使用 CQRS 在中介管道中处理 errors/exceptions？

Question

我正在尝试关注 this post by Jimmy Bogard to implement a mediator pipeline so I can use pre/post request handlers to do some work. From the comments on that article I come to this github gist。我不太明白如何连接所有这些，所以这是我的第一次尝试。仅供参考 - 我将 Autofac 用于 DI 和 Web Api 2. 按照 CQRS，这是一个查询。

public class GetAccountRequest : IAsyncRequest<GetAccountResponse>
{
    public int Id { get; set; }
}

//try using fluent validation
public class GetAccountRequestValidationHandler 
    : AbstractValidator<GetAccountRequest>, IAsyncPreRequestHandler<GetAccountRequest>
{
    public GetAccountRequestValidationHandler() {
        RuleFor(m => m.Id).GreaterThan(0).WithMessage("Please specify an id.");
    }

    public Task Handle(GetAccountRequest request) {
        Debug.WriteLine("GetAccountPreProcessor Handler");   
        return Task.FromResult(true);
    }
}

public class GetAccountResponse
{
    public int AccountId { get; set; }
    public string Name { get; set; }
    public string AccountNumber { get; set; }
    public string Nickname { get; set; }
    public string PhoneNumber { get; set; }
    public List<OrderAckNotification> OrderAckNotifications { get; set; }

    public class OrderAckNotification {
        public int Id { get; set; }
        public bool IsDefault { get; set; }
        public string Description { get; set; }
        public string Type { get; set; }
    }
}

GetAccountRequestHandler:

public class GetAccountRequestHandler 
    : IAsyncRequestHandler<GetAccountRequest, GetAccountResponse>
{
    private readonly IRedStripeDbContext _dbContext;

    public GetAccountRequestHandler(IRedStripeDbContext redStripeDbContext)
    {
        _dbContext = redStripeDbContext;
    }

    public async Task<GetAccountResponse> Handle(GetAccountRequest message)
    {
        //some mapping code here.. omitted for brevity
        Mapper.AssertConfigurationIsValid();

        return await _dbContext.Accounts.Where(a => a.AccountId == message.Id)
            .ProjectToSingleOrDefaultAsync<GetAccountResponse>();
    }

这是显示 HttpGet 的当前 Web api 2 控制器。

[RoutePrefix("api/Accounts")]
public class AccountsController : ApiController
{
    private readonly IMediator _mediator;

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

    // GET: api/Accounts/2
    [Route("{id:int}")]
    [HttpGet]
    public async Task<IHttpActionResult> GetById([FromUri] GetAccountRequest request)
    {
        var model = await _mediator.SendAsync<GetAccountResponse>(request);

        return Ok(model);
    }
}

最后是依赖解析代码：

public void Configuration(IAppBuilder app)
{
    var config = new HttpConfiguration();

    ConfigureDependencyInjection(app, config);

    WebApiConfig.Register(config);
    app.UseWebApi(config);
}

private static void ConfigureDependencyInjection(IAppBuilder app, 
    HttpConfiguration config)
{
    var builder = new ContainerBuilder();
    builder.RegisterSource(new ContravariantRegistrationSource());
    builder.RegisterAssemblyTypes(typeof(IMediator).Assembly).AsImplementedInterfaces();

    builder.Register<SingleInstanceFactory>(ctx =>
    {
        var c = ctx.Resolve<IComponentContext>();
        return t => c.Resolve(t);
    });

    builder.Register<MultiInstanceFactory>(ctx =>
    {
        var c = ctx.Resolve<IComponentContext>();
        return t => (IEnumerable<object>)c.Resolve(
            typeof(IEnumerable<>).MakeGenericType(t));
    });

    //register all pre handlers
    builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
        .As(type => type.GetInterfaces()
            .Where(t => t.IsClosedTypeOf(typeof(IAsyncPreRequestHandler<>))));

    //register all post handlers
    builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
        .As(type => type.GetInterfaces()
            .Where(t => t.IsClosedTypeOf(typeof(IAsyncPostRequestHandler<,>))));


    //register all handlers
    builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly())
        .As(type => type.GetInterfaces()
            .Where(t => t.IsClosedTypeOf(typeof(IAsyncRequestHandler<,>)))
            .Select(t => new KeyedService("asyncRequestHandler", t)));

    //register pipeline decorator
    builder.RegisterGenericDecorator(typeof(AsyncMediatorPipeline<,>), 
        typeof(IAsyncRequestHandler<,>), "asyncRequestHandler");

