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Article:
  Wiring Your Web Application with Open Source Java
Subject:   Not a good template for building a domain layer
Date:   2004-04-13 12:31:34
From:   Groo
Response to: Not a good template for building a domain layer

Sorry, I didn't mean to imply a service layer is not a good thing. What I meant to say was that the business logic logic should be in the domain objects rather than in the service layer.


In a simple example such as this it may not be as obvious, but as you get a more complex domain model it becomes more apparent. For example, even something as trivial as polymorphism is hard to take advantage of without a proper domain model.

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Showing messages 1 through 5 of 5.

  • Not a good template for building a domain layer
    2004-04-27 09:29:50  thlee [View]

    IMHO the separation of domain model and business logic
    has became an inevitable trend in most business application
    development.

    The reason is that domain model often refers to conceptual/ER model, which is essentially
    the data/bean objects. Thus persistence is part of the requirement of the domain objects.
    While ideally POJO is preferred for domain objects (like Martin and other OO purist wished for),
    in most cases they have been converted to accomodate the persistent layer
    such as EJB or other O/R mapping tool due to the persistence requirement.
    To embed business logic inside the domain model implies that
    more (non-bean)APIs will be added to the domain model and persistent layer specific
    transaction-handling logic will need to be coded inside the domain objects
    which will make the domain model and implementation even uglier.

    Thus, separating the domain object from the business logic does have some advantages:
    1. Cleaner domain model and implementation
    2. Stability of domain model on changing/different implementations of business logic
    3. Separation of concern (bean persistence and business logic are separated)

    Just my 2 cents.

    BTW, nice article and comments.
    • Not a good template for building a domain layer
      2004-04-28 12:53:42  gaoy [View]

      No DTOs (data transfer objects) are maintained. This may be feasible for certain type of domain models, such as without inheritance and polymorphism. If it is running on a rich client such as Swing or C#, or B2B through web services, I wonder one can avoid using DTOs. Are there frameworks facilitating management of DTOs?

      Very nice article, and good usage of Spring IoC for decoupling layers via interfaces.
      • Not a good template for building a domain layer
        2005-01-01 18:18:31  wrschneider99 [View]

        I stand corrected on my original comment;
        Hibernate does indeed let you persist private fields
        , and this feature can be used to move business rules into the domain layer when appropriate.

        Also, I have contributed to an XDoclet module to generate DTOs from metadata in the domain model, called XSnapshot.
    • Not a good template for building a domain layer
      2004-05-24 22:06:55  lachlanj [View]

      I see the domain layer & the service layer a little differently.

      Basically anything related to the problem domain, e.g. entities, value objects & the business rules related to them should go into the domain layer. This allows you to express domain concepts including business rules through objects.

      Anything related to the application (technical stuff) like transaction management, message sending, logging etc should not be in the service layer (sometimes application layer). This layer uses the domain layer to call the business rules. This layer can also be used to integrate with other applications.

      I think it is very important to make a clear separation between the two, after all they are two completely different things when you think about it. One is related to technical related implementation specific stuff, and the other is related directly to the problem domain in which the application is trying to solve.

      Without this separation it becomes too difficult to extend a system that has complex business rules and thus eliminates any of the benefits of using OOP.

      Check out http://www.domaindrivendesign.org/ for detailed information about why domain models are so important.

      Lachlan

      Lachlan
      • Not a good template for building a domain layer
        2004-05-24 22:09:50  lachlanj [View]

        Sorry, I meant to say...

        "Anything related to the application (technical stuff) like transaction management, message sending, logging etc should be in the service layer" :)