An Aggregate is an Entity that may hold other Entities and Value Objects. The parent Entity is known as the root Entity.

A single Entity without any child Entities or Value Objects is also an Aggregate by itself. That's why in some books, the term Aggregates is used instead of the term Entity. When we use them here, Entity and Aggregate mean the same thing.

The main goal of an Aggregate is to keep your Domain Model consistent. Aggregates centralize most of the business rules. Aggregates are persisted atomically in your persistence mechanism. No matter how many child Entities and Value Objects live inside the root Entity, all of them will be persisted atomically, as a single unit. Let's see an example.

Consider an e-commerce application, website, and so on. Users can place orders, which have multiple lines that define what product was bought, the price, the quantity, and the line total amount. An order has a total amount too, which is the sum of all line amounts.

What...