Exploring CQRS and MediatR in Software Development

CQRS (Command Query Responsibility Segregation) and MediatR are architectural patterns commonly used in software development, particularly in the context of building complex applications. While they are not directly related to each other, they can be used together to achieve separation of concerns and improve the maintainability and scalability of an application.

CQRS separates the responsibilities for handling read and write operations in an application. It suggests that the code for handling queries (read operations) should be separate from the code for handling commands (write operations). By segregating these responsibilities, CQRS aims to optimize each side for its specific task, allowing for different scalability and optimization techniques.

In a CQRS architecture, the write side typically uses commands to modify the application’s state. These commands are often represented as simple objects that encapsulate the intention to perform a specific action. On the other hand, the read side handles queries to retrieve data from the application’s state. The read side can have dedicated data models optimized for querying, which may differ from the models used on the write side.

By separating the read and write responsibilities, CQRS enables you to apply different patterns and techniques to each side independently. For example, you can use more complex and optimized data storage mechanisms for the read side, such as materialized views or denormalized data structures, to improve query performance.

MediatR is a popular library in the .NET ecosystem that provides a simple mediator implementation, which is commonly used with the CQRS pattern. It acts as a mediator or dispatcher between commands and their respective handlers, as well as between queries and their corresponding query handlers.

The MediatR library abstracts the logic of resolving and invoking handlers based on the received commands or queries. It decouples the sender of a request from its handler, making it easier to add or modify handlers without affecting the code that initiates the requests.

With MediatR, you can define commands and queries as individual classes, and then create separate handler classes for each command or query. The mediator dispatches the requests to their corresponding handlers based on their type. Handlers are responsible for performing the necessary actions, such as updating the application’s state for commands or retrieving data for queries.

When used together, CQRS and MediatR allow you to decouple the different parts of your application and provide a clear separation between read and write operations. MediatR acts as the mediator for dispatching commands and queries, while CQRS provides the overall architectural pattern for segregating the responsibilities of handling writes and reads. This combination can lead to more maintainable and scalable applications by allowing independent optimization and scaling strategies for each side.

CQRS and MediatR solve several common challenges in software development:

  1. Scalability: Traditional monolithic architectures often struggle to scale as the application grows in complexity and user load. CQRS provides a way to scale the read and write sides independently. By segregating the read and write responsibilities, you can optimize each side for its specific needs. For example, you can use caching and denormalization techniques on the read side to improve query performance, while scaling the write side to handle a higher volume of commands.
  2. Performance: In many applications, read operations outnumber write operations. By separating the read and write models and optimizing the read side independently, CQRS allows you to focus on performance improvements for the most frequently executed read queries. This can lead to faster response times and a better user experience.
  3. Maintainability: As applications grow, the complexity of handling both read and write operations within a single codebase can become overwhelming. CQRS promotes a separation of concerns, making it easier to understand, modify, and maintain the codebase. The read and write sides can be developed and modified independently, reducing the risk of unintended side effects when making changes.
  4. Domain modeling: CQRS encourages you to model your application’s domain separately for read and write operations. This can help in designing a more expressive and focused domain model for each side. The write side can be designed around the behavior and business rules of the application, while the read side can be optimized for efficient querying and reporting.
  5. Flexibility: By using MediatR as a mediator between commands/queries and their handlers, you gain flexibility in adding, removing, or modifying handlers without impacting the sender or consumer of the requests. This decoupling simplifies the introduction of new features and changes to the application without significant ripple effects.

Overall, CQRS and MediatR provide architectural patterns and tools that help address scalability, performance, maintainability, and flexibility challenges commonly encountered in complex software applications.

I have developed a CQRS template project using .NET Core and MSSQL, along with components like fluent validation and an error handling middleware for it to show proper error “API client friendly” messages. Click on the GitHub link below to access the code. Feel free to use it as your boilerplate.