簡介

  目前的.net 生態中，最終一致性元件的選擇一直是一個問題。本地事務表（cap）需要在每個服務的資料庫中插入訊息表，而且做不了此類事務 比如：建立訂單需要 餘額滿足+庫存滿足，庫存和餘額處於兩個服務中。masstransit 是我目前主要用的方案。以往一般都用 masstransit 中的 sagas 來實現 最終一致性，但是隨著併發的增加必定會對sagas 持久化的資料庫造成很大的壓力，根據stackoverflow 中的一個回答 我發現了 一個用  Request/Response 與 Courier 功能 實現最終一致性的方案 Demo地址。

Masstransit 中 Resquest/Response 功能 

 訊息DTO

    public class SampleMessageCommand
    {
    }

 消費者

    public class SampleMessageCommandHandler : IConsumer<SampleMessageCommand>
    {
        public async Task Consume(ConsumeContext<SampleMessageCommand> context)
        {
            await context.RespondAsync(new SampleMessageCommandResult() { Data = "Sample" });
        }
    }

 返回結果DTO

    public class SampleMessageCommandResult
    {
        public string Data { get; set; }
    }

 呼叫方式與註冊方式略過，詳情請看 官方文件。

  本質上使用訊息佇列實現 Resquest/Response，客戶端（生產者）將請求訊息傳送至指定訊息佇列並賦予RequestId和ResponseAddress（臨時佇列 rabbitmq），服務端（消費者）消費訊息並把 需要返回的訊息放入指定ResponseAddress,客戶端收到 Response message  通過匹配 RequestId 找到 指定Request，最後返回資訊。

Masstransit 中 Courier  功能

  通過有序組合一系列的Activity，得到一個routing slip。每個 activity(忽略 Execute Activities) 都有 Execute 和 Compensate 兩個方法。Compensate 用來執撤銷 Execute 方法產生的影響（就是回退 Execute 方法）。每個 Activity Execute 最後都會 呼叫 Completed 方法把 回退所需要的的資訊記錄在message中，最後持久化到訊息佇列的某一個訊息中。

 餘額扣減的Activity ，這裡的 DeductBalanceModel 是請求扣減的資料模型，DeductBalanceLog 是回退時需要用到的資訊。

public class DeductBalanceActivity : IActivity<DeductBalanceModel, DeductBalanceLog>
    {
        private readonly ILogger<DeductBalanceActivity> logger;
        public DeductBalanceActivity(ILogger<DeductBalanceActivity> logger)
        {
            this.logger = logger;
        }
        public async Task<CompensationResult> Compensate(CompensateContext<DeductBalanceLog> context)
        {
            logger.LogInformation("還原餘額");
            var log = context.Log; //可以獲取 所有execute 完成時儲存的資訊
            //throw new ArgumentException("some things were wrong");
            return context.Compensated();
        }

        public async Task<ExecutionResult> Execute(ExecuteContext<DeductBalanceModel> context)
        {

            logger.LogInformation("扣減餘額");
            await Task.Delay(100);
            return context.Completed(new DeductBalanceLog() { Price = 100 });
        }
    }

      扣減庫存 Activity

    public class DeductStockActivity : IActivity<DeductStockModel, DeductStockLog>
    {
        private readonly ILogger<DeductStockActivity> logger;
        public DeductStockActivity(ILogger<DeductStockActivity> logger)
        {
            this.logger = logger;
        }
        public async Task<CompensationResult> Compensate(CompensateContext<DeductStockLog> context)
        {
            var log = context.Log;
            logger.LogInformation("還原庫存");
            return context.Compensated();
        }

        public async Task<ExecutionResult> Execute(ExecuteContext<DeductStockModel> context)
        {
            var argument = context.Arguments;
            logger.LogInformation("扣減庫存");
            await Task.Delay(100);
            return context.Completed(new DeductStockLog() { ProductId = argument.ProductId, Amount = 1 });
        }
    }

       生成訂單 Execute Activity

    public class CreateOrderActivity : IExecuteActivity<CreateOrderModel>
    {
        private readonly ILogger<CreateOrderActivity> logger;
        public CreateOrderActivity(ILogger<CreateOrderActivity> logger)
        {
            this.logger = logger;
        }
        public async Task<ExecutionResult> Execute(ExecuteContext<CreateOrderModel> context)
        {
            logger.LogInformation("建立訂單");
            await Task.Delay(100);
            //throw new CommonActivityExecuteFaildException("當日訂單已達到上限");
            return context.CompletedWithVariables(new CreateOrderResult { OrderId="111122",Message="建立訂單成功" });
        }
    }