    // Register Web API controller in executing assembly.
    builder.RegisterApiControllers(Assembly.GetExecutingAssembly()).InstancePerRequest();

    //register RedStripeDbContext
    builder.RegisterType<RedStripeDbContext>().As<IRedStripeDbContext>()
        .InstancePerRequest();

    builder.RegisterType<AutofacServiceLocator>().AsImplementedInterfaces();
    var container = builder.Build();

    config.DependencyResolver = new AutofacWebApiDependencyResolver(container);

    // This should be the first middleware added to the IAppBuilder.
    app.UseAutofacMiddleware(container);

    // Make sure the Autofac lifetime scope is passed to Web API.
    app.UseAutofacWebApi(config);
}

我正在进入 GetAccountRequestValidationHandler。但是，当验证失败时（传递的 id 为 0），如何抛出异常或停止管道的执行？我如何 return .WithMessage?

Answer 1

我也为此苦苦挣扎。似乎有 two/three 个选项：

使用预处理器...

1) 您可以将错误加载到请求中，并让主处理程序在处理 command/query

之前检查错误

或

2) 让预处理器抛出异常。似乎围绕这种做法存在一些分歧。一方面，这感觉就像管理带有异常的控制流，但 "pro" 阵营认为客户端应该负责发送一个有效的命令。 IE。在让用户单击 "Create Account" 之前，它可以发送 ajax 查询以确认用户名可用。在这种情况下，违反此规则的异常将是由于竞争条件。

将验证处理程序直接放入管道中。

我相信这更符合@jbogard 的想法。我目前没有使用它，但我已经勾勒出它可能的样子——可能有更好的例子，当然你想要如何定义和处理事情可能会有所不同。它的要点是，作为管道的一部分，验证运行器可以 return 到调用者，而无需调用主处理程序。

public class AsyncValidationPipeline<TRequest, TResponse> : IAsyncRequestHandler<TRequest, TResponse>
    where TRequest : IAsyncRequest<TResponse>
{
    private IAsyncRequestHandler<TRequest, TResponse> _inner;
    private IValidator<TRequest>[] _validators;

    public AsyncValidationPipeline(IAsyncRequestHandler<TRequest, TResponse> inner,
        IValidator<TRequest>[] validators)
    {
        _inner = inner;
        _validators = validators;
    }
    public Task<TResponse> Handle(TRequest message)
    {
        List<string> errors = new List<string>();
        if (_validators != null && _validators.Any())
        {
            errors = _validators.Where(v => v.Fails(message))
                .Select(v => v.ErrorMessage);
        }

        if (errors.Any())
        {
            throw new ValidationException(errors);
        }
        return _inner.Handle(message);
    }
}

这是将其与 AutoFac 连接起来的代码：

            //register all pre handlers
            builder.RegisterAssemblyTypes(assembliesToScan)
                .AsClosedTypesOf(typeof(IAsyncPreRequestHandler<>));

            //register all post handlers
            builder.RegisterAssemblyTypes(assembliesToScan)
                .AsClosedTypesOf(typeof(IAsyncPostRequestHandler<,>));

            const string handlerKey = "async-service-handlers";
            const string pipelineKey = "async-service-pipelines";

            // Request/Response for Query

            builder.RegisterAssemblyTypes(assembliesToScan)
                .AsKeyedClosedTypesOf(typeof(IAsyncRequestHandler<,>), handlerKey)
                ;

            // Decorate All Services with our Pipeline
            //builder.RegisterGenericDecorator(typeof(MediatorPipeline<,>), typeof(IRequestHandler<,>), fromKey: "service-handlers", toKey: "pipeline-handlers");
           builder.RegisterGenericDecorator(typeof(AsyncMediatorPipeline<,>), typeof(IAsyncRequestHandler<,>), fromKey: handlerKey, toKey: pipelineKey);

            // Decorate All Pipelines with our Validator
           builder.RegisterGenericDecorator(typeof(AsyncValidationHandler<,>), typeof(IAsyncRequestHandler<,>), fromKey: pipelineKey);//, toKey: "async-validator-handlers");

           // Add as many pipelines as you want, but the to/from keys must be kept in order and unique

希望这对您有所帮助....

使用 CQRS 在中介管道中处理 errors/exceptions？

Handling errors/exceptions in a mediator pipeline using CQRS?

c#

asp.net-web-api2

autofac

cqrs

mediatr