Question

When setting up mongodb persistence for Oliver’s EventStore, is registering of BsonClassMap for each event type required? If I don’t do this, I get the exception Unknown discriminator value 'InventoryItemCreated'. I register all event types with MongoDB, like I described here, and then configure my event store:

var types = Assembly.GetAssembly(typeof(SimpleCQRS.Event))
                    .GetTypes()
                    .Where(type => type.IsSubclassOf(typeof(SimpleCQRS.Event)));
foreach (var t in types)
    BsonClassMap.LookupClassMap(t);

var store = Wireup.Init()
            .UsingMongoPersistence("mongo", new DocumentObjectSerializer())
            .UsingSynchronousDispatchScheduler()
            .DispatchTo(new DelegateMessageDispatcher(DispatchCommit))
            .Build();

Is this really necessarily to do?

I ran into this problem while modifying Greg Young’s “simplest thing” to support Oliver’s event store with MongoDB persistence, code is on github.

Some related observations (tldr)

If I don’t register the event types, I can start with an empty event store (because the events don’t need to be deserialized). I can add a single inventory item and my event store has a single commit:

/* 0 */
{
  "CommitId" : new BinData(3, "4iXrzvzNYEyKmGweCKkOVQ=="),
  "CommitStamp" : ISODate("2012-08-01T08:08:35.795Z"),
  "Dispatched" : true,
  "Events" : [{
      "StreamRevision" : 1,
      "Payload" : {
        "Headers" : { },
        "Body" : {
          "_t" : "InventoryItemCreated",
          "Version" : 0,
          "_id" : new BinData(3, "36qvklv+xU2+mDhVUlzmBg=="),
          "Name" : "abc"
        }
      }
    }],
  "Headers" : { },
  "_id" : {
    "StreamId" : new BinData(3, "36qvklv+xU2+mDhVUlzmBg=="),
    "CommitSequence" : 1
  }
}

When I restart the application and replay this single event, it can’t deserialize PayLoad.Body, because of Unknown discriminator value 'InventoryItemCreated'.

I found two alternative ways to successfully replay the InventoryItemCreated event:

• When I create another inventory item before replaying, I can replay
  all events; apparently MongoDB then knows of the discriminator.
• Manually change the discriminator to the full type name:

/* 0 */
{
  // ..
  "Events" : [{
      "StreamRevision" : 1,
      "Payload" : {
        "Headers" : { },
        "Body" : {
          "_t" : "SimpleCQRS.InventoryItemCreated, SimpleCQRS",
        // ..

It strikes me as a bit awkward that apparently the systems can work without registering all event types, but picks a default discriminator determining strategy that prevents this.

Answer 1

I’ve encountered the same situation. For right now, I perform the BSON registration in a similar manner that you’ve described. It doesn’t bother me to have the system perform this registration on startup.

Also, I keep my events and commands classes in a separate project from the actual domain logic as I can then ‘share’ the class structures in both the server and client projects.