What's an Aggregate Root?

I'm trying to get my head around how to properly use the repository pattern. The central concept of an Aggregate Root keeps coming up. When searching both the web and Stack Overflow for help with what an aggregate root is, I keep finding discussions about them and dead links to pages that are supposed to contain base definitions.

In the context of the repository pattern, what is an aggregate root?

Solution 1:

In the context of the repository pattern, aggregate roots are the only objects your client code loads from the repository.

The repository encapsulates access to child objects - from a caller's perspective it automatically loads them, either at the same time the root is loaded or when they're actually needed (as with lazy loading).

For example, you might have an Order object which encapsulates operations on multiple LineItem objects. Your client code would never load the LineItem objects directly, just the Order that contains them, which would be the aggregate root for that part of your domain.

Solution 2:

From Evans DDD:

An AGGREGATE is a cluster of associated objects that we treat as a unit for the purpose of data changes. Each AGGREGATE has a root and a boundary. The boundary defines what is inside the AGGREGATE. The root is a single, specific ENTITY contained in the AGGREGATE.

And:

The root is the only member of the AGGREGATE that outside objects are allowed to hold references to[.]

This means that aggregate roots are the only objects that can be loaded from a repository.

An example is a model containing a Customer entity and an Address entity. We would never access an Address entity directly from the model as it does not make sense without the context of an associated Customer. So we could say that Customer and Address together form an aggregate and that Customer is an aggregate root.