2.6.7 Masstransit详解

一、Consumer 消费者

在 MassTransit 中，一个消费者可以消费一种或多种消息

消费者的类型包括：普通消费者，saga，saga 状态机，路由活动（分布式追踪），处理器 handlers，工作消费者 job comsumers

1.1Consumer

public class Program
{
    public static async Task Main()
    {
        var busControl = Bus.Factory.CreateUsingRabbitMq(cfg =>
        {
            cfg.ReceiveEndpoint("order-service", e =>
            {
                e.Consumer<SubmitOrderConsumer>();
            });
        });
    }
}

继承 IConsumer，实现 Consume 方法

class SubmitOrderConsumer :
    IConsumer<SubmitOrder>
{
    public async Task Consume(ConsumeContext<SubmitOrder> context)
    {
        await context.Publish<OrderSubmitted>(new
        {
            context.Message.OrderId
        });
    }
}

三个原则：

• 拥抱 The Hollywood Principle, which states, "Dont't call us, we'll call you."
• Consume 方法是一个被等待的方法，在执行中时其他消费者无法接收到这个消息，当这个方法完成的时候，消息被 ack，并且从队列中移除
• Task 方法异常会导致消息触发 retry，如果没有配置重试，消息将被投递到失败队列

1.2Instance

public class Program
{
    public static async Task Main()
    {
        var submitOrderConsumer = new SubmitOrderConsumer();

        var busControl = Bus.Factory.CreateUsingRabbitMq(cfg =>
        {
            cfg.ReceiveEndpoint("order-service", e =>
            {
                e.Instance(submitOrderConsumer);
            });
        });
    }
}

所有接收到的消息都由一个消费者来实例来处理（请确保这个消费者类是线程安全）

Consumer 每次接收到消息都会 new 一个实例

1.3Handler

public class Program
{
    public static async Task Main()
    {
        var busControl = Bus.Factory.CreateUsingRabbitMq(cfg =>
        {
            cfg.ReceiveEndpoint("order-service", e =>
            {
                e.Handler<SubmitOrder>(async context =>
                {
                    await Console.Out.WriteLineAsync($"Submit Order Received: {context.Message.OrderId}");
                });
            });
        });
    }
}

通过一个委托 Lambda 方法，来消费消息

  1.4Others

• Saga<>
• StateMachineSaga<>

二、Producer 生产者

消息的生产可以通过两种方式产生：发送和发布

发送的时候需要指定一个具体的地址 DestinationAddress，发布的时候消息会被广播给所有订阅了这个消息类型的消费者

基于这两种规则，消息被定义为：命令 command 和事件 event

• send
• publish

2.1Send

可以调用以下对象的 send 方法来发送 command：

• ConsumeContext （在 Consumer 的 Consumer 方法参数中传递）

public class SubmitOrderConsumer : 
    IConsumer<SubmitOrder>
{
    private readonly IOrderSubmitter _orderSubmitter;

    public SubmitOrderConsumer(IOrderSubmitter submitter)
        => _orderSubmitter = submitter;

    public async Task Consume(IConsumeContext<SubmitOrder> context)
    {
        await _orderSubmitter.Process(context.Message);

        await context.Send(new StartDelivery(context.Message.OrderId, DateTime.UtcNow));
    }
}

• ISendEndpointProvider（可以从 DI 中获取）

public async Task SendOrder(ISendEndpointProvider sendEndpointProvider)
{
    var endpoint = await sendEndpointProvider.GetSendEndpoint(_serviceAddress);

    await endpoint.Send(new SubmitOrder { OrderId = "123" });
}

2.2Publish

2.2.1发送地址

rabbitmq://localhost/input-queue
rabbitmq://localhost/input-queue?durable=false

2.2.2短地址

GetSendEndpoint(new Uri("queue:input-queue"))

 2.2.3Convention Map

在配置文件中指定 map 规则

EndpointConvention.Map<StartDelivery>(new Uri(ConfigurationManager.AppSettings["deliveryServiceQueue"]));

直接发送

public class SubmitOrderConsumer : 
    IConsumer<SubmitOrder>
{
    private readonly IOrderSubmitter _orderSubmitter;

    public SubmitOrderConsumer(IOrderSubmitter submitter)
        => _orderSubmitter = submitter;

    public async Task Consume(IConsumeContext<SubmitOrder> context)
    {
        await _orderSubmitter.Process(context.Message);

        await context.Send(new StartDelivery(context.Message.OrderId, DateTime.UtcNow));
    }
}

可以调用以下对象的 publish 方法来发送 event：

• ConsumeContext （在 Consumer 的 Consumer 方法参数中传递）
• IPublishEndpoint（可以从 DI 中获取）
• IBusControl（最顶层的控制对象，用来启动和停止 masstransit 的控制器）

 IPublishEndpoint

public async Task NotifyOrderSubmitted(IPublishEndpoint publishEndpoint)
{
    await publishEndpoint.Publish<OrderSubmitted>(new
    {
        OrderId = "27",
        OrderDate = DateTime.UtcNow,
    });
}

三、Request-Response 请求-响应

Request-Response 模式让应用程序之间解耦之后，依然采用同步的方式

3.1Consumer

public async Task Consume(ConsumeContext<CheckOrderStatus> context)
{
    var order = await _orderRepository.Get(context.Message.OrderId);
    if (order == null)
        throw new InvalidOperationException("Order not found");
    
    await context.RespondAsync<OrderStatusResult>(new 
    {
        OrderId = order.Id,
        order.Timestamp,
        order.StatusCode,
        order.StatusText
    });
}

需要处理返回类型 OrderStatusResult，异步方式模拟同步，实际上同样有消息队列，消费者处理过程

3.2IClientFactory

public interface IClientFactory 
{
    IRequestClient<T> CreateRequestClient<T>(ConsumeContext context, Uri destinationAddress, RequestTimeout timeout);

    IRequestClient<T> CreateRequestClient<T>(Uri destinationAddress, RequestTimeout timeout);

    RequestHandle<T> CreateRequest<T>(T request, Uri destinationAddress, CancellationToken cancellationToken, RequestTimeout timeout);

    RequestHandle<T> CreateRequest<T>(ConsumeContext context, T request, Uri destinationAddress, CancellationToken cancellationToken, RequestTimeout timeout);
}

通过 IBusControl 的 CreateClientFactory 方法可以得到 ClientFactory

3.3IRequestClient

public interface IRequestClient<TRequest> where TRequest : class
{
    RequestHandle<TRequest> Create(TRequest request, CancellationToken cancellationToken, RequestTimeout timeout);

    Task<Response<T>> GetResponse<T>(TRequest request, CancellationToken cancellationToken, RequestTimeout timeout);
}

RequestClient 可以创建请求，或者直接获得响应

3.4Send a request

var serviceAddress = new Uri("rabbitmq://localhost/check-order-status");
var client = bus.CreateRequestClient<CheckOrderStatus>(serviceAddress);
var response = await client.GetResponse<OrderStatusResult>(new { OrderId = id});