Domain Model Design Pattern

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What is the Domain Model Design Pattern?

The Domain Model Pattern is one of the core domain logic patterns featured in Martin Fowler's Patterns of Enterprise Application Architecture (PofEAA).

At its core, a Domain Model is an object model of the business domain that incorporates both behavior and data. Instead of writing procedural scripts with dumb data records, you create rich objects that represent business entities and encapsulate their business rules.

Simple Domain Model vs. Rich Domain Model

• Simple Domain Model: Closely mirrors the relational database schema. If the database has 5 tables, the domain layer typically has 5 classes corresponding directly to those tables. It is quick to set up for straightforward systems.
• Rich (Complex) Domain Model: Employs true object-oriented concepts like inheritance, polymorphism, design strategies, and webs of small interconnected objects. It is designed to tackle complex, ever-evolving business logic even though mapping it to a relational database can be more challenging.

Real-World Hierarchy Example

A classic domain model utilizes clean object hierarchies to express relationships naturally:

• A Base class extended by Person and Task.
• Person is further extended by specific domain roles like Developer and Analyst.
• Task is extended by structures like Project.

Each entity encapsulates the operations and validations appropriate to its role, keeping business logic organized, reusable, and easy to maintain over time.

When Should You Use a Domain Model?

• When your business rules and validation logic are complex and interconnected.
• When business logic is subject to frequent change and evolution.
• When you need full object-oriented power—such as inheritance, strategy patterns, and polymorphism—to handle complicated business workflows cleanly.

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Recommended Design Pattern Tutorials:

• Builder Design Pattern: Overview and Basics
• Flyweight Design Pattern: Sequence Diagram Explained
• Adapter Design Pattern: Class Diagram Overview

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Domain Model Design Pattern