Software Architect's Handbook

Aggregates and root entities

Aggregates are groupings of entities and value objects that are treated as a single unit. A boundary is defined that groups them together. Without aggregates, complicated domain models can become unwieldy to manage, as the many entities and their dependencies grow large in number. Retrieving and saving an entity and all of its dependent objects can become difficult and error-prone.

An example of an aggregate is an order object that contains an address object and a collection of line item objects. The address object and the collection of line item objects are all separate objects, but they are treated as a single unit for data retrieval and changes.