Implementing CQRS in .NET Web Applications

Introduction

Modern .NET web applications are becoming more complex than ever, and CQRS in .NET web applications offers a structured way to manage this complexity. As features grow and user traffic increases, handling read and write operations using the same models and logic can quickly lead to performance issues and tightly coupled code.

This is where CQRS in .NET web applications plays a crucial role. Command Query Responsibility Segregation (CQRS) introduces a clear separation between operations that change data and those that simply read it. By splitting these responsibilities, developers can design systems that are easier to scale, easier to maintain, and better optimised for real-world workloads.

In this blog, we’ll explore the CQRS pattern, its key benefits, and a practical, high-level approach to implementing CQRS in .NET web applications using simple examples.

What is CQRS in .NET Web Applications?

CQRS stands for Command Query Responsibility Segregation. At its core, it’s an architectural pattern that separates write operations (commands) from read operations (queries).

Instead of using the same models and logic for both reading and writing data, CQRS encourages you to treat them as two distinct responsibilities. This separation allows each side to be designed, optimized, and scaled independently.

Key Concepts

• Commands: Commands represent actions that change the state of the system. Examples include creating a product, updating details, or deleting a record. A command focuses on what should happen, not on returning data.
• Queries: Queries are used to fetch data without modifying the application’s state. Their sole purpose is to return information cleanly and efficiently.

Benefits of CQRS

1. Clear Separation of Concerns
   By splitting commands and queries, the application becomes easier to understand. Each part has a single responsibility, which reduces complexity and improves code clarity.
2. Better Scalability
   Read and write operations often have very different workloads. With CQRS, you can scale the read side independently from the write side—ideal for applications with heavy read traffic.
3. Optimized Performance
   Since commands and queries are separate, each can be optimized for its specific use case. For example, read operations might use a database optimized for fast queries, while writes focus on consistency and validation.
4. Improved Maintainability
   When logic is clearly separated, changes become safer and easier to implement. Developers can work on query logic without worrying about impacting write operations, and vice versa.
5. Architectural Flexibility
   CQRS allows you to use different data models or even different data stores for reads and writes, making it easier to adapt the system as requirements evolve.

Implementing CQRS in .NET web applications

Step-by-Step Overview

1. Project Setup: Begin by creating a .NET project, such as an ASP.NET Core Web API, which will serve as the foundation for implementing CQRS.
2. Define Domain Models: Create simple domain models that represent your core business data. For example, a Product entity with properties like Id, Name, and Price.
3. Create Commands and Queries: Define command classes for actions that modify data and query classes for data retrieval. These typically implement interfaces such as IRequest<T> when using libraries like MediatR.
4. Implement Handlers: Handlers contain the business logic for processing commands and queries. For instance, a CreateProductCommandHandler would manage the logic required to add a new product.
5. Register MediatR: Configure MediatR in your application to route commands and queries to their respective handlers efficiently.
6. Create Controllers: API controllers act as entry points. They send commands and queries through MediatR instead of directly calling business logic, keeping controllers thin and focused.
7. Testing: Use tools like Postman or Swagger to test your endpoints and verify that commands and queries behave as expected.

Minimal Code Example

Below is a simple example that illustrates the structure of a command and a query in a CQRS in .NET web applications. (Code intentionally kept minimal for clarity.)

// Command to create a product
public class CreateProductCommand : IRequest<int>
{
    public string Name { get; set; }
    public decimal Price { get; set; }
}

// Query to get a product
public class GetProductQuery : IRequest<Product>
{
    public int Id { get; set; }
}

Conclusion

CQRS is a powerful architectural pattern for building scalable and maintainable CQRS in .NET web applications. By separating commands and queries, you reduce complexity, improve performance, and gain the flexibility to evolve your system as business requirements change.

While CQRS may not be necessary for every application, it becomes extremely valuable as .NET web applications grow in size and complexity. With the right approach and tooling, CQRS in .NET web applications enables developers to build cleaner, more resilient solutions that are easier to scale, maintain, and extend over time.

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