Martin Fowler, whom I respect, coined the term Anemic Domain Model (ADM) and called it anti-pattern.
At first, one will tend to agree with what Martin is saying. In OO design, objects must carry their state as well as their behavior. Having objects just with state (Entity Beans, Hibernate Entities) is same as C struct. Similarly having objects with just behavior (Stateless Session Beans) is transaction script pattern which is procedural style of programming.
In pure OO world, domain objects should have both state which is business information and behavior which are business rules. People call this Rich Domain Model (RDM).
Sounds good? Okay...sounds good only for small single tier desktop application.
What about multi tier and/or distributed applications? Well...then many questions pop in the mind.
What about separation of concerns? Aren't we mixing persistence concern with business logic concern?
Are Rich Domain Objects more reusable in different application? It doesn't look right. Generally Business Information is more reusable across different applications then the business rules. That is why integration between different applications is web services (XML) based. If that wasn't true, we should have seen Jini being more popular for enterprise integration then XML.
What about distributing the work in a large team? Doesn't Rich Domain Model requires all team members to be expert in all technologies? Doesn't this increase the cost of development?
I still don't have much trust in MDA and hence don't believe that complex Rich Domain objects can be auto-generated. I think C# has support for partial classes to solve such problem but there is no equivalent in Java.
In a real enterprise business application, business rules and business policies are more volatile than the business information. Well in a Rich Domain Model, both are combined into same classes, there is no exercise to distinguish and break down the classes into more stable package vs less stable package. Regardless of whether business information changes or business rules/policies change, same set of classes are going to be affected. Does it sound good approach? Not to me.
What about using Rules engine? Can these be still used with Rich Domain Model? If so how? Doesn't this require the rules and policies that alter the data to be separated from the data itself?
How are these Rich Domain objects implemented. Are these simply POJO or these are objects which are similar to classes that combine Stateful Session Bean and Persistent Entity? From the articles on internet and material in books, it appears that The answer here should be POJO.
In a multi-tier application, what does client/presentation tier see? Are Rich Domain objects exposed to the client? Can client invoke business methods locally?
What about the transactional boundaries? How do I ensure that business rules are executed as part of a transaction? If client is able to call the business rules methods locally, there is no transaction, there is no data source available. In real enterprise application, the connection to database server is protected and only the application server and few DBAs are allowed to connect to it. For a web only application, the data source issue shouldn't be there but what about the application that uses Rich Desktop client (Swing)?
Well even after reading many books that advocate Rich Domain Model, I don't seem to get clear answers to above questions.
Rich Domain Model is good from OO purism point of view. In fact its 'the' way of writing non-distributed applications. It should also be the choice for writing distributed application written in Jini.
However, for real enterprise applications, I will be more inclined to use ADM because:
I know that ADM works.
It allows good work distribution in a large team.
Model can be created in an iteration that precedes service objects development and presentation tier (and client tier) development.
- Project sponsors in a Corporate don't care about OO purism. The bottom line for them is to deliver an application that is easy for the developers to write, can be delivered on time and in budget and which works.