Nugets Used Code walk-through
When the default Saga message mappings do not satisfy the requirements, custom logic can be put in place to allow NServiceBus to find a saga data instance based on which logic best suites the environment.
NHibernate setup
This sample requires NHibernate persistence package and a running Microsoft SQL Server instance configured accordingly. The sample NHibernate setup can be configured according to the environment:
var persistence = endpointConfiguration.UsePersistence<NHibernatePersistence>();
persistence.ConnectionString(@"Data Source=.\SqlExpress;Database=NHCustomSagaFinder;Integrated Security=True");
The Saga
The saga shown in the sample is a very simple order management saga that:
- Handles the creation of an order.
- Offloads the payment process to a different handler.
- Handles the completion of the payment process.
- Completes the order.
public class OrderSaga :
Saga<OrderSagaData>,
IAmStartedByMessages<StartOrder>,
IHandleMessages<CompletePaymentTransaction>,
IHandleMessages<CompleteOrder>
{
static ILog log = LogManager.GetLogger<OrderSaga>();
protected override void ConfigureHowToFindSaga(SagaPropertyMapper<OrderSagaData> mapper)
{
mapper.ConfigureMapping<StartOrder>(_ => _.OrderId)
.ToSaga(_=> _.OrderId);
mapper.ConfigureMapping<CompleteOrder>(_ => _.OrderId)
.ToSaga(_ => _.OrderId);
}
public Task Handle(StartOrder message, IMessageHandlerContext context)
{
Data.PaymentTransactionId = Guid.NewGuid().ToString();
log.Info($"Saga with OrderId {Data.OrderId} received StartOrder with OrderId {message.OrderId}");
var issuePaymentRequest = new IssuePaymentRequest
{
PaymentTransactionId = Data.PaymentTransactionId
};
return context.SendLocal(issuePaymentRequest);
}
public Task Handle(CompletePaymentTransaction message, IMessageHandlerContext context)
{
log.Info($"Transaction with Id {Data.PaymentTransactionId} completed for order id {Data.OrderId}");
var completeOrder = new CompleteOrder
{
OrderId = Data.OrderId
};
return context.SendLocal(completeOrder);
}
public Task Handle(CompleteOrder message, IMessageHandlerContext context)
{
log.Info($"Saga with OrderId {Data.OrderId} received CompleteOrder with OrderId {message.OrderId}");
MarkAsComplete();
return Task.CompletedTask;
}
}
From the process point of view it is important to note that the saga is not sending to the payment processor the order id, instead it is sending a payment transaction id. A saga can be correlated given more than one unique attribute, such as OrderId and PaymentTransactionId, requiring both to be treated as unique.
class CompletePaymentTransactionSagaFinder :
IFindSagas<OrderSagaData>.Using<CompletePaymentTransaction>
{
public Task<OrderSagaData> FindBy(CompletePaymentTransaction message, SynchronizedStorageSession storageSession, ReadOnlyContextBag context)
{
var session = storageSession.Session();
var orderSagaData = session.QueryOver<OrderSagaData>()
.Where(d => d.PaymentTransactionId == message.PaymentTransactionId)
.SingleOrDefault();
return Task.FromResult(orderSagaData);
}
}
Building a saga finder requires to define a class that implements the IFindSagas<TSagaData>.Using<TMessage> interface. The class will be automatically picked up by NServiceBus at configuration time and used each time a message of type TMessage, that is expected to load a saga of type TSagaData, is received. The FindBy method will be invoked by NServiceBus.
ConfigureHowToFindSaga method, that is required, is empty since a saga finder is provided for each message type that the saga is handling. It is not required to provide a saga finder for every message type, a mix of standard saga mappings and custom saga finding is a valid scenario.
