Aggregates

An aggregate is a cluster of domain objects that can be treated as a single unit for data changes.

Each aggregate has a root entity, known as the aggregate root, responsible for enforcing business rules and ensuring the consistency of changes within the aggregate. In Protean, aggregate and aggregate root are treated as synonymous.

Aggregates help to maintain the integrity of the data by defining boundaries within which invariants must be maintained.

Facts

Aggregates are black boxes.

The external world communicates with aggregates solely through their published API. Aggregates, in turn, communicate with the external world through domain events.

Aggregates are versioned.

The version is a simple incrementing number. Every aggregate instance's version starts at 0.

Aggregates have concurrency control.

Aggregates are persisted with optimistic concurrency. If the expected version of the aggregate does not match the version in the database, the transaction is aborted.

Aggregates enclose business invariants.

Aggregates contain invariants that should always be satisfied - they are checked before and after every change to the aggregate. Invariants can be specified at the level of an aggregate's fields, the entire aggregate cluster, individual entities, or domain services that operate on multiple aggregates.

Object Graphs

Aggregates compose a graph of enclosed elements. The objects themselves can nest other objects and so on infinitely, though it is recommended to not go beyond 2 levels.

Aggregates can hold two types of objects - Entities and Value Objects.

Entities are objects with an identity. Value objects don't have identity; their data defines their identity.

Entities are accessible only via aggregates.

Entities within aggregates are loaded and accessible only through the aggregate. All changes to entities should be driven through the aggregates.

Persistence

Data persistence and retrieval are always at the level of an aggregate. They internally load and manage the objects within their cluster.

Aggregates persist data with the help of Repositories.

Aggregates are persisted and retrieved with the help of repositories. Repositories are collection-oriented - they mimic how a collection data type, like list, dictionary, and set, would work. Repositories can be augmented with custom methods to perform business queries.

Aggregates are transaction boundaries.

All changes to aggregates are performed within a transaction. This means that all objects in the aggregates cluster are enclosed in a single transaction during persistence. This also means that all objects within an aggregate cluster are kept together in the same persistence store.

Aggregates can enclose up to 500 entities.

The object graph under an aggregate is loaded eagerly. The number of associations under an aggregate is limited to 500. If you expect the number of entities to exceed this limit, rethink your aggregate boundary. One way would be to split the aggregate into multiple aggregates. Another would be to make the underlying entity an aggregate by itself.