CODEWITHSUNDEEP
nservicebus
struhtanov
struhtanov

Reputation: 402

Impossible to have two sagas that handle the same message type

I have 2 different sagas (I mean saga types) that handle the same message.

     public class AttachMessageToBugSaga : TpSaga<AttachMessageToBugSagaData>, IAmStartedByMessages<MessageIsNotAttached>, IHandleMessages<MessageAttachedToGeneralMessage>
    {
        public override void ConfigureHowToFindSaga()
        {
            ConfigureMapping<MessageAttachedToGeneralMessage>(
             saga => saga.Id,
             message => message.SagaId
             );
        }
        public void Handle(MessageIsNotAttachedToBug message)
        {
            Send(new AttachMessageToGeneralCommand { MessageId = 66, GeneralId = 13 });
        }

        public void Handle(MessageAttachedToGeneralMessage message)
        {
            //do some stuf fhere
        }
    }

    public class AttachMessageToBugSagaData : IContainSagaData
    {
        public Guid Id { get; set; }
        public string Originator { get; set; }
        public string OriginalMessageId { get; set; }
    }

    public class AttachMessageToRequestSaga : TpSaga<AttachMessageToRequestSagaData>, IAmStartedByMessages<MessageIsNotAttachedToRequest>, IHandleMessages<MessageAttachedToGeneralMessage>
    {
        public override void ConfigureHowToFindSaga()
        {
            ConfigureMapping<MessageAttachedToGeneralMessage>(
             saga => saga.Id,
             message => message.SagaId
             );
        }

        public void Handle(MessageIsNotAttachedMessageToRequest message)
        {
            //do some stuff here
        }

        public void Handle(MessageAttachedToGeneralMessage message)
        {
            //do some stuff here
        }
    }

    public class AttachMessageToRequestSagaData : IContainSagaData
    {
        public Guid Id { get; set; }
        public string Originator { get; set; }
        public string OriginalMessageId { get; set; }
    }

When I run the sample I get an exception :

System.InvalidCastException: Unable to cast object of type 'MyCustomPlugin.AttachMessageToGeneralSagaData' to type 'MyCustomPlugin.AttachMessageToRequestSagaData'.

I understand why it happens, but I still need some workaround. I tried to implement my own IFindSagas class : 

public class SagaFinder : IFindSagas<AttachMessageToGeneralSagaData>.Using<MessageAttachedToGeneralMessage>,
IFindSagas<AttachMessageToRequestSagaData>.Using<MessageAttachedToGeneralMessage>,
IFindSagas<AttachMessageToRequestSagaData>.Using<MessageIsNotAttachedToRequest>,
IFindSagas<AttachMessageToRequestSagaData>.Using<MessageIsNotAttachedToBug>
{
    AttachMessageToGeneralSagaData IFindSagas<AttachMessageToGeneralSagaData>.Using<MessageAttachedToGeneralMessage>.FindBy(MessageAttachedToGeneralMessage message)
    {
        return ObjectFactory.GetInstance<AttachMessageToGeneralSagaData>();
    }

    AttachMessageToRequestSagaData IFindSagas<AttachMessageToRequestSagaData>.Using<MessageAttachedToGeneralMessage>.FindBy(MessageAttachedToGeneralMessage message)
    {
        return ObjectFactory.GetInstance<AttachMessageToRequestSagaData>();
    }

    public AttachMessageToRequestSagaData FindBy(MessageIsNotAttachedToRequest message)
    {
        return new AttachMessageToRequestSagaData();
    }

    public AttachMessageToRequestSagaData FindBy(MessageIsNotAttachedToBug message)
    {
        return new AttachMessageToRequestSagaData();
    }
}

But I do not get into my finders for "MessageAttachedToGeneralMessage". Please tell me if there is some other workaround, or how to make this example working.

Upvotes: 1

Views: 953

Answers (2)

struhtanov
struhtanov

Reputation: 402

Finally (after digging into the source code) I've found the solution. It seems the only way is to implement my own SagaPersister, where I can do anything I want.

Default implementation in NserviceBus of InMemorySagaPersister has the following code : 

    T ISagaPersister.Get<T>(Guid sagaId)
    {
        ISagaEntity result;
        data.TryGetValue(sagaId, out result);

        return (T)result;
    }

And exception occurs while casting .

Upvotes: 0

David Boike
David Boike

Reputation: 18625

I'm not sure that having more than one Saga within the same process boundary works very well - at least, I've had problems with it too. It's probably better (in general) to have Sagas separated into two different processes anyway, because otherwise it would cause a lot of locking and potentially deadlocks on your saga storage.

Is your message that is handled by 2 Sagas Sent or Published? If it's published (or can be made to), it would be easy to separate the Sagas into two separate assemblies. Just be sure to manually call Bus.Subscribe() for the message type in each Saga, since Sagas don't auto-subscribe to messages listed in the app.config.

If your message is Sent, and there's nothing you can do to change it, then create a central handler for your existing message type that either Publishes a second message type to go to both Sagas, or Sends two separate messages to each saga.

Upvotes: 2

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