Handling transition to state for multiple events

Question

I have a MassTransitStateMachine that orchestrates a process which involves creating multiple events.

Once all of the events are done, I want the state to transition

Answer 1

The solution proposed by Chris won't work in my situation because I have multiple events of the same type arriving. I need to transition only when all of those events have arrived. The CompositeEvent construct doesn't work for this use case.

My solution to this was to raise a new AllDataImported event during the MarkImportCompletedForLocation method. This method now handles determining whether all sub-imports are complete in a thread safe way.

So my state machine definition is:

            During(ImportingData,
            When(DataImported)
                // When we get a data imported event, mark the URI in the locations list as done. 
                .Then(MarkImportCompletedForLocation),

            When(AllDataImported)
                // Once all are done, we can transition to cleaning up...
                .TransitionTo(CleaningUp)
                .Then(CleanUpSources)
        );

The IsAllDataImported method is no longer needed as a filter.

The saga state has a Locations property:

public Dictionary<Uri, bool> Locations { get; set; }

And the MarkImportCompletedForLocation method is defined as follows:

    private void MarkImportCompletedForLocation(BehaviorContext<DataImportSagaState, DataImportedEvent> ctx)
    {
        lock (ctx.Instance.Locations)
        {
            ctx.Instance.Locations[ctx.Data.ImportSource] = true;
            if (ctx.Instance.Locations.Values.All(x => x))
            {
                var allDataImported = new AllDataImportedEvent {CorrelationId = ctx.Instance.CorrelationId};
                this.CreateEventLift(AllDataImported).Raise(ctx.Instance, allDataImported);
            }
        }
    }

(I've just written this so that I understand how the general flow will work; I recognise that the MarkImportCompletedForLocation method needs to be more defensive by verifying that keys exist in the dictionary.)

Answer 2

You can use a composite event to accumulate multiple events into a subsequent event that fires when the dependent events have fired. This is defined using:

CompositeEvent(() => AllDataImported, x => x.ImportStatus, DataImported, MoreDataImported);

During(ImportingData,
    When(DataImported)
        .Then(context => { do something with data }),
    When(MoreDataImported)
        .Then(context => { do smoething with more data}),
    When(AllDataImported)
        .Then(context => { okay, have all data now}));

Then, in your state machine state instance:

class DataImportSagaState :
    SagaStateMachineInstance
{
    public int ImportStatus { get; set; }
}

This should address the problem you are trying to solve, so give it a shot. Note that event order doesn't matter, they can arrive in any order as the state of which events have been received is in the ImportStatus property of the instance.

The data of the individual events is not saved, so you'll need to capture that into the state instance yourself using .Then() methods.