Aggregates

Aggregates group entities and value objects behind one consistency boundary with a single aggregate root as the only external mutation entry. They define transaction scope—invariants that must hold atomically—and prevent tangled references across boundaries. Cross-aggregate updates use IDs and eventual coordination.

On interviews: choose aggregate roots for an e-commerce cart or invoice. Explain small aggregates versus large ones and invariant enforcement.

Common pitfalls: aggregate roots that span entire object graphs; direct mutation of child entities from outside; distributed transactions trying to lock multiple aggregates.

The trade-off is flexibility versus complexity—know when the simpler path is enough.

Checklist:

• One root controls internal consistency.
• External references use identifiers only.
• Size aggregates for real transaction needs.
• Domain events signal cross-aggregate reactions.