Eventual consistency across aggregate roots in the same bounded context using a process manager aka saga

Question

Suppose you've got two aggregates in your bounded context which have some constraints amongst each other. Using DDD these inter aggregate constraints can't be enforced in the same transaction i.e. the aggregate boundaries are transactional boundaries.

Would you consider using what in the Microsoft CQRS journey is called a "process manager" to coordinate two aggregates in the same bounded context or is a process manager only used to coordinate between two bounded contexts? What would the equivalent of a process manager that coordinates two or more aggregate roots within the same bounded context be?

Answer 1

An aggregate root defines a bounded context by default, albeit a lower level one (btw the lowest level bounded context you can find is an object, any object). The process manager is the name they used instead of a saga, proabably you can come up with other names too, it doesn't matter, they all have the same purpose.

And yes, I would consider using a saga to achieve eventual consistency. In fact, I think this is the best way and this is exactly what I'm doing in my own apps. Anyway, I'm using a message driven architecture (yes, in a local, non-distributed application) and I have automatically saga support via the service bus (my own, not released yet).

What is important when dealing with eventual consistency is to ensure idempotency everywhere. That is the aggregate roots should reject a duplicate operation and of course the event handler should be able to cope with the fact that the same event can be published more than once. However, be aware that you can't guarantee 100% idempotency but you can get very close to.