Aggregates represent a concept in the Domain-Driven Design language, which is sometimes confusing to developers. Aggregates are complex models that consist of several entities or value objects, respectively, which form a whole. For consistency reasons, this conglomerate of objects should be accessed and managed as a whole as well. Accessing methods of some contained objects directly could lead to inconsistencies and potential errors. The idea behind aggregates it to represent a root objects for all operations. A good example is a car consisting of four wheels, an engine, a chassis, and so on. Whenever some operation, such as drive is required, it will be invoked on the whole car, potentially involving several objects at once.

Aggregates are entities that also define the root of an object hierarchy. They are implemented as plain Java classes containing business domain functionality and holding reference onto entities and value objects, respectively.

Therefore, aggregates can be persisted using JPA as well. All persistence operations are invoked on the aggregate, the root object, and cascaded to its contained objects. JPA supports persistence of complex object hierarchies, as we will see in later sub-chapters.