MassTransit 知多少 | 基于MassTransit Courier实现Saga 编排式分布式事务

时间:2022-12-12 09:09:02

Saga 最初出现在1987年Hector Garcaa-Molrna & Kenneth Salem发表的一篇名为《Sagas》的论文里。其核心思想是将长事务拆分为多个短事务,借助Saga事务协调器的协调,来保证要么所有操作都成功完成,要么运行相应的补偿事务以撤消先前完成的工作,从而维护多个服务之间的数据一致性。举例而言,假设有个在线购物网站,其后端服务划分为订单服务、支付服务和库存服务。那么一次下订单的Saga流程如下图所示:

MassTransit 知多少 | 基于MassTransit Courier实现Saga 编排式分布式事务

在Saga模式中本地事务是Saga 参与者执行的工作单元,每个本地事务都会更新数据库并发布消息或事件以触发 Saga 中的下一个本地事务。如果本地事务失败,Saga 会执行一系列补偿事务,以撤消先前本地事务所做的更改。
对于Saga模式的实现又分为两种形式:

  1. 协同式:把Saga 的决策和执行顺序逻辑分布在Saga的每个参与方中,通过交换事件的方式进行流转。示例图如下所示:

MassTransit 知多少 | 基于MassTransit Courier实现Saga 编排式分布式事务

  1. 编排式:把Saga的决策和执行顺序逻辑集中定义在一个Saga 编排器中。Saga 编排器发出命令式消息给各个Saga 参与方,指示这些参与方执行怎样的操作。

MassTransit 知多少 | 基于MassTransit Courier实现Saga 编排式分布式事务

从上图可以看出,对于协同式Saga 存在一个致命的弊端,那就是存在循环依赖的问题,每个Saga参与方都需要订阅所有影响它们的事件,耦合性较高,且由于Saga 逻辑分散在各参与方,不便维护。相对而言,编排式Saga 则实现了关注点分离,协调逻辑集中在编排器中定义,Saga 参与者仅需实现供编排器调用的API 即可。
在.NET 中也有开箱即用的开源框架实现了编排式的Saga事务模型,也就是MassTransit Courier,接下来就来实际探索一番。

MassTransit Courier 简介

MassTransit Courier 是对Routing Slip(路由单) 模式的实现。该模式用于运行时动态指定消息处理步骤,解决不同消息可能有不同消息处理步骤的问题。实现机制是消息处理流程的开始,创建一个路由单,这个路由单定义消息的处理步骤,并附加到消息中,消息按路由单进行传输,每个处理步骤都会查看_路由单_并将消息传递到路由单中指定的下一个处理步骤。
在MassTransit Courier中是通过抽象IActivityRoutingSlip来实现了Routing Slip模式。通过按需有序组合一系列的Activity,得到一个用来限定消息处理顺序的Routing Slip。而每个Activity的具体抽象就是IActivityIExecuteActivity。二者的差别在于IActivity定义了ExecuteCompensate两个方法,而IExecuteActivitiy仅定义了Execute方法。其中Execute代表正向操作,Compensate代表反向补偿操作。用一个简单的下单流程:创建订单->扣减库存->支付订单举例而言,使用Courier的实现示意图如下所示:
MassTransit 知多少 | 基于MassTransit Courier实现Saga 编排式分布式事务

基于Courier 实现编排式Saga事务

那具体如何使用MassTransit Courier来应用编排式Saga 模式呢,接下来就来创建解决方案来实现以上下单流程示例。

创建解决方案

依次创建以下项目,除共享类库项目外,均安装MassTransitMassTransit.RabbitMQNuGet包。

项目 项目名 项目类型
订单服务 MassTransit.CourierDemo.OrderService ASP.NET Core Web API
库存服务 MassTransit.CourierDemo.InventoryService Worker Service
支付服务 MassTransit.CourierDemo.PaymentService Worker Service
共享类库 MassTransit.CourierDemo.Shared Class Library

三个服务都添加扩展类MassTransitServiceExtensions,并在Program.cs类中调用services.AddMassTransitWithRabbitMq();注册服务。

using System.Reflection;
using MassTransit.CourierDemo.Shared.Models;

namespace MassTransit.CourierDemo.InventoryService;

public static class MassTransitServiceExtensions
{
    public static IServiceCollection AddMassTransitWithRabbitMq(this IServiceCollection services)
    {
        return services.AddMassTransit(x =>
        {
            x.SetKebabCaseEndpointNameFormatter();

