基于Abp VNext框架设计

abp 通过IDistributedEventBus接口集成自IEventBus实现分布式事件消息的发布订阅。

IEventBus在什么时机触发PublishAsync?

1. 当前UnitOfWork完成时，触发IEventBus的PublishAsync
2. 在没有事务环境下，同步调用IEventBus的PublishAsync

abp 默认实现基于RabbitMq消息队列Volo.Abp.EventBus.RabbitMQ实现分布式消息的发布与订阅。

消息治理核心问题：

1. 生产端如何保证投递成功的消息不能丢失。
2. Mq自身如何保证消息不丢失。
3. 消费段如何保证消费端的消息不丢失。

基于abp 默认实现的DistributedEventBus不能满足以下场景：

1. Publisher 生产者无法保证消息一定能投递到MQ。
2. Consumer 消费端在消息消费时，出现异常时，没有异常错误处理机制（确保消费失败的消息能重新被消费）。

我们引入Masstransit，来提升abp对消息治理能力。

Masstransit提供以下开箱即用功能：

1. Publish/Send/Request-Response等几种消息投递机制。
2. 多种IOC容器支持。
3. 异常机制。
4. Saga事务管理。
5. 事务活动补偿机制（Courier）
6. 消息审计
7. 消息管道处理机制

Abp 框架下事件消息集成

1. 使用MassTransit重新实现IDistributedEventBus。
2. 在消费端Consumer传递用户身份信息。
3. 使用Asp.Net Core Web Host 作消费端Consumer宿主。

集成MassTransit

在Module初始化时，注入MassTransit实例，并启动。

/// <summary>
/// 配置DistributedEventBus
/// </summary>
/// <param name="context"></param>
/// <param name="configuration"></param>
/// <param name="hostingEnvironment"></param>
private void ConfigureDistributedEventBus(ServiceConfigurationContext context, IConfiguration configuration, IWebHostEnvironment hostingEnvironment)
{
    var options = context.Services.GetConfiguration().GetSection("Rabbitmq").Get<MassTransitEventBusOptions>();

    var mqConnectionString = "rabbitmq://" + options.ConnectionString;


    context.Services.AddMassTransit(mtConfig =>
    {
        //inject consumers into IOC from assembly
        mtConfig.AddConsumers(typeof(AuthCenterEventBusHostModule));


        mtConfig.AddBus(provider =>
        {
            var bus = Bus.Factory.CreateUsingRabbitMq(mqConfig =>
                {
                    var host = mqConfig.Host(new Uri(mqConnectionString), h =>
                    {
                        h.Username(options.UserName);
                        h.Password(options.Password);
                    });
                // set special message serializer
                    mqConfig.UseBsonSerializer();

                    // integrated existed logger compontent
                    mqConfig.UseExtensionsLogging(provider.GetService<ILoggerFactory>());

                    mqConfig.ReceiveEndpoint(host, "authcenter-queue", q =>
                    {
                        //set rabbitmq prefetch count
                        q.PrefetchCount = 200;

                        //set message retry policy
                        q.UseMessageRetry(r => r.Interval(3, 100));

                        q.Consumer<SmsTokenValidationCreatedEventConsumer>(provider);
                        EndpointConvention.Map<SmsTokenValidationCreatedEvent>(q.InputAddress);

                    });

                    mqConfig.ReceiveEndpoint(host, "user-synchronization", q =>
                    {
                        //set rabbitmq prefetch count
                        q.PrefetchCount = 50;
                        //q.UseRateLimit(100, TimeSpan.FromSeconds(1));
                        //q.UseConcurrencyLimit(2);

                        //set message retry policy
                        q.UseMessageRetry(r => r.Interval(3, 100));

                        q.Consumer<UserSyncEventConsumer>(provider);
                        EndpointConvention.Map<UserSyncEvent>(q.InputAddress);
                    });

                    mqConfig.ConfigureEndpoints(provider);

                    mqConfig.UseAuditingFilter(provider, o =>
                    {
                        o.ReplaceAuditing = true;
                    });
                });

            // set authtication middleware for user identity
            bus.ConnectAuthenticationObservers(provider);

            return bus;
        });
    });
}

在MassTransit中，使用IBusControl接口 StartAsync 或 StopAsync 来启动或停止。

使用IPublishEndpoint重新实现IDistributedEventBus接口，实现与abp分布式事件总线集成。

 public class MassTransitDistributedEventBus : IDistributedEventBus, ISingletonDependency
    {
        private readonly IPublishEndpoint _publishEndpoint;


