原标题：MassTransit 知多少 | 基于StateMachine实现Saga编排式分布式事务

什么是状态机

状态机作为一种程序开发范例，在实际的应用开发中有很多的应用场景，其中.NET 中的async/await 的核心底层实现就是基于状态机机制。状态机分为两种：有限状态机和无限状态机，本文介绍的就是有限状态机，有限状态机在任何时候都可以准确地处于有限状态中的一种，其可以根据一些输入从一个状态转换到另一个状态。一个有限状态机是由其状态列表、初始状态和触发每个转换的输入来定义的。如下图展示的就是一个闸机的状态机示意图：

从上图可以看出，状态机主要有以下核心概念：

1. State：状态，闸机有已开启（opened）和已关闭（closed）状态。


2. Transition：转移，即闸机从一个状态转移到另一个状态的过程。


3. Transition Condition：转移条件，也可理解为事件，即闸机在某一状态下只有触发了某个转移条件，才会执行状态转移。比如，闸机处于已关闭状态时，只有接收到开启事件才会执行转移动作，进而转移到开启状态。


4. Action：动作，即完成状态转移要执行的动作。比如要从关闭状态转移到开启状态，则需要执行开闸动作。

在.NET中，dotnet-state-machine/stateless和MassTransit都提供了开箱即用的状态机实现。本文将重点介绍MassTransit中的状态机在Saga 模式中的应用。

MassTransit StateMachine

在MassTransit 中MassTransitStateMachine就是状态机的具体抽象，可以用其编排一系列事件来实现状态的流转，也可以用来实现Saga模式的分布式事务。并支持与EF Core和Dapper集成将状态持久化到关系型数据库，也支持将状态持久化到MongoDB、Redis等数据库。是以简单的下单流程：创建订单->扣减库存->支付订单举例而言，其示意图如下所示。

基于状态机实现编排式Saga事务

那具体如何使用MassTransitStateMachine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。依次创建以下项目，除共享类库项目外，均安装MassTransit和MassTransit.RabbitMQNuGet包。

项目	项目名	项目类型
订单服务	MassTransit.SmDemo.OrderService	ASP.NET Core Web API
库存服务	MassTransit.SmDemo.InventoryService	Worker Service
支付服务	MassTransit.SmDemo.PaymentService	Worker Service
共享类库	MassTransit.SmDemo.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中的核心服务，主要职责包含接收创建订单请求和订单状态机的实现。先来定义OrderController如下：

namespace MassTransit.SmDemo.OrderService.Controllers;
[ApiController]
[Route("[controller]")]
public class OrderController : ControllerBase
{
private readonly IBus _bus;
public OrderController(IBus bus)
{
_bus = bus;
}
[HttpPost]
public async Task CreateOrder(CreateOrderDto createOrderDto)
{
await _bus.Publish(new
{
createOrderDto.CustomerId,
createOrderDto.ShoppingCartItems
});
return Ok();
}
}


紧接着，订阅ICreateOrderCommand，执行订单创建逻辑，订单创建完毕后会发布ICreateOrderSucceed事件。

public class CreateOrderConsumer : IConsumer
{
private readonly ILogger _logger;
public CreateOrderConsumer(ILogger logger)
{
_logger = logger;
}
public async Task Consume(ConsumeContext context)
{
var shoppingItems =
context.Message.ShoppingCartItems.Select(item => new ShoppingCartItem(item.SkuId, item.Price, item.Qty));
var order = new Order(context.Message.CustomerId).NewOrder(shoppingItems.ToArray());
await OrderRepository.Insert(order);

_logger.LogInformation($"Order {owww.yii666.comrder.OrderId} created successfully");
await context.Publish(new
{
order.OrderId,
order.OrderItems
});
}
}


