Inter-Aggregate Communication in CQRS + DDD + Event Sourcing

Question

How should separate aggregate roots communicate with one another in an environment built on DDD principles using an event-sourced aggregate back-end?

For instance, I have a Facility aggregate root (AR) which has a factory method responsible for creating a Booking AR. The Booking is a time-sensitive combination of a Person AR and a Facility AR. A Person can only be booked in a single Facility.

In DDD, I would have held references to the Booking in Person, and Person in Facility. However, when generating events for use in event-sourcing I think that trying to handle the event deserialization from the back-end would become prohibitive. Therefore, I've taken to only holding references to the value object-based unique id's. This brings up a new problem, however, when a method on an AR needs to call another method on another AR--how do you handle that situation? Hit the event source repository from the domain AR?

What is the general use case in this scenario? Am I approaching this all wrong?

Answer 1

When using Event Sorcing and CQRS the most elegant (at least in my opinion) way of inter-AR communication is messaging. You can look at Ncqrs project (it will be easier if you are a .NET guy), particularly 'Messaging' branch. The idea is, ARs implement IMessageHandler interface for every message type they handle and AR base class exposes method Send for sending there messages. By means of this API clients can invoke model behavior and model itself can communicate (between ARs).