        //protected IHybridServiceScopeFactory ServiceScopeFactory { get; }
        protected AbpDistributedEventBusOptions DistributedEventBusOptions { get; }

        public MassTransitDistributedEventBus(
            IOptions<AbpDistributedEventBusOptions> distributedEventBusOptions,
            IPublishEndpoint publishEndpoint)
        {
            //ServiceScopeFactory = serviceScopeFactory;
            _publishEndpoint = publishEndpoint;
            DistributedEventBusOptions = distributedEventBusOptions.Value;
            //Subscribe(distributedEventBusOptions.Value.Handlers);
        }

        /*
        *  Not Implementation
        */
      

        public Task PublishAsync<TEvent>(TEvent eventData)
            where TEvent : class
        {
            return _publishEndpoint.Publish(eventData);
        }

        public Task PublishAsync(Type eventType, object eventData)
        {
            return _publishEndpoint.Publish(eventData, eventType);
        }
    }

到此，我们实现了MassTransit与Abp集成。

事件消息传递User Claims

在实际业务实现过程中，我们会用消息队列实现“削峰填谷”的效果。异步消息队列中传递用户身份信息如何实现呢？

我们先看看abp在WebApi中，如何确定当前用户？

ICurrentUser 提供当前User Claims抽象。而ICurrentUser依赖于ICurrentPrincipalAccessor，在Asp.Net core中利用HttpContext User 来记录当前用户身份。

在MassTransit中，利用IPublishObserver > IConsumeObserver 生产者/消费端的观察者，来实现传递已认证的用户Claims。

    /// <summary>
    /// 生产者传递当前用户Principal
    /// </summary>
 public class AuthPublishObserver : IPublishObserver
    {
        private readonly ICurrentPrincipalAccessor _currentPrincipalAccessor;
        private readonly IClaimsPrincipalFactory _claimsPrincipalFactory;

        public AuthPublishObserver(
            ICurrentPrincipalAccessor currentPrincipalAccessor,
            IClaimsPrincipalFactory claimsPrincipalFactory)
        {
            _currentPrincipalAccessor = currentPrincipalAccessor;
            _claimsPrincipalFactory = claimsPrincipalFactory;
        }

        public Task PrePublish<T>(PublishContext<T> context) where T : class
        {
            var claimsPrincipal = _claimsPrincipalFactory
                .CreateClaimsPrincipal(
                    _currentPrincipalAccessor.Principal
                    );

            if (claimsPrincipal != null)
            {
                context.Headers.SetAuthenticationHeaders(claimsPrincipal);
            }


            return TaskUtil.Completed;

        }
        public Task PostPublish<T>(PublishContext<T> context) where T : class => TaskUtil.Completed;
        public Task PublishFault<T>(PublishContext<T> context, Exception exception) where T : class => TaskUtil.Completed;

    }

    /// <summary>
    /// 消费端从MqMessage Heads 中获取当前用户Principal，并赋值给HttpContext
    /// </summary>
 public class AuthConsumeObserver : IConsumeObserver
    {
        private readonly IHttpContextAccessor _httpContextAccessor;
        private readonly IServiceScopeFactory _factory;


        public AuthConsumeObserver(IHttpContextAccessor httpContextAccessor, IServiceScopeFactory factory)
        {
            _httpContextAccessor = httpContextAccessor;
            _factory = factory;
        }

        public Task PreConsume<T>(ConsumeContext<T> context) where T : class
        {
            if (_httpContextAccessor.HttpContext == null)
            {
                _httpContextAccessor.HttpContext = new DefaultHttpContext
                {
                    RequestServices = _factory.CreateScope().ServiceProvider
                };
            }

            var abpClaimsPrincipal = context.Headers.TryGetAbpClaimsPrincipal();

            if (abpClaimsPrincipal != null && abpClaimsPrincipal.IsAuthenticated)
            {
                var claimsPrincipal = abpClaimsPrincipal.ToClaimsPrincipal();

                _httpContextAccessor.HttpContext.User = claimsPrincipal;
                Thread.CurrentPrincipal = claimsPrincipal;
            }

            return TaskUtil.Completed;
        }

        public Task PostConsume<T>(ConsumeContext<T> context) where T : class
        {
            _httpContextAccessor.HttpContext = null;

            return TaskUtil.Completed;
        }

        public Task ConsumeFault<T>(ConsumeContext<T> context, Exception exception) where T : class
        {
            _httpContextAccessor.HttpContext = null;

            return TaskUtil.Completed;
        }
    }

使用Asp.Net Core Web Host 作消费端Consumer宿主

基于以下几点原因，我们使用Asp.Net Core Web Host 作为消息端Consumer宿主

1. 部署在Linux环境下，Asp.Net Core Web Host 通常使用守护进程来启动服务实例，这样可以保证服务不被中断。
2. 根据abp vnext DDD 项目分层，最大程度利用Application层应用方法，复用abp vnext 框架机制。

MassTransit 深入研究

1. 延迟消息
2. 限流、熔断降级
3. 批量消费
4. Saga

References

1. abp vnext disctributed event bus
2. MassTransit