　　組裝 以上 Activity 生成一個 Routing Slip，這是一個有序的組合，扣減庫存=》扣減餘額=》生成訂單

            var builder = new RoutingSlipBuilder(NewId.NextGuid());

            builder.AddActivity("DeductStock", new Uri($"{configuration["RabbitmqConfig:HostUri"]}/DeductStock_execute"), new DeductStockModel { ProductId = request.Message.ProductId });

            builder.AddActivity("DeductBalance", new Uri($"{configuration["RabbitmqConfig:HostUri"]}/DeductBalance_execute"), new DeductBalanceModel { CustomerId = request.Message.CustomerId, Price = request.Message.Price });

            builder.AddActivity("CreateOrder", new Uri($"{configuration["RabbitmqConfig:HostUri"]}/CreateOrder_execute"), new CreateOrderModel { Price = request.Message.Price, CustomerId = request.Message.CustomerId, ProductId = request.Message.ProductId });

            var routingSlip = builder.Build();

　　執行 Routing Slip

await bus.Execute(routingSlip);

      這裡是沒有任何返回值的，所有activity都是 非同步執行，雖然所有的activity可以執行完成或者由於某個Activity執行出錯而全部回退。（其實這裡有一種更壞的情況就是 Compensate 出錯，預設情況下 Masstransit 只會傳送一個回退錯誤的訊息，後面講到建立訂單的時候我會把它塞到錯誤佇列裡，這樣我們可以通過修改 Compensate bug後重新匯入到正常佇列來修正資料），這個功能完全滿足不了 建立訂單這個需求，執行 await bus.Execute(routingSlip) 後我們完全不知道訂單到底建立成功，還是由於庫存或餘額不足而失敗了（非同步）。

     還好 routing slip 在執行過程中產生很多訊息，比如 RoutingSlipCompleted ，RoutingSlipCompensationFailed ，RoutingSlipActivityCompleted，RoutingSlipActivityFaulted 等，具體文件，我們可以訂閱這些事件，再結合Request/Response 實現 建立訂單的功能。

）

實現建立訂單（庫存滿足+餘額滿足）長流程

建立訂單 command 

    /// <summary>
    /// 長流程 分散式事務
    /// </summary>
    public class CreateOrderCommand
    {
        public string ProductId { get; set; }
        public string CustomerId { get; set; }
        public int Price { get; set; }
    }

  事務第一步，扣減庫存相關 程式碼

  public class DeductStockActivity : IActivity<DeductStockModel, DeductStockLog>
    {
        private readonly ILogger<DeductStockActivity> logger;
        public DeductStockActivity(ILogger<DeductStockActivity> logger)
        {
            this.logger = logger;
        }
        public async Task<CompensationResult> Compensate(CompensateContext<DeductStockLog> context)
        {
            var log = context.Log;
            logger.LogInformation("還原庫存");
            return context.Compensated();
        }

        public async Task<ExecutionResult> Execute(ExecuteContext<DeductStockModel> context)
        {
            var argument = context.Arguments;
            logger.LogInformation("扣減庫存");
            await Task.Delay(100);
            return context.Completed(new DeductStockLog() { ProductId = argument.ProductId, Amount = 1 });
        }
    }
    public class DeductStockModel
    {
        public string ProductId { get; set; }
    }
    public class DeductStockLog
    {
        public string ProductId { get; set; }
        public int Amount { get; set; }
    }

 事務第二步，扣減餘額相關程式碼

public class DeductBalanceActivity : IActivity<DeductBalanceModel, DeductBalanceLog>
    {
        private readonly ILogger<DeductBalanceActivity> logger;
        public DeductBalanceActivity(ILogger<DeductBalanceActivity> logger)
        {
            this.logger = logger;
        }
        public async Task<CompensationResult> Compensate(CompensateContext<DeductBalanceLog> context)
        {
            logger.LogInformation("還原餘額");
            var log = context.Log;
            //throw new ArgumentException("some things were wrong");
            return context.Compensated();
        }

        public async Task<ExecutionResult> Execute(ExecuteContext<DeductBalanceModel> context)
        {

            logger.LogInformation("扣減餘額");
            await Task.Delay(100);
            return context.Completed(new DeductBalanceLog() { Price = 100 });
        }
    }
    public class DeductBalanceModel
    {
        public string CustomerId { get; set; }
        public int Price { get; set; }
    }
    public class DeductBalanceLog
    {
        public int Price { get; set; }
    }

