How to use sagas in a CQRS architecture using DDD?

Question

I am designing a CQRS application using DDD, and am wondering how to implement the following scenario:

• a Participant aggregate can be referenced by multiple ParticipantEntry aggregates
• an AddParticipantInfoCommand is issued to the Command side, which contains all info of the Participant and one ParticipantEntry (similar to an Order and one OrderLineItem)

Where should the logic be implemented that checks whether the Participant already exists and if it doesn't exist, creates the Participant?

• Should it be done in a Saga that first checks the domain model for the existence of the Participant, and if it doesn't find it, issues an AddParticipantCommand and afterwards an AddParticipantEntry command containing the Participant ID?
• Should this be done entirely by the aggregateroots in the domain model itself?

Answer 1

You don't necessarily need sagas in order to deal with this situation. Take a look at my blog post on why not to create aggregate roots, and what to do instead:

http://udidahan.com/2009/06/29/dont-create-aggregate-roots/