What Is an Aggregate?
Aggregates are Entities that hold other Entities and Value Objects that help keep data consistent. From Vaughn Vernon's Implementing Domain-Driven Design:
Aggregates are carefully crafted consistency boundaries that cluster Entities and Value Objects.
Another amazing book that you should definitely buy and read is NoSQL Distilled: A Brief Guide to the Emerging World of Polyglot Persistence by Pramod J. Sadalage and Martin Fowler. This book says that:
In Domain-Driven Design, an aggregate is a collection of related objects that we wish to treat as a unit. In particular, it is a unit for data manipulation and management of consistency. Typically, we like to update aggregates with atomic operations and communicate with our data storage in terms of aggregates.
What Martin Fowler Says...
From http://martinfowler.com/bliki/DDD_Aggregate.html:
Aggregate is a pattern in Domain-Driven Design. A DDD aggregate is a cluster of domain objects that can be treated as a single unit. An example...
