Event Sourcing using NHibernate

Question

I'm moving from pure DDD paradigm to CQRS. My current concern is with Event Sourcing and, more specifically, organizing Event Store. I've read tons of blogs posts but still can't understand some things. So correct me if I'm wrong.

Each event basically consists of: - Event date/time - type of Event (we can figure out type of AggregateRoot from this as well) - AggregateRoot id (Guid) - AggregateRoot version (to maintain the order of updates) - Event data (some serialized class with data necessary to make update)

Now, if my Event data consists of simple value types (ints, strings, enums, etc.) then it's easy. But what if I have to pass another AggregateRoot? I can't serialize the whole AR as a part of Event data (think of all the data and lazy loading), basically I only need to store Id of that AR. But then, when I need to apply that event, I'd need to get that AR from database first. And it doesn't feel right to do so from my Domain Model (calling Repositories and working with AR Ids).

What's the best approach for this?

p.s. For a concrete example, let's assume there's a Model which consists of Task and User entities (both ARs). Task hold a reference to User responsible. But the responsible User can be changed.

Update: I think I've found the source of my confusion. I believe event sourcing should be used only for building read model. And in this case passing Ids and raw data is ok. But the same events used on aggregates themselves. And this I cannot understand.

Answer 1

In DDD an aggregate is a consistency/invariant boundary, so one may never depend on another to maintain its invariants. When we start using this design restriction we find very few situations where is necessary to store a full reference to the other, usually we store its id and (if necessary) version and a copy of the relevant attributes.

For example, using the usual Order/LineItem and Product problem we would copy the Product's id and price in the LineItem, instead of a full reference. This way prevents changes in the Product's price affect the Order/LineItem aggregate's invariants. If is necessary to update the LineItem price after Product price changes we need to keep track of the PriceChanged event from used Products and send a compensating command to the Order/LineItem. Usually this coordination/synchronization is handled by a saga.

Answer 2

In Event Sourcing, the state of the aggregate is defined by Events, and nothing more. All domain model stuff (ala DDD) is there just to decide what domain events should be raised. Event should know nothing about your Domain, it should be simple DTO. In fact, it is perfectly OK to have Event Sourcing without DDD.

Answer 3

As i understand Event Sourcing, it is supposed to help people get rid of relational data models and ORM like NHibernate or Entity Framework, since each of them is a science on its own. Programmers could then simply focus on business logic. I saw here some relational schemas used for event stores, and they were simply ID, Version, Timestamp plus an NClob or NVarchar(max) column to store the event payload schema-less.