C#, implementing CQRS often involves leveraging the language's object-oriented features, libraries, and frameworks like ASP.NET Core.

Core Concepts of CQRS

1. Segregation of Commands and Queries:
• Commands: Modify the application's state (e.g., creating, updating, or deleting data).
• Queries: Retrieve data without modifying it.

Each of these operations typically has its model and pathway in the system, avoiding a single model or service trying to do both.

2. Separate Models for Read and Write:
• Write Model: Handles the domain logic and is responsible for validating and processing commands.
• Read Model: Focuses on returning data optimised for client needs, often through simpler, denormalised models.
3. Event Sourcing (Optional):
• CQRS is often used alongside event sourcing, representing state changes as events stored in an event store. The system's current state can be reconstructed from these events.
4. Scalability:
• By separating the read and write operations, each side can be scaled independently, optimising performance and resource usage.

Benefits of CQRS in C#

1. Clear Separation of Concerns:
• Improves maintainability and reduces complexity by decoupling the read and write responsibilities.
2. Performance Optimisation:
• Queries can be optimised independently by leveraging caching, denormalised tables, or database views.
3. Flexibility for Complex Domains:
• CQRS shines in systems with complex business rules or scalability requirements, allowing each concern to evolve independently.
4. Enhanced Testing:
• Commands and queries are independently testable due to their separation.

Implementation in C#

1. Command Implementation

A command represents an intent to change the state. In C#, it is usually a simple class:

public class CreateOrderCommand

{

    public Guid OrderId { get; set; }

    public string CustomerName { get; set; }

    public List<OrderItem> Items { get; set; }

}

Command Handlers process commands:

public class CreateOrderCommandHandler : ICommandHandler<CreateOrderCommand>

{

    private readonly IOrderRepository _orderRepository;

    public CreateOrderCommandHandler(IOrderRepository orderRepository)

    {

        _orderRepository = orderRepository;

    }

    public async Task Handle(CreateOrderCommand command)

    {

        var order = new Order(command.OrderId, command.CustomerName, command.Items);

        await _orderRepository.AddAsync(order);

    }

}

2. Query Implementation

Queries retrieve data without modifying state. Similar to commands, they are represented by simple classes:

public class GetOrderByIdQuery

{

    public Guid OrderId { get; set; }

}

Query Handlers process queries:

public class GetOrderByIdQueryHandler : IQueryHandler<GetOrderByIdQuery, OrderDto>

{

    private readonly IOrderReadRepository _readRepository;

    public GetOrderByIdQueryHandler(IOrderReadRepository readRepository)

    {

        _readRepository = readRepository;

    }

    public async Task<OrderDto> Handle(GetOrderByIdQuery query)

    {

        return await _readRepository.GetOrderByIdAsync(query.OrderId);

    }

}

3. MediatR for CQRS (optional)

Using libraries like MediatR simplifies CQRS by abstracting the handler registration and dispatching:

• Command/Query Dispatching:

·         public class OrdersController : ControllerBase

·         {

·             private readonly IMediator _mediator;

·          

·             public OrdersController(IMediator mediator)

·             {

·                 _mediator = mediator;

·             }

·          

·             [HttpPost]

·             public async Task<IActionResult> CreateOrder(CreateOrderCommand command)

·             {

·                 await _mediator.Send(command);

·                 return Ok();

·             }

·          

·             [HttpGet("{id}")]

·             public async Task<ActionResult<OrderDto>> GetOrderById(Guid id)

·             {

·                 return Ok(await _mediator.Send(new GetOrderByIdQuery { OrderId = id }));

·             }

·         }

Best Practices

1. Keep Handlers Simple:
• Avoid mixing business logic in handlers. Delegate complex logic to domain services or entities.
2. Use Dependency Injection:
• Leverage DI in ASP.NET Core to manage services and repositories in handlers.
3. Optimise Read Models:
• Tailor read models to client requirements, ensuring they efficiently return the data in the desired format.
4. Consider Eventual Consistency:
• Be mindful that in distributed systems, read and write models might not always be immediately synchronised.
5. Logging and Monitoring:
• Track command and query execution for better debugging and monitoring.

When to Use CQRS

• Suitable for complex domains with high scalability or performance requirements.
• Ideal for systems with distinct read and write workloads.
• Overhead may not be justified for simple CRUD applications.

By using CQRS effectively in C#, developers can build scalable, maintainable, and optimised systems that cater to complex domain logic and high-performance requirements.