Event Store subscription service to MongoDB

Question

I was wondering if it was possible to create a subscription service to Mongo via Get Event Store? Perhaps I have phrased this incorrectly but let me explain. I currently have a process which writes events to a Mongo Database using NEventStore. What I would like to do is have a subscription service which subscribes to a Stream in Mongo.

Not been able to find anything on the interweb about this in particular however is this possible? My question in a nutshell perhaps is can you mix and match the two together or in order to do this I must write my events to eventstore as opposed to Mongo? Perhaps, I am going about this wrong and there is an alternative?

I can see my event being written however it fails to trigger EventAppeared. All of this is being done locally on my machine for now.

I have tried creating a stripped down app which does this:

1. Create a subscription using the following

       using (var connection = EventStoreConnection.Create(new IPEndPoint(IPAddress.Loopback, 1113)))
       {
           connection.SubscribeToStreamAsync(@"mongodb://localhost:27017/Test", false, EventAppeared, SubscriptionDropped);
   
           var repository = new NEventStoreRepository();
           repository.Write(new SomethingHasHappened("Hello"));
   
           Console.ReadLine();
       }
   
   private static void SubscriptionDropped(EventStoreSubscription arg1, SubscriptionDropReason arg2, Exception arg3)
   {
   }
   
   private static void EventAppeared(EventStoreSubscription arg1, ResolvedEvent arg2)
   {
   }
   

2. I write an event to my mongo database via NEventStore

   public void Write(object @event)
   {
       var id = Guid.NewGuid();
   
       using (var scope = new TransactionScope())
       {
           using (var store = WireupEventStore())
           {
               using (var stream = store.OpenStream(id.ToString(), 0, int.MaxValue))
               {
                   stream.Add(new EventMessage { Body = @event });
                   stream.CommitChanges(Guid.NewGuid());
                   scope.Complete();
               }
           }
       }
   
       Console.ReadKey();
   }
   
   private static IStoreEvents WireupEventStore()
   {
       return Wireup
           .Init()
           .LogToOutputWindow()
           .UsingMongoPersistence("NEventStore.MongoDB", new DocumentObjectSerializer())
           .InitializeStorageEngine()
           .UsingJsonSerialization()
           .Build();
   }
   

Answer 1

The normal flow of events for this would be as follows:

(given everything is installed and running...)

1. Register a subscriber to a stream in GetEventStore in your application code
2. Save events to the stream
3. The events appear in your subscriber

I think you are either confusing the flow of things or trying to do something totally unsupported (like having MongoDb subscriber to GetEventStore). What I think your code is doing is:

1. Setting up NEventStore to save to MongoDb
2. Subscribing to a stream in GetEventStore called "mongodb://localhost:27017/Test"
3. Saving an event to MongoDb

As far as I can see, you're never saving any events to GetEventStore, hence why nothing ever appears in the EventAppeared method. You're saving to MongoDb.

[UPDATE]

I want to subscribe to a Mongodb stream and populate GetEventStore which I believe isn't possible from what I gather from your answer.

MongoDb doesn't have streams, it has collections - it is a document database. Streams are a concept in GetEventStore. However, it looks like NEventStore allows you to hook up a message dispatcher which presumably means you can register handlers to listen to events. In these handlers, you can then save to GetEventStore.