            // By default, sagas are in-memory, but should be changed to a durable
            // saga repository.
            x.SetInMemorySagaRepositoryProvider();

            var entryAssembly = Assembly.GetEntryAssembly();
            x.AddConsumers(entryAssembly);
            x.AddSagaStateMachines(entryAssembly);
            x.AddSagas(entryAssembly);
            x.AddActivities(entryAssembly);
            x.UsingRabbitMq((context, busConfig) =>
            {
                busConfig.Host(
                    host: "localhost",
                    port: 5672,
                    virtualHost: "masstransit",
                    configure: hostConfig =>
                    {
                        hostConfig.Username("guest");
                        hostConfig.Password("guest");
                    });

                busConfig.ConfigureEndpoints(context);
            });
        });
    }
}

订单服务

订单服务作为下单流程的起点,需要承担构建RoutingSlip的职责,因此可以创建一个OrderRoutingSlipBuilder来构建RoutingSlip,代码如下:

using MassTransit.Courier.Contracts;
using MassTransit.CourierDemo.Shared.Models;

namespace MassTransit.CourierDemo.OrderService;
public static class OrderRoutingSlipBuilder
{
    public static RoutingSlip BuildOrderRoutingSlip(CreateOrderDto createOrderDto)
    {
        var createOrderAddress = new Uri("queue:create-order_execute");
        var deduceStockAddress = new Uri("queue:deduce-stock_execute");
        var payAddress = new Uri("queue:pay-order_execute");        
        var routingSlipBuilder = new RoutingSlipBuilder(Guid.NewGuid());

        routingSlipBuilder.AddActivity(
            name: "order-activity",
            executeAddress: createOrderAddress,
            arguments: createOrderDto);
        routingSlipBuilder.AddActivity(name: "deduce-stock-activity", executeAddress: deduceStockAddress);
        routingSlipBuilder.AddActivity(name: "pay-activity", executeAddress: payAddress);

        var routingSlip = routingSlipBuilder.Build();
        return routingSlip;
    }
}

从以上代码可知,构建一个路由单需要以下几步:

  1. 明确业务用例涉及的具体用例,本例中为:
    1. 创建订单:CreateOrder
    2. 扣减库存:DeduceStock
    3. 支付订单:PayOrder
  2. 根据用例名,按短横线隔开命名法(kebab-case)定义用例执行地址,格式为queue:<usecase>_execute,本例中为:
    1. 创建订单执行地址:queue:create-order_execute
    2. 扣减库存执行地址:queue:deduce-stock_execute
    3. 支付订单执行地址:queue:pay-order_execute
  3. 创建路由单:
    1. 通过RoutingSlipBuilder(Guid.NewGuid())创建路由单构建器实例
    2. 根据业务用例流转顺序,调用AddActivity()方法依次添加Activity用来执行用例,因为第一个创建订单用例需要入口参数,因此传入了一个CreateOrderDtoDTO(Data Transfer Object)对象
    3. 调用Build()方法创建路由单

对于本例而言,由于下单流程是固定流程,因此以上路由单的构建也是按业务用例进行定义的。而路由单的强大之处在于,可以按需动态组装。在实际电商场景中,有些订单是无需执行库存扣减的,比如充值订单,对于这种情况,仅需在创建路由单时判断若为充值订单则不添加扣减库存的Activity即可。
对于订单服务必然要承担创建订单的职责,定义CreateOrderActivity(Activity的命名要与上面定义的用例对应)如下,其中OrderRepository为一个静态订单仓储类:

public class CreateOrderActivity : IActivity<CreateOrderDto, CreateOrderLog>
{
    private readonly ILogger<CreateOrderActivity> _logger;
    public CreateOrderActivity(ILogger<CreateOrderActivity> logger)
    {
        _logger = logger;
    }

