Entity vs. VValue Objects
The primary difference between Entity and Value Objects lies in their identity and purpose within a software system, a core concept in Domain-Driven Design (DDD).
| Characteristic | Entity Object | Value Object |
| Identity | Has a unique, persistent identity that remains constant throughout its lifecycle. | Defined solely by its attributes; does not have its own distinct identity. |
| Equality | Equality is based on identity, not attribute values. Two entities with the same attributes are still different if their IDs differ. | Equality is based on structural (attribute) values. Two value objects with the same attributes are considered identical and interchangeable. |
| Mutability | Typically mutable; its state can change over time without affecting its unique identity. | Generally immutable; once created, its attributes cannot be changed. |
| Lifecycle | Has a long lifecycle and can be tracked over time (created, modified, deleted). | Has a short, transactional lifecycle, often belonging to an entity. It is created, used, and discarded as needed. |
| Examples | User, Order, Product, Post. | Address, DateRange, Color, GPS Coordinates. |
Key Distinctions Explained
- Identity vs. Attribute-based Equality: The core difference is the focus on who something is (Entity) versus what something is (Value Object). A person is an entity because they have a unique identity (e.g. social security number), even if their name or address changes. An address on the other hand, is generally a value object because you only care about its components (street, city, zip code), not a specific instance of that data in memory.
- Mutability: Value objects are typically immutable, which makes them simpler to manage and reason about, as their state is guaranteed not to change after creation. Entities are often mutable because tracking changes to their state is an essential part of business function.
- Usage: Entities represent core business concepts that need continuity and tracking. Value objects describe or measure an aspect of an entity or domain concept, ensuring data validity and simplifying the domain model where unique identity is unnecessary.
- Persistence: Entities typically have their own table in an database and are retrieved via their unique ID. Value objects are often stored as part of an entity's table (e.g. embedded fields) and do not have their own repositories.
In practice, the same concept can be an Entity in one context and a Value Object in another, depending entirely on the specific business domain and requirements.