最后来实现订单状态机，主要包含以下几步：

1. 定义状态机状态： 一个状态机从启动到结束可能会经历各种异常，包括程序异常或物理故障，为确保状态机能从异常中恢复，因此必须保存状态机的状态。本例中，定义OrderState以保存状态机实例状态数据：

using MassTransit.SmDemo.OrderService.Domains;
namespace MassTransit.SmDemo.OrderService;
public class OrderState : SagaStateMachineInstance
{
public Guid CorrelationId { get; set; }
public string CurrentState { get; set; }
public Guid OrderId { get; set; }
public decimal Amount { get; set; }
public List OrderItems { get; set; }
}


2. 定义状态机：直接继承自MassTransitStateMachine并同时指定状态实例即可：

namespace MassTransit.SmDemo.OrderService;
public class OrderStateMachine : MassTransitStateMachine
{
}


3. 注册状态机：这里指定内存持久化方式来持久化状态，也可指定诸如MongoDb、MySQL等数据库进行状态持久化：

return services.AddMassTransit(x =>
{
//...
x.AddSagaStateMachine()
.InMemoryRepository();
}


4. 定义状态列表：即状态机涉及到的系列状态，并通过State类型定义，本例中为：
   
   
   
   1. 已创建：public State Created { get; private set; }
      
   
   
   2. 库存已扣减：public State InventoryDeducted { get; private set; }
      
   
   
   3. 已支付：public State Paid { get; private set; }
      
   
   
   4. 已取消：public State Canceled { get; private set; }
      
   
   
   
   


5. 定义转移条件：即推动状态流转的事件，通过Event类型定义，本例涉及有：
   
   
   
   1. 订单成功创建事件：public Event OrderCreated {get; private set;}
      
   
   
   2. 库存扣减成功事件：public Event DeduceInventorySucceed {get; private set;}
      
   
   
   3. 库存扣减失败事件：public Event DeduceInventoryFailed {get; private set;}
      
   
   
   4. 订单支付成功事件：public Event PayOrderSucceed {get; private set;}
      
   
   
   5. 订单支付失败事件：public Event PayOrderFailed {get; private set;}
      
   
   
   6. 库存已返还事件：public Event ReturnInventorySucceed { get; private set; }
      
   
   
   7. 订单取消事件：public Event OrderCanceled { get; private set; }
      
   
   
   
   


6. 定义关联关系：由于每个事件都是孤立的，但相关联的事件终会作用到某个具体的状态机实例上，如何关联事件以推动状态机的转移呢？配置关联Id。以下就是将事件消息中的传递的OrderId作为关联ID。
   
   
   
   1. Event(() => OrderCreated, x => x.CorrelateById(m => m.Message.OrderId));
   
   
   2. Event(() => DeduceInventorySucceed, x => x.CorrelateById(m => m.Message.OrderId));
   
   
   3. Event(() => DeduceInventoryFailed, x => x.CorrelateById(m => m.Message.OrderId));
   
   
   4. Event(() => PayOrderSucceed, x => x.CorrelateById(m => m.Message.OrderId));
   
   
   
   


7. 定义状态转移：即状态在什么条件下做怎样的动作完成状态的转移，本例中涉及的正向状态转移有：

(1) 初始状态->已创建：触发条件为OrderCreated事件，同时要发送IDeduceInventoryCommand推动库存服务执行库存扣减。

Initially(
When(OrderCreated)
.Then(cOntext=>
{
context.Saga.OrderId = context.Message.OrderId;
context.Saga.OrderItems = context.Message.OrderItems;
context.Saga.Amount = context.Message.OrderItems.Sum(x => x.Price * x.Qty);
})
.PublishAsync(cOntext=> context.Init(new
{
context.Saga.OrderId,
DeduceInventoryItems =
context.Saga.OrderItems.Select(x => new DeduceInventoryItem(x.SkuId, x.Qty)).ToList()
}))
.TransitionTo(Created));


(2) 已创建-> 库存已扣减：触发条件为DeduceInventorySucceed事件，同时要发送IPayOrderCommand推动支付服务执行订单支付。

During(Created,
When(DeduceInventorySucceed)
.Then(cOntext=>
{
context.Publish(new
{
context.Saga.OrderId,
context.Saga.Amount
});
}).TransitionTo(InventoryDeducted),
When(DeduceInventoryFailed).Then(cOntext=>
{
context.Publish(new
{
context.Saga.OrderId
});
})
);