    // 订单创建
    public async Task<ExecutionResult> Execute(ExecuteContext<CreateOrderDto> context)
    {
        var order = await CreateOrder(context.Arguments);
        var log = new CreateOrderLog(order.OrderId, order.CreatedTime);
        _logger.LogInformation($"Order [{order.OrderId}] created successfully!");
        return context.CompletedWithVariables(log, new {order.OrderId});
    }

    private async Task<Order> CreateOrder(CreateOrderDto orderDto)
    {
        var shoppingItems =
            orderDto.ShoppingCartItems.Select(item => new ShoppingCartItem(item.SkuId, item.Price, item.Qty));
        var order = new Order(orderDto.CustomerId).NewOrder(shoppingItems.ToArray());
        await OrderRepository.Insert(order);
        return order;
    }

    // 订单补偿(取消订单)
    public async Task<CompensationResult> Compensate(CompensateContext<CreateOrderLog> context)
    {
        var order = await OrderRepository.Get(context.Log.OrderId);
        order.CancelOrder();
        var exception = context.Message.ActivityExceptions.FirstOrDefault();
        _logger.LogWarning(
            $"Order [{order.OrderId} has been canceled duo to {exception.ExceptionInfo.Message}!");
        return context.Compensated();
    }
}

从以上代码可知,实现一个Activity,需要以下步骤:

  1. 定义实现IActivity<in TArguments, in TLog>需要的参数类:
    1. TArguments对应正向执行入口参数,会在Execute方法中使用,本例中为CreateOrderDto,用于订单创建。
    2. TLog对应反向补偿参数,会在Compensate方法中使用,本例中为CreateOrderLog,用于订单取消。
  2. 实现IActivity<in TArguments, in TLog>接口中的Execute方法:
    1. 具体用例的实现,本例中对应订单创建逻辑
    2. 创建TLog反向补偿参数实例,以便业务异常时能够按需补偿
    3. 返回Activity执行结果,并按需传递参数至下一个Activity,本例仅传递订单Id至下一流程。
  3. 实现IActivity<in TArguments, in TLog>接口中的Compensate方法:
    1. 具体反向补偿逻辑的实现,本例中对应取消订单
    2. 返回反向补偿执行结果

订单服务的最后一步就是定义WebApi来接收创建订单请求,为简要起便创建OrderController如下:

using MassTransit.CourierDemo.Shared.Models;
using Microsoft.AspNetCore.Mvc;

namespace MassTransit.CourierDemo.OrderService.Controllers;

[ApiController]
[Route("[controller]")]
public class OrderController : ControllerBase
{
    private readonly IBus _bus;
    public OrderController(IBus bus)
    {
        _bus = bus;
    }

    [HttpPost]
    public async Task<IActionResult> CreateOrder(CreateOrderDto createOrderDto)
    {
        // 创建订单路由单
        var orderRoutingSlip = OrderRoutingSlipBuilder.BuildOrderRoutingSlip(createOrderDto);
    	// 执行订单流程
        await _bus.Execute(orderRoutingSlip);

        return Ok();
    }
}

库存服务

库存服务在整个下单流程的职责主要是库存的扣减和返还,但由于从上游用例仅传递了OrderId参数到库存扣减Activity,因此在库存服务需要根据OrderId 去请求订单服务获取要扣减的库存项才能执行扣减逻辑。而这可以通过使用MassTransit的Reqeust/Response 模式来实现,具体步骤如下:

  1. 在共享类库MassTransit.CourierDemo.Shared中定义IOrderItemsRequestIOrderItemsResponse
namespace MassTransit.CourierDemo.Shared.Models;

public interface IOrderItemsRequest
{
    public string OrderId { get; }
}
public interface IOrderItemsResponse
{
    public List<DeduceStockItem> DeduceStockItems { get; set; }
    public string OrderId { get; set; }
}
  1. 在订单服务中实现IConsumer<IOrderItemsRequest:
using MassTransit.CourierDemo.OrderService.Repositories;
using MassTransit.CourierDemo.Shared.Models;

namespace MassTransit.CourierDemo.OrderService.Consumers;

public class OrderItemsRequestConsumer : IConsumer<IOrderItemsRequest>
{
    public async Task Consume(ConsumeContext<IOrderItemsRequest> context)
    {
        var order = await OrderRepository.Get(context.Message.OrderId);
        await context.RespondAsync<IOrderItemsResponse>(new
        {
            order.OrderId, 
            DeduceStockItems = order.OrderItems.Select(
                item => new DeduceStockItem(item.SkuId, item.Qty)).ToList()
        });
    }
}
  1. 在库存服务注册service.AddMassTransit()中注册x.AddRequestClient<IOrderItemsRequest>();
using System.Reflection;
using MassTransit.CourierDemo.Shared.Models;

namespace MassTransit.CourierDemo.InventoryService;

public static class MassTransitServiceExtensions
{
    public static IServiceCollection AddMassTransitWithRabbitMq(this IServiceCollection services)
    {
        return services.AddMassTransit(x =>
        {
            //...            
            x.AddRequestClient<IOrderItemsRequest>();
            //...
        });
    }
}
  1. 在需要的类中注册IRequestClient<OrderItemsRequest>服务即可。

