Entities

Entities are domain objects with a unique identity that persists over time. Unlike Value Objects, entities are identified by their ID rather than their attributes.

Characteristics

• Identity: Each entity has a unique identifier
• Lifecycle: Entities are created, modified, and deleted
• Equality by ID: Two entities with the same ID are the same entity
• Encapsulation: Entities protect invariants through behavior methods

Structure

Entities extend EntityAbstract and implement EntityInterface:

class Book extends EntityAbstract implements EntityInterface
{
    public function __construct(
        private readonly IdInterface $id,
        private readonly Title $title,
        private readonly Author $author,
        private readonly ISBN $isbn,
        private BookStatus $status = BookStatus::Available
    ) {}

    public function getId(): IdInterface
    {
        return $this->id;
    }

    public function markAsBorrowed(): void
    {
        $this->status = BookStatus::Borrowed;
    }
}

Identity Types

Available implementations via IdInterface:

• UuidV4 - Random UUID (version 4)
• UuidV7 - Timestamp-ordered UUID (version 7)
• Ulid - Lexicographically sortable identifier
• TimestampId - Base32-encoded timestamp ID

Entity vs Value Object

Best Practices

• Compose with Value Objects for entity properties
• Protect invariants in behavior methods
• Name methods after domain actions (markAsBorrowed, not setStatus)
• One repository per aggregate root

See Also

• ID Generator - Generating unique identifiers