(3) 库存已扣减->已支付：触发条件为PayOrderSucceed事件，转移到已支付后，流程结束。

During(InventoryDeducted,
When(PayOrderFailed).Then(cOntext=>
{
context.Publish(new
{
context.Message.OrderId,
ReturnInventoryItems =
context.Saga.OrderItems.Select(x => new ReturnInventoryItem(x.SkuId, x.Qty)).ToList()
});
}),
When(PayOrderSucceed).TransitionTo(Paid).Then(cOntext=> context.SetCompleted()));


最终完整版的OrderStateMachine如下所示：

using MassTransit.SmDemo.OrderService.Events;
using MassTransit.SmDemo.Shared.Contracts;
namespace MassTransit.SmDemo.OrderService;
public class OrderStateMachine : MassTransitStateMachine
{
public State Created { get; private set; }
public State InventoryDeducted { get; private set; }
public State Paid { get; private set; }
public State Canceled { get; private set; }
public Event OrderCreated { get; private set; }
public Event DeduceInventorySucceed { get; private set; }
public Event DeduceInventoryFailed { get; private set; }
public Event OrderCanceled { get; private set; }
public Event PayOrderSucceed { get; private set; }
public Event PayOrderFailed { get; private set; }
public Event ReturnInventorySucceed { get; private set; }
public Event OrderStateRequested { get; private set; }

public OrderStateMachine()
{
Event(() => OrderCreated, x => x.CorrelateById(m => m.Message.OrderId));
Event(() => DeduceInventorySucceed, x => x.CorrelateById(m => m.Message.OrderId));
Event(() => DeduceInventoryFailed, x => x.CorrelateById(m => m.Message.OrderId));
Event(() => ReturnInventorySucceed, x => x.CorrelateById(m => m.Message.OrderId));
Event(() => PayOrderSucceed, x => x.CorrelateById(m => m.Message.OrderId));
Event(() => PayOrderFailed, x => x.CorrelateById(m => m.Message.OrderId));
Event(() => OrderCanceled, x => x.CorrelateById(m => m.Message.OrderId));
Event(() => OrderStateRequested, x =>
{
x.CorrelateById(m => m.Message.OrderId);
x.OnM文章来源地址19328.htmlissingInstance(m =>
{
return m.ExecuteAsync(x => x.RespondAsync(new { x.Message.OrderId }));
});
});
InstanceState(x => x.CurrentState);
Initially(
When(OrderCreated)
.Then(cOntext=>
{
context.Saga.OrderId = context.Message.OrderId;
context.Saga.OrderItems = context.Message.OrderItems;
var amount = context.Message.OrderItems.Sum(x => x.Price * x.Qty);
context.Saga.Amount = amount;
})
.PublishAsync(cOntext=> context.Init(new
{
context.Saga.OrderId,
DeduceInventoryItems =
context.Saga.OrderItems.Select(x => new DeduceInventoryItem(x.SkuId, x.Qty)).ToList()
}))
.TransitionTo(Created));
During(Created,
When(DeduceInventorySucceed)
.Then(cOntext=>
{
context.Publish(new
{
contex文章来源地址19328.htmlt.Saga.OrderId,
context.Saga.Amount
});
}).TransitionTo(InventoryDeducted),
When(DeduceInventoryFailed).Then(cOntext=>
{
context.Publish(new
{
context.Saga.OrderId
www.yii666.com });
})
);
During(InventoryDeducted,
When(PayOrderFailed).Then(cOntext=>
{
context.Publish(new
{
context.Message.OrderId,
ReturnInventoryItems =
context.Saga.OrderItems.Select(x => new ReturnInventoryItem(x.SkuId, x.Qty)).ToList()
});
}),
When(PayOrderSucceed).TransitionTo(Paid).Then(cOntext=> context.SetCompleted()),
When(ReturnInventorySucceed)
.ThenAsync(cOntext=> context.Publish(new
{
context.Saga.OrderId
})).TransitionTo(Created));
DuringAny(When(OrderCanceled).TransitionTo(Canceled).ThenAsync(async cOntext=>
{
await Task.Delay(TimeSpan.FromSeconds(10));
await context.SetCompleted();
}));
DuringAny(
When(OrderStateRequested)
.RespondAsync(x => x.Init(new
{
x.Saga.OrderId,
State = x.Saga.CurrentState
}))
);
}
}