最终扣减库存的Activity实现如下:

public class DeduceStockActivity : IActivity<DeduceOrderStockDto, DeduceStockLog>
{
    private readonly IRequestClient<IOrderItemsRequest> _orderItemsRequestClient;
    private readonly ILogger<DeduceStockActivity> _logger;

    public DeduceStockActivity(IRequestClient<IOrderItemsRequest> orderItemsRequestClient,
        ILogger<DeduceStockActivity> logger)
    {
        _orderItemsRequestClient = orderItemsRequestClient;
        _logger = logger;
    }
	// 库存扣减
    public async Task<ExecutionResult> Execute(ExecuteContext<DeduceOrderStockDto> context)
    {
        var deduceStockDto = context.Arguments;
        var orderResponse =
            await _orderItemsRequestClient.GetResponse<IOrderItemsResponse>(new { deduceStockDto.OrderId });

        if (!CheckStock(orderResponse.Message.DeduceStockItems))
            return context.Faulted(new Exception("insufficient stock"));
        
        DeduceStocks(orderResponse.Message.DeduceStockItems);

        var log = new DeduceStockLog(deduceStockDto.OrderId, orderResponse.Message.DeduceStockItems);

        _logger.LogInformation($"Inventory has been deducted for order [{deduceStockDto.OrderId}]!");
        return context.CompletedWithVariables(log, new { log.OrderId });
    }
	// 库存检查
    private bool CheckStock(List<DeduceStockItem> deduceItems)
    {
        foreach (var stockItem in deduceItems)
        {
            if (InventoryRepository.GetStock(stockItem.SkuId) < stockItem.Qty) return false;
        }

        return true;
    }

    private void DeduceStocks(List<DeduceStockItem> deduceItems)
    {
        foreach (var stockItem in deduceItems)
        {
            InventoryRepository.TryDeduceStock(stockItem.SkuId, stockItem.Qty);
        }
    }
	//库存补偿
    public Task<CompensationResult> Compensate(CompensateContext<DeduceStockLog> context)
    {
        foreach (var deduceStockItem in context.Log.DeduceStockItems)
        {
            InventoryRepository.ReturnStock(deduceStockItem.SkuId, deduceStockItem.Qty);
        }

        _logger.LogWarning($"Inventory has been returned for order [{context.Log.OrderId}]!");
        return Task.FromResult(context.Compensated());
    }
}

支付服务

对于下单流程的支付用例来说,要么成功要么失败,并不需要像以上两个服务一样定义补偿逻辑,因此仅需要实现IExecuteActivity<in TArguments>接口即可,该接口仅定义了Execute接口方法,具体PayOrderActivity实现如下:

using MassTransit.CourierDemo.Shared;
using MassTransit.CourierDemo.Shared.Models;

namespace MassTransit.CourierDemo.PaymentService.Activities;

public class PayOrderActivity : IExecuteActivity<PayDto>
{
    private readonly IBus _bus;
    private readonly IRequestClient<IOrderAmountRequest> _client;
    private readonly ILogger<PayOrderActivity> _logger;

    public PayOrderActivity(IBus bus,IRequestClient<IOrderAmountRequest> client,ILogger<PayOrderActivity> logger)
    {
        _bus = bus;
        _client = client;
        _logger = logger;
    }

    public async Task<ExecutionResult> Execute(ExecuteContext<PayDto> context)
    {
        var response = await _client.GetResponse<IOrderAmountResponse>(new { context.Arguments.OrderId });        
        // do payment...

        if (response.Message.Amount % 2 == 0)
        {
            _logger.LogInformation($"Order [{context.Arguments.OrderId}] paid successfully!");
            return context.Completed();
        }
        _logger.LogWarning($"Order [{context.Arguments.OrderId}] payment failed!");
        return context.Faulted(new Exception("Order payment failed due to insufficient account balance."));
    }
}

