In Domain-Driven Design (DDD), a domain model is a representation of the problem domain in the form of a software model. It is a collection of domain objects and the relationships between them, and is used to model the business logic and behavior of the domain.

The domain model is the central focus of DDD, and serves as a communication tool between domain experts and software developers. It provides a shared understanding of the domain and its concepts, and helps to ensure that the software system accurately reflects the business requirements and constraints.

The domain model is constructed using a combination of object-oriented techniques, such as classes and objects, and domain-specific techniques, such as entities, value objects, and domain services. The model is iteratively refined as the project progresses, and evolves to accurately reflect the changing needs of the business.

The ultimate goal of the domain model is to provide a clear, concise, and accurate representation of the problem domain, and to serve as a basis for implementing the software system that solves the business problems.

Is DDD tied to Object-Oriented Programming?

No, DDD is not tied specifically to object-oriented programming. DDD provides a set of principles and patterns for designing software systems that can be applied to various programming paradigms, including object-oriented, functional, and procedural. While OOP is a common way to implement DDD, the concepts can also be applied in other programming paradigms such as functional programming or procedural programming. The important thing is to capture the domain concepts accurately, not just to use a specific programming paradigm.

An example

Imagine a retail business that sells products to customers. A domain model for this business might include the following concepts:

• Product: Represents a product that the business sells, such as a t-shirt or a pair of shoes. Attributes might include the product’s name, description, price, and availability.
• Customer: Represents a customer who makes purchases from the business. Attributes might include the customer’s name, address, email, and purchase history.
• Order: Represents an order that a customer places for one or more products. Attributes might include the date of the order, the products included in the order, and the total price of the order.
• Payment: Represents a payment made by a customer for an order. Attributes might include the date of the payment, the payment method (such as credit card or PayPal), and the amount paid.
• Inventory: Represents the business’s inventory of products. Attributes might include the number of each product in stock and the minimum number of each product that should be kept in stock.

These concepts form a simple domain model that captures the core concepts of a retail business. The domain model can be used to implement the business logic and data storage for the retail business.

Multiple domain models

It is possible for more than one domain model to co-exist within an organization or system. Each domain model focuses on a specific area of the business and represents its concepts, rules, and relationships in a distinct way. The different domain models may interact with each other, but they maintain their own autonomy and consistency boundaries. This allows for the modeling of complex systems with multiple business domains, each with its own unique requirements and perspectives.