How to correctly correlate a controller saga which starts multiple instances of another controller saga?

Question

I have a controller saga which used to have a step starting a process containing 3 actions in one transaction. I am now in the process of refactoring this sub-process into a separate saga. The result of this will be that the original saga will start multiple instances of the new "sub-saga" (This sub-saga will also be started by other non-saga processes, through the same command). My problem is how to correlate this hierarchy of sagas in the best possible way?

In the following example, the main saga will try to start three instances of the sub-saga with the same correlationId. Even if this was to work, the 3 instances would interfere with eachother by handling "completed events" originating from all instances.

public class MyMainSaga : Saga<MyMainSagaData>, 
    IAmStartedByMessages<MyMainCommand>,
    IHandleMessage<MySubProcessCommandCompletedEvent>
{
    protected override void ConfigureHowToFindSaga(SagaPropertyMapper<MyMainSagaData> mapper)
    {
        mapper.ConfigureMapping<MyMainCommandCompletedEvent>(message => message.CorrelationId).ToSaga(data => data.CorrelationId);
    }

    public void Handle(MyMainCommand message)
    {
        Data.CorrelationId = message.CorrelationId;
        foreach (var item in message.ListOfObjectsToProcess)
        {
            Bus.Send(new MySubProcessCommand{
                CorrelationId = Data.CorrelationId,
                ObjectId = item.Id
            });
        }
    }

    public void Handle(MySubProcessCommandCompletedEvent message)
    {
        SetHandledStatus(message.ObjectId);
        if(AllObjectsWhereProcessed())
            MarkAsComplete();     
    }       
}


public class MySubSaga : Saga<MySubSagaData>, 
    IAmStartedByMessages<MySubProcessCommand>,
    IHandleMessage<Step1CommandCompletedEvent>,
    IHandleMessage<Step2CommandCompletedEvent>,
    IHandleMessage<Step3CommandCompletedEvent>
{
    protected override voidConfigureHowToFindSaga(SagaPropertyMapper<MySubSagaData> mapper)
    {
        mapper.ConfigureMapping<Step1CommandCompletedEvent>(message => message.CorrelationId).ToSaga(data => data.CorrelationId);
        mapper.ConfigureMapping<Step2CommandCompletedEvent>(message => message.CorrelationId).ToSaga(data => data.CorrelationId);
        mapper.ConfigureMapping<Step3CommandCompletedEvent>(message => message.CorrelationId).ToSaga(data => data.CorrelationId);
    }

    public void Handle(MySubProcessCommand message)
    {
        Data.CorrelationId = message.CorrelationId;
        Data.ObjectId = message.ObjectId;
        Bus.Send(new Step1Command{
            CorrelationId = Data.CorrelationId;  
        });
    }

    public void Handle(Step1CommandCompletedEvent message)
    {
        Bus.Send(new Step2Command{
            CorrelationId = Data.CorrelationId;  
        });
    }

    public void Handle(Step2CommandCompletedEvent message)
    {
        Bus.Send(new Step3Command{
            CorrelationId = Data.CorrelationId;  
        });
    }

    public void Handle(Step3CommandCompletedEvent message)
    {
        Bus.Publish<MySubProcessCommandCompletedEvent>(e => {
            e.CorrelationId = Data.CorrelationId;
            e.ObjectId = Data.ObjectId;
        });
        MarkAsComplete();
    }   
}

The only sollution I see is to change the sub-saga to generate a separate correlationId as well as keeping the originator id. E.g:

    public void Handle(MySubProcessCommand message)
    {
        Data.CorrelationId = Guid.NewGuid();
        Data.OriginatorCorrelationId = message.CorrelationId;
        Data.ObjectId = message.ObjectId;
        Bus.Send(new Step1Command{
            CorrelationId = Data.CorrelationId;  
        });
    }
    public void Handle(Step1CommandCompletedEvent message)
    {
        Bus.Send(new Step2Command{
            CorrelationId = Data.CorrelationId;  
        });
    }

    public void Handle(Step2CommandCompletedEvent message)
    {
        Bus.Send(new Step3Command{
            CorrelationId = Data.CorrelationId;  
        });
    }

    public void Handle(Step3CommandCompletedEvent message)
    {
        Bus.Publish<MySubProcessCommandCompletedEvent>(e => {
            e.CorrelationId = Data.OriginatorCorrelationId;
            e.ObjectId = Data.ObjectId;
        });
        MarkAsComplete();
    } 

Is there a "best practice" solution to this problem? I have been thinking of using Bus.Reply, notifying the MainSaga when the sub-saga has completed. Problem with this is that another consumer is also sending the MySubProcessCommand without waiting for a completed event/reply.

Answer 1

The best practice is to use ReplyToOriginator() in the sub-saga to communicate back to the main saga. This method is exposed on the Saga base class.

There are two ways to fix the issue of starting the sub-saga both by the main saga and a different initiator.

1. Use two different commands.

Let two different commands start the sub-saga, like MySubProcessFromMainSagaCommand and MySubProcessFromSomewhereElseCommand. It is fine to have multiple IAmStartedByMessages<> for a Saga.

2. Extend MySubProcessCommand

Include some data in MySubProcessCommand to indicate whether it came from the main saga or the other initiator.

Either way will give you enough information to store how the sub-saga was started, for example Data.WasInitatedByMainSaga. Check this in the sub-saga completion logic. If it is true, do a ReplyToOriginator() to communicate back to the originating main saga. If not, skip the reply.