 事務第三步，建立訂單相關程式碼

 public class CreateOrderActivity : IExecuteActivity<CreateOrderModel>
    {
        private readonly ILogger<CreateOrderActivity> logger;
        public CreateOrderActivity(ILogger<CreateOrderActivity> logger)
        {
            this.logger = logger;
        }
        public async Task<ExecutionResult> Execute(ExecuteContext<CreateOrderModel> context)
        {
            logger.LogInformation("建立訂單");
            await Task.Delay(100);
            //throw new CommonActivityExecuteFaildException("當日訂單已達到上限");
            return context.CompletedWithVariables(new CreateOrderResult { OrderId="111122",Message="建立訂單成功" });
        }
    }
    public class CreateOrderModel
    {
        public string ProductId { get; set; }
        public string CustomerId { get; set; }
        public int Price { get; set; }
    }
    public class CreateOrderResult
    {
        public string OrderId { get; set; }
        public string Message { get; set; }
    }

   我通過 消費 建立訂單 request，獲取 request 的 response 地址與 RequestId，這兩個值 返回 response 時需要用到，我把這些資訊存到 RoutingSlip中，並且訂閱 RoutingSlipEvents.Completed | RoutingSlipEvents.Faulted | RoutingSlipEvents.CompensationFailed 三種事件，當這三種訊息出現時 我會根據 事件類別 和RoutingSlip中 之前加入的 （response 地址與 RequestId）生成 Response ，整個過程大概就是這麼個意思，沒理解可以看demo。這裡由於每一個事物所需要用到的 RoutingSlip + Request/Response 步驟都類似 可以抽象一下（模板方法），把Activity 的組裝 延遲到派生類去解決，這個代理類Masstransit有 ，但是官方沒有顧及到 CompensationFailed 的情況，所以我乾脆自己再寫一個。

    public abstract class RoutingSlipDefaultRequestProxy<TRequest> :
        IConsumer<TRequest>
        where TRequest : class
    {
        public async Task Consume(ConsumeContext<TRequest> context)
        {
            var builder = new RoutingSlipBuilder(NewId.NextGuid());

            builder.AddSubscription(context.ReceiveContext.InputAddress, RoutingSlipEvents.Completed | RoutingSlipEvents.Faulted | RoutingSlipEvents.CompensationFailed);
            
            builder.AddVariable("RequestId", context.RequestId);
            builder.AddVariable("ResponseAddress", context.ResponseAddress);
            builder.AddVariable("FaultAddress", context.FaultAddress);
            builder.AddVariable("Request", context.Message);

            await BuildRoutingSlip(builder, context);

            var routingSlip = builder.Build();

            await context.Execute(routingSlip).ConfigureAwait(false);
        }

        protected abstract Task BuildRoutingSlip(RoutingSlipBuilder builder, ConsumeContext<TRequest> request);
    }

 這個 是派生類 Routing slip 的拼裝過程 

    public class CreateOrderRequestProxy : RoutingSlipDefaultRequestProxy<CreateOrderCommand>

    {
        private readonly IConfiguration configuration;
        public CreateOrderRequestProxy(IConfiguration configuration)
        {
            this.configuration = configuration;
        }
        protected override Task BuildRoutingSlip(RoutingSlipBuilder builder, ConsumeContext<CreateOrderCommand> request)
        {
            builder.AddActivity("DeductStock", new Uri($"{configuration["RabbitmqConfig:HostUri"]}/DeductStock_execute"), new DeductStockModel { ProductId = request.Message.ProductId });

            builder.AddActivity("DeductBalance", new Uri($"{configuration["RabbitmqConfig:HostUri"]}/DeductBalance_execute"), new DeductBalanceModel { CustomerId = request.Message.CustomerId, Price = request.Message.Price });

            builder.AddActivity("CreateOrder", new Uri($"{configuration["RabbitmqConfig:HostUri"]}/CreateOrder_execute"), new CreateOrderModel { Price = request.Message.Price, CustomerId = request.Message.CustomerId, ProductId = request.Message.ProductId });