以上代码中也使用了MassTransit的Reqeust/Response 模式来获取订单要支付的余额,并根据订单金额是否为偶数来模拟支付失败。

运行结果

启动三个项目,并在Swagger中发起订单创建请求,如下图所示:

MassTransit 知多少 | 基于MassTransit Courier实现Saga 编排式分布式事务

由于订单总额为奇数,因此支付会失败,最终控制台输出如下图所示:

MassTransit 知多少 | 基于MassTransit Courier实现Saga 编排式分布式事务
打开RabbitMQ后台,可以看见MassTransit按照约定创建了以下队列用于服务间的消息传递:

MassTransit 知多少 | 基于MassTransit Courier实现Saga 编排式分布式事务

但你肯定好奇本文中使用的路由单具体是怎样实现的?简单,停掉库存服务,再发送一个订单创建请求,然后从队列获取未消费的消息即可解开谜底。以下是抓取的一条消息示例:

{
    "messageId": "ac5d0000-e330-482a-b7bc-08dada7915ab",
    "requestId": null,
    "correlationId": "ce8af31b-a65c-4dfa-915c-4ae5174820f9",
    "conversationId": "ac5d0000-e330-482a-28a5-08dada7915ad",
    "initiatorId": null,
    "sourceAddress": "rabbitmq://localhost/masstransit/THINKPAD_MassTransitCourierDemoOrderService_bus_itqoyy8dgbrniyeobdppw6engn?temporary=true",
    "destinationAddress": "rabbitmq://localhost/masstransit/deduce-stock_execute?bind=true",
    "responseAddress": null,
    "faultAddress": null,
    "messageType": [
        "urn:message:MassTransit.Courier.Contracts:RoutingSlip"
    ],
    "message": {
        "trackingNumber": "ce8af31b-a65c-4dfa-915c-4ae5174820f9",
        "createTimestamp": "2022-12-10T06:38:01.5452768Z",
        "itinerary": [
            {
                "name": "deduce-stock-activity",
                "address": "queue:deduce-stock_execute",
                "arguments": {}
            },
            {
                "name": "pay-activity",
                "address": "queue:pay-order_execute",
                "arguments": {}
            }
        ],
        "activityLogs": [
            {
                "executionId": "ac5d0000-e330-482a-7cb2-08dada7915bf",
                "name": "order-activity",
                "timestamp": "2022-12-10T06:38:01.7115314Z",
                "duration": "00:00:00.0183136",
                "host": {
                    "machineName": "THINKPAD",
                    "processName": "MassTransit.CourierDemo.OrderService",
                    "processId": 23980,
                    "assembly": "MassTransit.CourierDemo.OrderService",
                    "assemblyVersion": "1.0.0.0",
                    "frameworkVersion": "6.0.9",
                    "massTransitVersion": "8.0.7.0",
                    "operatingSystemVersion": "Microsoft Windows NT 10.0.19044.0"
                }
            }
        ],
        "compensateLogs": [
            {
                "executionId": "ac5d0000-e330-482a-7cb2-08dada7915bf",
                "address": "rabbitmq://localhost/masstransit/create-order_compensate",
                "data": {
                    "orderId": "8c47a1db-cde3-43bb-a809-644f36e7ca99",
                    "createdTime": "2022-12-10T14:38:01.7272895+08:00"
                }
            }
        ],
        "variables": {
            "orderId": "8c47a1db-cde3-43bb-a809-644f36e7ca99"
        },
        "activityExceptions": [],
        "subscriptions": []
    },
    "expirationTime": null,
    "sentTime": "2022-12-10T06:38:01.774618Z",
    "headers": {
        "MT-Forwarder-Address": "rabbitmq://localhost/masstransit/create-order_execute"
    }
}

从中可以看到信封中的message.itinerary定义了消息的行程,从而确保消息按照定义的流程进行流转。同时通过message.compensateLogs来指引若失败将如何回滚。

总结

通过以上示例的讲解,相信了解到MassTransit Courier的强大之处。Courier中的RoutingSlip充当着事务编排器的角色,将Saga的决策和执行顺序逻辑封装在消息体内随着消息进行流转,从而确保各服务仅需关注自己的业务逻辑,而无需关心事务的流转,真正实现了关注点分离。