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，其仅需要订阅IDeduceInventoryCommand和IReturnInventoryCommand两个命令并实现即可。代码如下所示：

using MassTransit.SmDemo.InventoryService.Repositories;
using MassTransit.SmDemo.Shared.Contracts;
namespace MassTransit.SmDemo.InventoryService.Consumers;
public class DeduceInventoryConsumer : IConsumer
{
private readonly ILogger _logger;
public DeduceInventoryConsumer(ILogger logger)
{
_logger = logger;
}
public async Task Consume(ConsumeContext context)
{
if (!CheckStock(context.Message.DeduceInventoryItems))
{
_logger.LogWarning($"Insufficient stock for order [{context.Message.OrderId}]!");
await context.Publish(
new { context.Message.OrderId, Reason = "insufficient stock" });
}
else
{
_logger.LogInformation($"Inventory has been deducted for order [{context.Message.OrderId}]!");
DeduceStocks(context.Message.DeduceInventoryItems);
await context.Publish(new { context.Message.OrderId });
}
}
private bool CheckStock(List deduceItems)
{
foreach (var stockItem in deduceItems)
{
if (InventoryRepository.GetStock(stockItem.SkuId) }
return true;
}
private void DeduceStocks(List deduceItems)
{
foreach (var stockItem in deduceItems)
文章来源站点https://www.yii666.com/ {
InventoryRepository.TryDeduceStock(stockItem.SkuId, stockItem.Qty);
}
}
}


namespace MassTransit.SmDemo.InventoryService.Consumers;
public class ReturnInventoryConsumer : IConsumer
{
private readonly ILogger _logger;
public ReturnInventoryConsumer(ILogger logger)
{
_logger = logger;
}
public async Task Consume(ConsumeContext context)
{
foreach (var returnInventoryItem in context.Message.ReturnInventoryItems)
{
InventoryRepository.ReturnStock(returnInventoryItem.SkuId, returnInventoryItem.Qty);
}
_logger.LogInformation($"Inventory has been returned for order [{context.Message.OrderId}]!");
await context.Publish(new { context.Message.OrderId });
}
}


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，因此仅需要订阅IPayOrderCommand命令即可，具体PayOrderConsumer实现如下：

using MassTransit.SmDemo.Shared.Contracts;
namespace MassTransit.SmDemo.PaymentService.Consumers;
public class PayOrderConsumer : IConsumer
{
private readonly ILogger _logger;
public PayOrderConsumer(ILogger logger)
{
_logger = logger;
}
public async Task Consume(ConsumeContext context)
{
await Task.Delay(TimeSpan.FromSeconds(10));
if (context.Message.Amount % 2 == 0)
{_logger.LogInformation($"Order [{context.Message.OrderId}] paid successfully!");
await context.Publish(new { context.Message.OrderId });
}
else
{
_logger.LogWarning($"Order [{context.Message.OrderId}] payment failed!");
await context.Publish(new
{
context.Message.OrderId,
Reason = "Insufficient account balance"
});
}
}
}


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：

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：

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：

其中order-state队列绑定到类型为fanout的同名order-stateExchange，其绑定关系如下图所示，该Exchange负责从其他同名事件的Exchange转发事件。

总结

通过以上示例的讲解，相信了解到MassTransit StateMachine的强大之处。StateMachine充当着事务编排器的角色，通过集中定义状态、转移条件和状态转移的执行顺序，实现高内聚的事务流转控制，也确保了其他伴生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。

来源于：MassTransit 知多少 | 基于StateMachine实现Saga编排式分布式事务