            return Task.CompletedTask;
        }
    }

  構造response 基類，主要是對三種情況做處理。

    public abstract class RoutingSlipDefaultResponseProxy<TRequest, TResponse, TFaultResponse> : IConsumer<RoutingSlipCompensationFailed>, IConsumer<RoutingSlipCompleted>,
        IConsumer<RoutingSlipFaulted>
        where TRequest : class
        where TResponse : class
        where TFaultResponse : class
    {
        public async Task Consume(ConsumeContext<RoutingSlipCompleted> context)
        {
            var request = context.Message.GetVariable<TRequest>("Request");
            var requestId = context.Message.GetVariable<Guid>("RequestId");

            Uri responseAddress = null;
            if (context.Message.Variables.ContainsKey("ResponseAddress"))
                responseAddress = context.Message.GetVariable<Uri>("ResponseAddress");

            if (responseAddress == null)
                throw new ArgumentException($"The response address could not be found for the faulted routing slip: {context.Message.TrackingNumber}");

            var endpoint = await context.GetResponseEndpoint<TResponse>(responseAddress, requestId).ConfigureAwait(false);

            var response = await CreateResponseMessage(context, request);

            await endpoint.Send(response).ConfigureAwait(false);
        }

        public async Task Consume(ConsumeContext<RoutingSlipFaulted> context)
        {
            var request = context.Message.GetVariable<TRequest>("Request");
            var requestId = context.Message.GetVariable<Guid>("RequestId");

            Uri faultAddress = null;
            if (context.Message.Variables.ContainsKey("FaultAddress"))
                faultAddress = context.Message.GetVariable<Uri>("FaultAddress");
            if (faultAddress == null && context.Message.Variables.ContainsKey("ResponseAddress"))
                faultAddress = context.Message.GetVariable<Uri>("ResponseAddress");

            if (faultAddress == null)
                throw new ArgumentException($"The fault/response address could not be found for the faulted routing slip: {context.Message.TrackingNumber}");

            var endpoint = await context.GetFaultEndpoint<TResponse>(faultAddress, requestId).ConfigureAwait(false);

            var response = await CreateFaultedResponseMessage(context, request, requestId);

            await endpoint.Send(response).ConfigureAwait(false);
        }
        public async Task Consume(ConsumeContext<RoutingSlipCompensationFailed> context)
        {
            var request = context.Message.GetVariable<TRequest>("Request");
            var requestId = context.Message.GetVariable<Guid>("RequestId");

            Uri faultAddress = null;
            if (context.Message.Variables.ContainsKey("FaultAddress"))
                faultAddress = context.Message.GetVariable<Uri>("FaultAddress");
            if (faultAddress == null && context.Message.Variables.ContainsKey("ResponseAddress"))
                faultAddress = context.Message.GetVariable<Uri>("ResponseAddress");

            if (faultAddress == null)
                throw new ArgumentException($"The fault/response address could not be found for the faulted routing slip: {context.Message.TrackingNumber}");

            var endpoint = await context.GetFaultEndpoint<TResponse>(faultAddress, requestId).ConfigureAwait(false);

            var response = await CreateCompensationFaultedResponseMessage(context, request, requestId);

            await endpoint.Send(response).ConfigureAwait(false);
        }
        protected abstract Task<TResponse> CreateResponseMessage(ConsumeContext<RoutingSlipCompleted> context, TRequest request);

        protected abstract Task<TFaultResponse> CreateFaultedResponseMessage(ConsumeContext<RoutingSlipFaulted> context, TRequest request, Guid requestId);
        protected abstract Task<TFaultResponse> CreateCompensationFaultedResponseMessage(ConsumeContext<RoutingSlipCompensationFailed> context, TRequest request, Guid requestId);
    }

