Aggregates

Aggregates

It is difficult to guarantee the consistency of changes to objects in a model with complex associations. Many times invariants apply to closely related objects, not just discrete ones. Yet cautious locking schemes cause multiple users to interfere pointlessly with each other and make a system unusable.

Therefore, use Aggregates. An Aggregate is a group of associated objects which are considered as one unit with regard to data changes. The Aggregate is demarcated by a boundary which separates the objects inside from those outside. Each Aggregate has one root. The root is an Entity, and it is the only object accessible from outside. The root can hold references to any of the aggregate objects, and the other objects can hold references to each other, but an outside object can hold references only to the root object. If there are other Entities inside the boundary, the identity of those entities is local, making sense only inside the aggregate.

Objects inside an Aggregate should be allowed to hold references to roots of other Aggregates.

The root Entity has global identity, and is responsible for maintaining the invariants. Internal Entities have local identity.

Cluster the Entities and Value Objects into Aggregates and define boundaries around each. Choose one Entity to be the root of each Aggregate, and control all access to the objects inside the boundary through the root. Allow external objects to hold references to the root only.

Transient references to internal members can be passed out for use within a single operation only. Because the root controls access, it cannot be blindsided by changes to the internals. This arrangement makes it practical to enforce all invariants for objects in the Aggregate and for the Aggregate as a whole in any state change.