Anemic domain model

Anemic domain model is the use of a software domain model where the domain objects contain little or no business logic (validations, calculations, business rules etc.).

Overview

This pattern was first described by Martin Fowler, who considers the practice an anti-pattern. He says:

The fundamental horror of this anti-pattern is that it's so contrary to the basic idea of object-oriented designing; which is to combine data and process them together. The anemic domain model is just a procedural style design, exactly the kind of thing that object bigots like me ... have been fighting since our early days in Smalltalk. What's worse, many people think that anemic objects are real objects, and thus completely miss the point of what object-oriented design is all about.

In an anemic domain design, business logic is typically implemented in separate classes which transform the state of the domain objects. Fowler calls such external classes transaction scripts. This pattern is a common approach in Java applications, possibly encouraged by technologies such as early versions of EJB's Entity Beans, as well as in .NET applications following the Three-Layered Services Application architecture where such objects fall into the category of "Business Entities" (although Business Entities can also contain behavior).

Fowler describes the transaction script pattern thus: "Most business applications can be thought of as a series of transactions. A transaction may view some information as organized in a particular way, another will make changes to it. Each interaction between a client system and a server system contains a certain amount of logic. In some cases this can be as simple as displaying information in the database. In others it may involve many steps of validations and calculations. A Transaction Script organizes all this logic primarily as a single procedure, making calls directly to the database or through a thin database wrapper. Each transaction will have its own Transaction Script, although common subtasks can be broken into subprocedures." In his book "Enterprise Application Patterns", Fowler noted that the transaction script pattern is OK for many simple business applications, and avoids the need for a complex OO-database mapping layer.

Benefits

• Clear separation between logic and data (Procedural programming).
• Works well for simple applications.
• Results in stateless logic, which facilitates scaling out
• Avoids the need for a complex OO-Database mapping layer.
• It follows the Single Responsibility principle giving a class no more than one reason to change (the data changes).

Liabilities

• Logic cannot be implemented in a truly object-oriented way.
• Violation of the encapsulation and information hiding principles.
• Needs a separate business layer to contain the logic otherwise located in a domain model. It also means that domain model's objects cannot guarantee their correctness at any moment, because their validation and mutation logic is placed somewhere outside (most likely in multiple places).
• Needs a service layer when sharing domain logic across differing consumers of an object model.
• Makes a model less expressive.

See also

• Domain-driven design
• Plain Old Java Object
• GRASP information expert, an anemic domain model is the typical result of not applying the information expert principle, i.e. you can avoid an anemic domain model by trying to assign responsibilities to the same classes that contain the data

External links

• Anemic Domain Model by Martin Fowler
• Three-Layered Services Application
• Application Architecture for .NET: Designing Applications and Services