 Response 派生類 ，這裡邏輯可以隨自己定義，我也是隨便寫了個 CommonResponse和一個業務錯誤拋錯（犧牲了一點效能）。

    public class CreateOrderResponseProxy :
            RoutingSlipDefaultResponseProxy<CreateOrderCommand, CommonCommandResponse<CreateOrderResult>, CommonCommandResponse<CreateOrderResult>>
    {

        protected override Task<CommonCommandResponse<CreateOrderResult>> CreateResponseMessage(ConsumeContext<RoutingSlipCompleted> context, CreateOrderCommand request)
        {

            return Task.FromResult(new CommonCommandResponse<CreateOrderResult>
            {
                Status = 1,
                Result = new CreateOrderResult
                {
                    Message = context.Message.Variables.TryGetAndReturn(nameof(CreateOrderResult.Message))?.ToString(),
                    OrderId = context.Message.Variables.TryGetAndReturn(nameof(CreateOrderResult.OrderId))?.ToString(),
                }
            });
        }
        protected override Task<CommonCommandResponse<CreateOrderResult>> CreateFaultedResponseMessage(ConsumeContext<RoutingSlipFaulted> context, CreateOrderCommand request, Guid requestId)
        {
            var commonActivityExecuteFaildException = context.Message.ActivityExceptions.FirstOrDefault(m => m.ExceptionInfo.ExceptionType == typeof(CommonActivityExecuteFaildException).FullName);
            if (commonActivityExecuteFaildException != null)
            {
                return Task.FromResult(new CommonCommandResponse<CreateOrderResult>
                {
                    Status = 2,
                    Message = commonActivityExecuteFaildException.ExceptionInfo.Message
                });
            }
            // system error  log here
            return Task.FromResult(new CommonCommandResponse<CreateOrderResult>
            {
                Status = 3,
                Message = "System error"
            });
        }

        protected override Task<CommonCommandResponse<CreateOrderResult>> CreateCompensationFaultedResponseMessage(ConsumeContext<RoutingSlipCompensationFailed> context, CreateOrderCommand request, Guid requestId)
        {
            var exception = context.Message.ExceptionInfo;
            // lg here context.Message.ExceptionInfo
            return Task.FromResult(new CommonCommandResponse<CreateOrderResult>
            {
                Status = 3,
                Message = "System error"
            });           
        }
    }

對於  CompensationFailed 的處理 通過 ActivityCompensateErrorTransportFilter 實現 傳送到錯誤訊息佇列，後續通過prometheus + rabbitmq-exporter + alertmanager 觸發告警 通知相關人員處理。

  public class ActivityCompensateErrorTransportFilter<TActivity, TLog> : IFilter<CompensateActivityContext<TActivity, TLog>>
        where TActivity : class, ICompensateActivity<TLog>
        where TLog : class
    {
        public void Probe(ProbeContext context)
        {
            context.CreateFilterScope("moveFault");
        }

        public async Task Send(CompensateActivityContext<TActivity, TLog> context, IPipe<CompensateActivityContext<TActivity, TLog>> next)
        {
            try
            {
                await next.Send(context).ConfigureAwait(false);
            }
            catch(Exception ex)
            {
                if (!context.TryGetPayload(out IErrorTransport transport))
                    throw new TransportException(context.ReceiveContext.InputAddress, $"The {nameof(IErrorTransport)} was not available on the {nameof(ReceiveContext)}.");
                var exceptionReceiveContext = new RescueExceptionReceiveContext(context.ReceiveContext, ex);
                await transport.Send(exceptionReceiveContext);
            }
        }
    }

註冊 filter 

    public class RoutingSlipCompensateErrorSpecification<TActivity, TLog> : IPipeSpecification<CompensateActivityContext<TActivity, TLog>>
        where TActivity : class, ICompensateActivity<TLog>
        where TLog : class
    {
        public void Apply(IPipeBuilder<CompensateActivityContext<TActivity, TLog>> builder)
        {
            builder.AddFilter(new ActivityCompensateErrorTransportFilter<TActivity, TLog>());
        }

        public IEnumerable<ValidationResult> Validate()
        {
           yield return this.Success("success");
        }
    }


            cfg.ReceiveEndpoint("DeductStock_compensate", ep =>
            {
                ep.PrefetchCount = 100;
                ep.CompensateActivityHost<DeductStockActivity, DeductStockLog>(context.Container, conf =>
                 {
                     conf.AddPipeSpecification(new RoutingSlipCompensateErrorSpecification<DeductStockActivity, DeductStockLog>());
                 });

            });

實現建立產品（建立完成+新增庫存）

實現了 建立訂單的功能，整個流程其實是同步的，我在想能不能實現最為簡單的最終一致性 比如 建立一個產品 ，然後非同步生成它的庫存 ，我發現是可以的，因為我們可以監聽到每一個Execute Activity 的完成事件,並且把出錯時的資訊通過 filter 塞到 錯誤佇列中。

這裡的程式碼就不貼了，詳情請看 demo