Anemic Domain Model

The Anemic Domain Model is a pejorative term used to describe the use of a domain model where the business logic is implemented outside the domain objects. This pattern was first described by Martin Fowler who considers the practice an anti-pattern. With this pattern, 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 enterprise Java applications, possibly encouraged by technologies such as J2EE's Entity Beans, [http://www.martinfowler.com/bliki/AnemicDomainModel.html] as well as in .NET applications following the Three-Layered Services Application architecture where such objects fall into the category of "Business Entities" (though Business Entities may also contain behavior). [http://msdn2.microsoft.com/en-us/library/ms954595.aspx]

Benefits

* Encourages and enforces a clear separation of concerns between presentation layers, business layers, and resource access layers of an application.
* Allows for the development of a common object model which can be applied across an Enterprise while at the same time allowing for flexibility in the domain logic implementation used by differing applications.
* Enables auto-generation through modeling tools which may render a common object model in various languages used across an Enterprise.

Liabilities

* Necessitates 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 any moment, because their validation and mutation logic is placed somewhere outside (most likely in multiple places).
* Necessitates a global access to internals of shared business entities increasing coupling and fragility.
* Facilitates code duplication among transactional scripts and similar use cases, reduces code reuse.
* Necessitates a service layer when sharing domain logic across differing consumers of an object model.
* Makes a model less expressive and harder to understand.

External links

* [http://www.martinfowler.com/bliki/AnemicDomainModel.html Anemic Domain Model] by Martin Fowler
* [http://msdn.microsoft.com/en-us/library/ms978689.aspx Three-Layered Services Application]
* [http://msdn2.microsoft.com/en-us/library/ms954595.aspx Application Architecture for .NET: Designing Applications and Services]