Retrieval of child objects of aggregates in DDD

Question

In DDD root of an aggregate is the only reference to retrieve its child objects. Repository of root of an aggregate is responsible for giving the root object reference only. If I need child objects then need to call a getter method of the aggregate to retrieve the child objects which results in a DB query.

Consider a case where I am retrieving multiple aggregates from DB. So in my case this situation results in multiple DB queries which leads a very slow request. How to avoid this in terms of DDD. For persisting I came across a pattern called Unit Of Work. Is there any pattern for the search which resolves my problem or any other way to do this.

Answer 1

First of all, 95% of problems are solved by your ORM (if you happen to use relational database).

Aggregate root repository should (in most cases) return a fully loaded object with all child objects (entities). Lazy loading children should be an exception, not a rule.

Another thing is, you should avoid loading and persisting multiple aggregates at a time. Try repartitioning you domain so that each user interaction deals with only one aggregate.

And consider a document database solution. It really makes sanes to store whole aggregates as documents in a doc database.

Answer 2

Okey it seems like you have a scenario where you in a single use case want to read from several AR and also savee their state into DB. Is the read operation taking to long? or is it both read and write that takes time?

Your domain model and Aggregate roots should be partly defined through interation from use cases. What I'm saying is that, the model should be designed so it suits your clients needs. This scenario seems not like one that fits your model well.

Reports or other operations that uses a large data view should be bypasses the domain model. Don't use DDD for reports etc. Just do a fast data access.

Second. Unit of work is one way to go, if you want all aggregates to participate in a transaction.

Third. I would say, Use Lazy loading, but some use cases that need performance boost you can do a loading strategy which means you let the root load some child collections without having sql-sub-selects firing... look at this article http://weblogs.asp.net/fredriknormen/archive/2010/07/25/loading-strategy-for-entity-framework-4-0.aspx (even its for EF pattern works well for NH ORM)

Then at last you can always provide db indexes, caching etc to boost perfomance, but given the scenario info, you have takensome kind of wrong design desicion. I don't havee all the facts but maybe some use cases aren't suitable for