While searching the web, I came across a list of rules from Eric Evans' book that should be enforced for aggregates:
The root Entity has global identity and is ultimately responsible for checking invariants
Root Entities have global identity. Entities inside the boundary have local identity, unique only within the Aggregate.
Nothing outside the Aggregate boundary can hold a reference to anything inside, except to the root Entity. The root Entity can hand references to the internal Entities to other objects, but they can only use them transiently (within a single method or block).
Only Aggregate Roots can be obtained directly with database queries. Everything else must be done through traversal.
Objects within the Aggregate can hold references to other Aggregate roots.
A delete operation must remove everything within the Aggregate boundary all at once
When a change to any object within the Aggregate boundary is committed, all invariants of the whole Aggregate must be satisfied.
This all seems fine in theory, but I don't see how these rules would be enforced in the real world.
Take rule 3 for example. Once the root entity has given an exteral object a reference to an internal entity, what's to keep that external object from holding on to the reference beyond the single method or block?
(If the enforcement of this is platform-specific, I would be interested in knowing how this would be enforced within a C#/.NET/NHibernate environment.)
