Exploring the REPR Design Pattern in .NET

The REPR (Request-Endpoint-Response) design pattern, pronounced “Reaper,” is an innovative approach to designing APIs by focusing on endpoints rather than traditional controllers. This pattern, created by Steve "ardalis" Smith, simplifies the organization of endpoints in an application, making them easier to manage, modify, and scale. In this article, we’ll delve into the REPR pattern, discuss how to implement it in a .NET project, and examine its advantages and drawbacks.

What is the REPR Pattern?

The REPR pattern is designed around the concept of breaking down an API into individual endpoints rather than lumping multiple action methods into a single controller. Each endpoint is defined as a separate class with a single method that handles incoming requests. This endpoint-centric approach allows for more modular, maintainable, and scalable APIs.

The pattern is based on three main components:

1. Request: The object that represents the data sent by the client.
2. Endpoint: The class responsible for processing the request and generating a response.
3. Response: The object sent back to the client, containing the result of the request.

By adopting this structure, APIs become more straightforward and adhere to the Single Responsibility Principle (SRP). Each endpoint class focuses on a specific task, making it easier to maintain and test.

Why Do We Need the REPR Pattern?

Traditional MVC architecture in .NET typically organizes logic within controllers. At the start of API development, controllers tend to be small and concise. However, as the application grows, controllers often become bloated with numerous action methods and dependencies, leading to challenges in maintainability and readability.

Some of the issues with bloated controllers include:

• Multiple responsibilities: Controllers often handle various tasks, violating SRP.
• Difficulties in testing: Large controllers become hard to unit test as they rely on many services.
• Merge conflicts: Multiple developers working on the same controller can introduce synchronization issues.

To solve these problems, Steve Smith developed the REPR pattern. By separating endpoints into individual classes, each endpoint is isolated, making the codebase more manageable and modular. This also minimizes the risks of merge conflicts and allows for more granular testing.

Implementing the REPR Pattern in .NET Using FastEndpoints

To implement the REPR pattern, one of the easiest approaches is by using the FastEndpoints library, which provides an elegant way to organize endpoints in a .NET application. Let’s walk through setting up a basic API using the REPR pattern and FastEndpoints.

Step 1: Create a New ASP.NET Core Web App

First, we create an empty ASP.NET Core project:

dotnet new web -n REPRPattern
        

Step 2: Install the FastEndpoints Package

Next, we add the FastEndpoints library:

dotnet add package FastEndpoints
        

Step 3: Configure the Program.cs File

We configure our Program.cs file to integrate FastEndpoints into the middleware pipeline:

using FastEndpoints;

var builder = WebApplication.CreateBuilder();
builder.Services.AddFastEndpoints();

var app = builder.Build();
app.UseFastEndpoints();
app.Run();
        

Step 4: Define the Request, Response, and Endpoint Classes

Now, let’s define the core components of our application—Request, Response, and Endpoint classes. In this example, we’ll create an endpoint that accepts book details and returns a description.

• Book Request:

public record BookEndpointRequest(string? Title, string? AuthorName);
        

• Book Response:

public record BookEndpointResponse(string? Description);
        

• Book Endpoint:

public class BookEndpoint : Endpoint<BookEndpointRequest, BookEndpointResponse>
{
    public override void Configure()
    {
        Post("/api/book/create");
        AllowAnonymous();
    }

    public override async Task HandleAsync(BookEndpointRequest request, CancellationToken cancellationToken)
        => await SendAsync(new BookEndpointResponse($"{request.Title} was written by {request.AuthorName}"), 
           cancellation: cancellationToken);
}
        

In this implementation:

• The request object is passed to the HandleAsync method.
• The endpoint processes the request and returns a description as the response.

The Configure() method defines the endpoint’s route and specifies that it handles HTTP POST requests.

Benefits of the REPR Pattern

1. Improved Organization: With each endpoint residing in its own class, it’s easier to locate and modify the logic for a particular API operation.
2. Adherence to SRP: By assigning each class a single responsibility, we reduce the chance of having complex controllers that handle too many tasks.
3. Enhanced Testability: Since each endpoint is isolated, unit tests can target individual functionalities more effectively.
4. Modularity: Endpoints are highly modular and can be refactored or extended without affecting other parts of the application.
5. Performance: According to the FastEndpoints documentation, APIs built using this library are faster than traditional controller-based APIs and only slightly slower than minimal APIs.

Drawbacks of the REPR Pattern

Despite its many advantages, there are some potential downsides to using the REPR pattern:

1. Increased File Count: Breaking endpoints into individual classes can lead to a proliferation of files, making the project more complex and requiring more effort to maintain an organized structure.
2. Relatively New Pattern: As the REPR pattern is still gaining adoption, the community support is smaller, and there may be fewer resources and examples to refer to.

Conclusion

The REPR design pattern offers a fresh approach to structuring APIs in .NET by focusing on endpoint-centric development. By splitting action methods into separate classes and following the Request-Endpoint-Response flow, this pattern enhances the modularity, maintainability, and testability of APIs. While the pattern introduces some complexity in file management, its benefits in terms of scalability and organization outweigh the drawbacks. Additionally, with libraries like FastEndpoints, implementing the REPR pattern is straightforward and brings performance improvements compared to traditional controller-based APIs.

Ultimately, the REPR pattern offers a valuable alternative to MVC controllers, especially for projects that prioritize clean, maintainable code and modular design.