Aggregates, Repositories and Factories

Once you have entities and value objects, you need patterns to keep clusters of them consistent and to persist them cleanly. DDD answers with three tactical patterns: the Aggregate (a consistency boundary), the Repository (a collection-like persistence abstraction), and the Factory (complex construction). This lesson shows how they fit together in PHP.

An Aggregate is a cluster of entities and value objects treated as a single unit for data changes. One entity is the Aggregate Root — the only member outside code may hold a reference to. All modifications go through the root, which enforces the aggregate's invariants. The aggregate is also the transactional boundary: it is loaded and saved atomically.