使用 MediatR 實作 CQRS

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在 Clean Architecture 上，使用 MediatR 實作 CQRS (Command Query Responsibility Segregation)。

關於 CQRS 模式，推薦閱讀 CQRS (Command Query Responsibility Segregation). By Ajay Kumar。書中的範例專案是用 Java 撰寫，但只要熟悉 C# 或物件導向程式語言的讀者，都可以很容易理解。此書以學生選課管理系統為範例，透過重構為任務型介面，再進一步重構為 CQRS 模式，來說明主要概念。此書也是以實務的角度出發，提到了很多權衡，例如建立查詢專用資料庫的必要性。

基礎配置

  1. 建立 .NET 8 專案

    • Purchasing.Core (Class Library)

    • Purchasing.Infrastructure (Class Library)

    • Purchasing.UseCases (Class Library)

    • Purchasing.Api (Web API)

  2. 安裝 MediatR Nuget Packages

    版本為 12.2.0，安裝到 Purchasing.UseCases、Purchasing.Api。

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    Install-Package MediatR -Project Purchasing.UseCases
    Install-Package MediatR -Project Purchasing.Api

  3. 配置 MediatR

    在 Purchasing.UseCases 新增下列程式碼：

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    // Extensions/DependencyInjection/ServiceCollectionExtensions.cs

    using Microsoft.Extensions.DependencyInjection;

    namespace Purchasing.UseCases.Extensions.DependencyInjection
    {
        public static class ServiceCollectionExtensions
        {
            public static IServiceCollection AddMediatR(this IServiceCollection services)
            {
                services.AddMediatR(cfg =>
                    cfg.RegisterServicesFromAssembly(typeof(ServiceCollectionExtensions).Assembly));

                return services;
            }
        }
    }

    在 Purchasing.Api 新增下列程式碼：

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    // Program.cs

    // ...

    using Purchasing.UseCases.Extensions.DependencyInjection

    // ...

    builder.Services.AddMediatR();

    // ...

建立 Query

  1. 在 Purchasing.UseCases 新增 Query

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    // Queries/ListPurchaseOrdersQuery.cs

    using MediatR;

    namespace Purchasing.UseCases.Queries
    {
        public record ListPurchaseOrdersQuery(string? UserId) : IRequest<List<PurchaseOrderDto>>;
    }
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    // Handlers/ListPurchaseOrdersHandler.cs

    using MediatR;
    using Purchasing.UseCases.Contracts;
    using Purchasing.UseCases.Queries;

    namespace Purchasing.UseCases.Handlers
    {
        public class ListPurchaseOrdersHandler: IRequestHandler<ListPurchaseOrdersQuery, List<PurchaseOrderDto>>
        {
            public async Task<List<PurchaseOrderDto>> Handle(ListPurchaseOrdersQuery request, CancellationToken cancellationToken)
            {
                // ...
            }
        }
    }

建立 Command

  1. 在 Purchasing.UseCases 新增 Command

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    // Commands/CreatePurchaseOrderCommand.cs

    using MediatR;

    namespace Purchasing.UseCases.Commands
    {
        public record CreatePurchaseOrderCommand(string UserId) : IRequest<int>;
    }
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    // Handlers/CreatePurchaseOrderHandler.cs

    using MediatR;
    using Purchasing.UseCases.Commands;

    namespace Purchasing.UseCases.Handlers
    {
        public class CreatePurchaseOrderHandler: IRequestHandler<CreatePurchaseOrderCommand, int>
        {
            public async Task<int> Handle(CreatePurchaseOrderCommand request, CancellationToken cancellationToken)
            {
                // ...
            }
        }
    }

建立 Controller

  1. 在 Purchasing.Api 新增 Controller

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    // Controllers/PurchaseOrdersController.cs

    using Purchasing.UseCases.Commands;
    using Purchasing.UseCases.Contracts;
    using Purchasing.UseCases.Queries;
    using MediatR;
    using Microsoft.AspNetCore.Mvc;

    namespace Purchasing.Api.Controllers
    {
        [Route("[controller]")]
        [ApiController]
        public class PurchaseOrdersController : ControllerBase
        {
            private readonly IMediator _mediator;

            public PurchaseOrdersController(IMediator mediator)
            {
                _mediator = mediator;
            }

            [HttpGet]
            public async Task<ActionResult<List<PurchaseOrderDto>>> ListPurchaseOrdersAsync([FromQuery] ListPurchaseOrdersQuery query)
            {
                var purchaseOrders = await _mediator.Send(query);

                return purchaseOrders;
            }

            [HttpPost]
            public async Task<ActionResult<int>> CreatePurchaseOrderAsync([FromBody] CreatePurchaseOrderCommand command)
            {
                var purchaseOrderId = await _mediator.Send(command);

                return purchaseOrderId;
            }
        }
    }