原文地址:Consumer Sagas
consumer saga是一個由CorrelationId標識的類,它定義了由saga repository持久化的狀態。除了狀態之外,還可以向saga類添加接口,定義由saga處理的事件。這種狀態和行爲在單個類中的組合就是一個consumer saga。在下面的示例中,定義了由SubmitOrder消息發起的order saga。
Interfaces
InitiatedBy
public record SubmitOrder :
CorrelatedBy<Guid>
{
public Guid CorrelationId { get; init; }
public DateTime OrderDate { get; init; }
}
public class OrderSaga :
ISaga,
InitiatedBy<SubmitOrder>
{
public Guid CorrelationId { get; set; }
public DateTime? SubmitDate { get; set; }
public DateTime? AcceptDate { get; set; }
public async Task Consume(ConsumeContext<SubmitOrder> context)
{
SubmitDate = context.Message.OrderDate;
}
}
當Saga的接收端點接收到SubmitOrder消息時,將使用CorrelationId屬性來確定是否存在具有該CorrelationId的現有Saga實例。如果沒有找到現有的實例,repository將創建一個新的saga實例,並在新實例上調用Consume方法。在Consume方法完成之後,repository保存新創建的實例。
Orchestrates
要定義由現有的saga實例(如OrderAccepted)編排的事件,需要指定一個額外的接口和方法。
public record OrderAccepted :
CorrelatedBy<Guid>
{
public Guid CorrelationId { get; init; }
public DateTime Timestamp { get; init; }
}
public class OrderSaga :
ISaga,
InitiatedBy<SubmitOrder>,
Orchestrates<OrderAccepted>,
{
public Guid CorrelationId { get; set; }
public DateTime? SubmitDate { get; set; }
public DateTime? AcceptDate { get; set; }
public async Task Consume(ConsumeContext<SubmitOrder> context) {...}
public async Task Consume(ConsumeContext<OrderAccepted> context)
{
AcceptDate = context.Message.Timestamp;
}
}
InitiatedByOrOrchestrates
要定義一個可以啓動一個新的或編排一個現有的saga實例(如OrderInvoiced)的事件,需要指定一個額外的接口和方法。
public record OrderInvoiced :
CorrelatedBy<Guid>
{
public Guid CorrelationId { get; init; }
public DateTime Timestamp { get; init; }
public decimal Amount { get; init; }
}
public class OrderPaymentSaga :
ISaga,
InitiatedByOrOrchestrates<OrderInvoiced>
{
public Guid CorrelationId { get; set; }
public DateTime? InvoiceDate { get; set; }
public decimal? Amount { get; set; }
public async Task Consume(ConsumeContext<OrderInvoiced> context)
{
InvoiceDate = context.Message.Timestamp;
Amount = context.Message.Amount;
}
}
Observes
要定義由未實現CorrelatedBy接口(如OrderShipped)的現有saga實例觀察到的事件,需要指定一個額外的接口和方法。
public record OrderShipped
{
public Guid OrderId { get; init; }
public DateTime ShipDate { get; init; }
}
public class OrderSaga :
ISaga,
InitiatedBy<SubmitOrder>,
Orchestrates<OrderAccepted>,
Observes<OrderShipped, OrderSaga>
{
public Guid CorrelationId { get; set; }
public DateTime? SubmitDate { get; set; }
public DateTime? AcceptDate { get; set; }
public DateTime? ShipDate { get; set; }
public async Task Consume(ConsumeContext<SubmitOrder> context) {...}
public async Task Consume(ConsumeContext<OrderAccepted> context) {...}
public async Task Consume(ConsumeContext<OrderShipped> context)
{
ShipDate = context.Message.ShipDate;
}
public Expression<Func<OrderSaga, OrderShipped, bool>> CorrelationExpression =>
(saga,message) => saga.CorrelationId == message.OrderId;
}
Configuration
要在配置MassTransit時添加一個saga,請使用如下所示的AddSaga方法
services.AddMassTransit(x =>
{
x.AddSaga<OrderSaga>()
.InMemoryRepository();
});