Entities

Entities are very similar to Aggregates in nature and purpose. In fact, an Aggregate is nothing but an entity chosen to represent the entire cluster.

Entities are mutable and encapsulate both state and behavior. They have distinct identity. This identity remains consistent throughout the life of the entity, regardless of changes to its attributes.

Facts

Entities have an identity.

The primary characteristic of an entity is its unique identity, which distinguishes it from other objects. This identity is independent of the entity's attributes and remains constant throughout the entity's lifecycle.

Entities are mutable.

Entities can change state over time while maintaining their identity. This mutability is essential for modeling real-world objects that undergo various state changes.

Entities can contain other entities or value objects.

Entities can compose other entities and value objects, enabling the modeling of complex domain relationships. However, care should be taken to manage the complexity and maintain the boundaries of aggregates.

Entities enforce business rules.

Entities are responsible for ensuring that business rules and invariants are upheld within their scope. They encapsulate the logic necessary to maintain domain integrity.

Entities in one domain can be value objects in another.

Entities in one domain can be a Value Object in another.

For example, a seat is an Entity in the Theatre domain if the theatre allocate seat numbers to patrons. If visitors are not allotted specific seats, then a seat can be considered a ValueObject, as one seat can be exchanged with another.

Aggregates handle entity persistence.

Entities are persisted and retrieved through their aggregates. They should not be queried or updated directly, as it can lead to an invalid state because of unsatisfied domain invariants.

Entities are related through associations.

Associations define how entities are connected. These associations are typically managed within the context of an aggregate to ensure consistency and integrity.

Entities can contain nested entities and value objects.

Entities can nest other entities and value objects to form hierarchical structures. These nested objects are managed as part of the aggregate's lifecycle and contribute to the overall state of the entity.

Entity graphs should be kept manageable.

While entities can form complex graphs, it is crucial to maintain manageable structures to avoid excessive complexity. To ensure performance and manageability, entities should be part of aggregates that are appropriately sized. If an aggregate becomes too large, consider splitting it into smaller, more manageable aggregates.