Guidelines for identifying and designing entities in a DDD

Here are some tips for identifying and designing entities in your DDD application:

• Look for objects in your domain that are important to the business and that need to be tracked over time.

  Entities are the core of the domain model in DDD. They represent the fundamental business objects in the domain, such as products, customers, orders, and invoices. Entities should be important to the business and should need to be tracked over time. This means that entities should be things that the business needs to know about and keep track of, such as the products that it sells, the customers that it has, and the orders that it receives.

• Consider the identity of each object.

  Every entity must have a unique identity. This identity is used to distinguish one entity from another of the same type. For example, the identity of a product might be its product ID, the identity of a customer might be their customer ID, and the identity of an order might be its order ID. The identity of an entity should be stable and should not change over time.

• Consider the lifecycle of each object.

  Every entity has a lifecycle. The lifecycle of an entity describes how the entity can be created, updated, and deleted. For example, a product can be created, updated with a new price, or deleted. A customer can be created, updated with a new address, or deleted. An order can be created, updated with a new shipping address, or deleted. The lifecycle of an entity should be modeled in the domain model.

• Encapsulate the state and behavior of each object.

  Entities should encapsulate their own state and behavior. This means that the state and behavior of an entity should be hidden from the outside world. The only way to interact with an entity is through its public methods. Encapsulating the state and behavior of entities helps to ensure that the state of the entities is consistent and that they are only modified by authorized code.

• Use value objects to represent immutable data.

  Value objects are similar to entities, but they are immutable. This means that their state cannot be changed once they are created. Value objects are useful for representing data that does not change over time, such as a customer's name, address, or phone number.