Anemic Domain Model
Encyclopedia
Anemic domain model is a term used to describe the use of a software domain model
where the business logic
is implemented outside the domain objects.
, 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 early versions of EJB's Entity Bean
s, 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).
Domain model
A domain model in problem solving and software engineering can be thought of as a conceptual model of a domain of interest which describes the various entities, their attributes, roles and relationships, plus the constraints that govern the integrity of the model elements comprising that problem...
where the business logic
Business logic
Business logic, or domain logic, is a non-technical term generally used to describe the functional algorithms that handle information exchange between a database and a user interface.- Scope of business logic :Business logic:...
is implemented outside the domain objects.
Overview
This pattern was first described by Martin FowlerMartin Fowler
-Online presentations:* at RailsConf 2006* at JAOO 2006* at QCon London 2007 * at QCon London 2008 * at ThoughtWorks Quarterly Technology Briefing, October 2008...
, who considers the practice an anti-pattern
Anti-pattern
In software engineering, an anti-pattern is a pattern that may be commonly used but is ineffective and/or counterproductive in practice.The term was coined in 1995 by Andrew Koenig,...
. 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 early versions of EJB's Entity Bean
Entity Bean
An Entity Bean is a type of Enterprise JavaBean, a server-side J2EE component, that represents persistent data maintained in a database. An entity bean can manage its own persistence or can delegate this function to its EJB Container . An entity bean is identified by a primary key...
s, as well as in .NET
.NET Framework
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
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).
Liabilities
- Logic cannot be implemented in a truly object-oriented way unless wrappers are used, which hide the anemic data structure.
- Violation of the encapsulation and information hidingInformation hidingIn computer science, information hiding is the principle of segregation of the design decisions in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decision is changed...
principles. - Necessitates a separate business layer to contain the logic otherwise located in a domain modelDomain modelA domain model in problem solving and software engineering can be thought of as a conceptual model of a domain of interest which describes the various entities, their attributes, roles and relationships, plus the constraints that govern the integrity of the model elements comprising that problem...
. It also means that domain modelDomain modelA domain model in problem solving and software engineering can be thought of as a conceptual model of a domain of interest which describes the various entities, their attributes, roles and relationships, plus the constraints that govern the integrity of the model elements comprising that problem...
's objects cannot guarantee their correctness at 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.
See also
- Domain-driven designDomain-driven designDomain-driven design is an approach to developing software for complex needs by deeply connecting the implementation to an evolving model of the core business concepts...
- Plain Old Java ObjectPlain Old Java ObjectIn computing software, POJO is an acronym for Plain Old Java Object. The name is used to emphasize that a given object is an ordinary Java Object, not a special 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 FowlerMartin Fowler-Online presentations:* at RailsConf 2006* at JAOO 2006* at QCon London 2007 * at QCon London 2008 * at ThoughtWorks Quarterly Technology Briefing, October 2008...
- Three-Layered Services Application
- Application Architecture for .NET: Designing Applications and Services