Domain driven design and aggregate references

Question

I am designing the domain model, but there is something that doesn't seem to be ok.

I start with a main aggregate. It has references to other aggregates and those other aggregates reference more aggregates too. I can travel the hole domain model starting from the main aggregate.

The problem I see is that I will be holding all instances of aggregates in memory.

Is that a good design? I can solve the memory problem with lazy loading but I think that I have a deeper problem.

I have another question regarding aggregate references. Should I load the references to other aggregates lazily? If that is the case I would almost never user their repositories. Is that ok?

Answer 1

Having direct referenced between aggregate roots (ARs) can lead to problems that cannot be solved by lazy loading. Moreover, it forces all connected ARs to be in the same database and makes it more difficult to reason about and enforce invariants which is the primary purpose of an AR in the first place. It is better to limit or eliminate direct references between ARs. A great resource to learn about aggregate design is a series of articles by Vaughn Vernon. The basic idea is to make your ARs lean and focused all while keeping in mind their function - enforcing business constraints and forging a boundary around the root entity. If an AR needs data from another AR to perform its work, this data can be provided to it by an application service via repository. Also, if references are only needed to fulfill UI requirements, then consider using the read-model pattern.