Domain Modeling Made Functional

A Question of Identity: Entities

However, we often model things that, in the real world, do have a unique identity, even as their components change. For example, even if I change my name or my address, I am still the same person.

In DDD terminology, we call such things Entities.

In a business context, Entities are often a document of some kind: orders, quotes, invoices, customer profiles, product sheets, and so on. They have a life cycle and are transformed from one state to another by various business processes.

The distinction between “Value Object” and “Entity” is context-dependent. For example, consider the life cycle of a cell phone. During manufacturing, each phone is given a unique serial number—a unique identity—so in that context, the ...