Capturing creation of large number of aggregates at once in DDD/CQRS using event sourcing

Question

I have an aggregate that consumes a file and creates a large number of other aggregates as a result.

e.g.
Factory aggregate (event sourced)
Product aggregate (event sourced)

List<Product> Factory.CreateProducts(specifications);

the call to CreateProducts produces one event : FactoryCreatedProducts, which will be saved to the event stream once I save the factory aggregate. The call also results in 10000+ Product aggregates, which, upon instantiation, each will contain one ProductCreated event that will be saved to event stream.

Currently, I have this coded as below:

var factory eventstream.get(command.FactoryId);
var products = factory.CreateProducts(command.specs);

foreach(var product in products) {eventstream.save(product.PendingEvents);}

eventstream.save(factory.PendingEvents);

To me this approach has some fundamental issues, the biggest of which are:

multiple aggregates are being modified at once.
the overhead of carrying 10000+ messages over the message bus and handling them independently in the read model generators.

As an alternative, I could potentially push the entire thing as one event in the message bus, but that would mean a potentially huge event that will grow larger as the number of generated aggregates grow.

Is my approach to this problem common, or have I completely missed the mark on this? What is the proper way to handle thing in a DDD/CQRS architecture where event sourcing is used for persistance of aggregates?

p.s. not relevant to the question but I am using C#, MongoDb for persistence, Windows Service Bus but switching to RabbitMQ.

Idcmp Idcmp · Accepted Answer · 2015-04-22T03:42:43

Technically what you're doing fits nicely in the idea of a Saga; but even as a Saga you would generate 10k+ events to create the 10k+ products. Have you considered lazily creating your products the first time a command is dispatched?

var product = aggregateRepository.find(productId);
if (!product) {
   var productSpecs = db.fetchProductSpecs(productId);
   product = factory.CreateProduct(command.specs);
}
product.someHandler(...

Alternatively creating products more slowly in the background so it doesn't saturate your read model updaters.

Assuming your modelling of Product is correct, you really do want to have 1 event per product to announce its creation.

Capturing creation of large number of aggregates at once in DDD/CQRS using event sourcing

1 Answers